USB, short for UNIVERSAL SERIAL BUS, is an industry standard that
defines cables, connectors and communications protocols for
connection, communication, and power supply between computers and
USB was designed to standardize the connection of computer
peripherals (including keyboards, pointing devices , digital cameras,
printers, portable media players , disk drives and network adapters )
to personal computers , both to communicate and to supply electric
power . It has largely replaced a variety of earlier interfaces, such
as serial ports and parallel ports , as well as separate power
chargers for portable devices – and has become commonplace on a wide
range of devices.
Created in the mid-1990s, it is currently developed by the USB
Implementers Forum (
* 1 Overview
* 2 History
* 2.1 Version history
* 2.1.1 Overview
* 2.1.2 Power related specifications
* 3 System design
* 4 Device classes
USB mass storage /
Media Transfer Protocol
* 4.3 Human interface devices
* 4.4 Device
* 5 Connectors
* 5.1 Connector properties
* 5.1.1 Receptacles and plugs
* 5.1.2 Usability and orientation
* 5.1.3 Power-use topology
* 5.1.4 Durability
* 5.1.5 Compatibility
* 5.2 Connector types
* 5.2.1 Standard connectors
* 5.2.2 Mini connectors
* 5.2.3 Micro connectors
* 126.96.36.199 OMTP standard
* 5.2.4 Non-standard cables
USB 3.0 connectors and backward compatibility
USB On-The-Go connectors
* 5.2.8 Host and device interface receptacles
* 5.3 Pinouts
* 5.3.1 Proprietary connectors and formats
* 5.4 Colors
* 6 Cabling
* 7 Power
USB Battery Charging
* 7.1.1 Accessory charging adaptors (ACA)
* 7.2 Power Delivery (PD)
* 7.3 Sleep-and-charge ports
* 7.4 Mobile device charger standards
* 7.4.1 In
* 7.4.2 OMTP/GSMA Universal Charging Solution
* 7.4.3 EU smartphone power supply standard
* 7.5 Non-standard devices
* 8 Signalling (
* 8.1 Signalling rate (transmission rate)
* 8.1.1 Transaction latency
* 8.2 Electrical specification
* 8.3 Signalling state
* 8.3.1 Line transition state
* 8.3.2 Line State (covering
USB 1.x and 2.x)
* 8.4 Transmission
* 8.4.1 Transmission example on a
USB 1.1 Full Speed Device
USB 2.0 Speed Negotiation
* 9 Protocol layer
* 9.1 Handshake packets
* 9.2 Token packets
* 9.2.1 OUT, IN, SETUP and PING Token Packets
* 9.2.2 SOF : Start-of-Frame
* 9.2.3 SSPLIT and CSPLIT: Start-Split Transaction and Complete
* 9.3 Data packets
* 9.4 PRE packet (tells hubs to temporarily switch to low speed
* 10 Transaction
* 10.1 OUT Transaction
* 10.2 IN Transaction
* 10.3 SETUP Transaction
* 10.3.1 Setup Packet
* 10.4 Control Transfer Exchange
* 11 Audio streaming
* 12 Comparisons with other connection methods
* 12.4 eSATA/eSATAp
* 12.5 Thunderbolt
* 13 Interoperability
* 14 Related standards
* 15 See also
* 16 Notes
* 17 References
* 18 Further reading
* 19 External links
In general, there are three basic formats of
USB connectors: the
default or _standard_ format intended for desktop or portable
equipment (for example, on
USB flash drives ), the _mini_ intended for
mobile equipment (now deprecated except the Mini-B, which is used on
many cameras), and the thinner _micro_ size, for low-profile mobile
equipment (most modern mobile phones). Also, there are 5 modes of USB
data transfer, in order of increasing bandwidth: Low Speed (from 1.0),
Full Speed (from 1.0), High Speed (from 2.0),
SuperSpeed (from 3.0),
and SuperSpeed+ (from 3.1); modes have differing hardware and cabling
USB devices have some choice of implemented modes, and
USB version is not a reliable statement of implemented modes. Modes
_are_ identified by their names and icons, and the specifications
suggests that plugs and receptacles be colour-coded (
identified by blue).
Unlike other data buses (e.g., Ethernet, HDMI),
USB connections are
directed, with both upstream and downstream ports emanating from a
single host. This applies to electrical power, with only downstream
facing ports providing power; this topology was chosen to easily
prevent electrical overloads and damaged equipment. Thus,
have different ends: A and B, with different physical connectors for
each. Therefore, in general, each different format requires four
different connectors: a plug and receptacle for each of the A and B
USB cables have the plugs, and the corresponding receptacles are
on the computers or electronic devices. In common practice, the A end
is usually the standard format, and the B side varies over standard,
mini, and micro. The mini and micro formats also provide for USB
On-The-Go with a hermaphroditic AB receptacle, which accepts either an
A or a B plug. On-the-Go allows
USB between peers without discarding
the directed topology by choosing the host at connection time; it also
allows one receptacle to perform double duty in space-constrained
There are cables with A plugs on both ends, which may be valid if the
cable includes, for example, a
USB host-to-host transfer device with 2
ports, but they could also be non-standard and erroneous and should be
The micro format is the most durable from the point of view of
designed insertion lifetime. The standard and mini connectors have a
design lifetime of 1,500 insertion-removal cycles, the improved
Mini-B connectors increased this to 5,000. The micro connectors were
designed with frequent charging of portable devices in mind, so have a
design life of 10,000 cycles and also place the flexible contacts,
which wear out sooner, on the easily replaced cable, while the more
durable rigid contacts are located in the receptacles. Likewise, the
springy component of the retention mechanism, parts that provide
required gripping force, were also moved into plugs on the cable side.
_ The basic
USB trident_ logo _
USB logo on the head of a
standard A plug (the most common
USB plug ).
A group of seven companies began the development of
USB in 1994:
Compaq , DEC ,
NEC , and
Nortel . The goal
was to make it fundamentally easier to connect external devices to PCs
by replacing the multitude of connectors at the back of PCs,
addressing the usability issues of existing interfaces, and
simplifying software configuration of all devices connected to USB, as
well as permitting greater data rates for external devices. A team
Ajay Bhatt worked on the standard at Intel; the first
integrated circuits supporting
USB were produced by
Intel in 1995.
USB 1.0 specification, which was introduced in January
1996, defined data transfer rates of 1.5
Mbit/s _Low Speed_ and 12
Mbit/s _Full Speed_.
Microsoft Windows 95, OSR 2.1 provided OEM
support for the devices. The first widely used version of
USB was 1.1,
which was released in September 1998. The 12
Mbit/s data rate was
intended for higher-speed devices such as disk drives, and the lower
Mbit/s rate for low data rate devices such as joysticks . Apple
Inc. 's iMac was the first mainstream product with
USB and the iMac's
USB itself. Following Apple's design decision to
remove all legacy ports from the iMac, many PC manufacturers began
building legacy-free PCs , which led to the broader PC market using
USB as a standard.
USB 2.0 specification was released in April 2000 and was ratified
USB Implementers Forum (USB-IF) at the end of 2001.
Hewlett-Packard , Intel,
Lucent Technologies (now Nokia), NEC, and
Philips jointly led the initiative to develop a higher data transfer
rate, with the resulting specification achieving 480 Mbit/s, 40 times
as fast as the original
USB 1.1 specification.
USB 3.0 specification was published on 12 November 2008. Its main
goals were to increase the data transfer rate (up to 5 Gbit/s),
decrease power consumption, increase power output, and be backward
USB 3.0 includes a new, higher speed bus
SuperSpeed in parallel with the
USB 2.0 bus. For this reason,
the new version is also called SuperSpeed. The first
USB 3.0 equipped
devices were presented in January 2010.
As of 2008 , approximately 6 billion
USB ports and interfaces were in
the global marketplace, and about 2 billion were being sold each year.
In December 2014, USB-IF submitted
USB Power Delivery 2.0
USB Type-C specifications to the IEC (TC 100 – Audio, video and
multimedia systems and equipment) for inclusion in the international
standard IEC 62680 _Universal Serial Bus interfaces for data and
power_, which is currently based on
MAXIMUM TRANSFER RATE
USB 1.0 Release Candidate
Low Speed (1.5 Mbit/s)
Full Speed (12 Mbit/s)
High Speed (480 Mbit/s)
SuperSpeed (5 Gbit/s)
Also referred to as
USB 3.1 Gen 1 by
USB 3.1 standard
SuperSpeed+ (10 Gbit/s)
Also referred to as
USB 3.1 Gen 2 by
USB 3.1 standard
SuperSpeed++ (20 Gbit/s)
Also referred to as
USB 3.1 Gen 3 by
USB 3.1 standard
Power Related Specifications
USB Battery Charging 1.0
5 V, 1.5 A
USB Battery Charging 1.1
USB Battery Charging 1.2
5 V, 5 A
USB Power Delivery revision 1.0 (version 1.0)
20 V, 5 A
Using FSK protocol over bus power (VBUS)
USB Power Delivery revision 1.0 (version 1.3)
USB Type-C 1.0
5 V, 3 A
New connector and cable specification
USB Power Delivery revision 2.0 (version 1.0)
20 V, 5 A
Using BMC protocol over communication channel (CC) on type-C
USB Type-C 1.1
5 V, 3 A
USB Power Delivery revision 2.0 (version 1.1)
20 V, 5 A
USB Power Delivery revision 2.0 (version 1.2)
20 V, 5 A
Released in January 1996,
USB 1.0 specified a data rate of 1.5 Mbit/s
_(Low Bandwidth_ or _Low Speed)_. It did not allow for extension
cables or pass-through monitors, due to timing and power limitations.
USB devices made it to the market until
USB 1.1 was released in
August 1998, which introduced the speed of 12
Mbit/s _(full speed)_.
USB 1.1 was the earliest revision that was widely adopted and led to
Microsoft designated the "
Legacy-free PC ".
USB 1.0 nor 1.1 specified a design for any connector smaller
than the standard type A or type B. Though many designs for a
miniaturised type B connector appeared on many peripheral devices,
conformance to the
USB 1.x standard was fudged by treating peripherals
that had miniature connectors as though they had a tethered connection
(that is: no plug or socket at the peripheral end). There was no known
miniature type A connector until
USB 2.0 (rev 1.01) introduced one.
USB logo A
USB 2.0 PCI expansion card
USB 2.0 was released in April 2000, adding a higher maximum signaling
rate of 480
Mbit/s _(High Speed_ or _High Bandwidth)_, in addition to
USB 1.x _Full Speed_ signaling rate of 12 Mbit/s. Due to bus
access constraints, the effective throughput of the _High Speed_
signaling rate is limited to 280
Mbit/s or 35 MB/s.
Further modifications to the
USB specification have been made via
Engineering Change Notices (ECN). The most important of these ECNs are
included into the
USB 2.0 specification package available from
* _Mini-A and Mini-B Connector ECN_: Released in October 2000.
Specifications for Mini-A and Mini-B plug and receptacle. Also
receptacle that accepts both plugs for On-The-Go. These should not be
confused with Micro-B plug and receptacle. * _Pull-up/Pull-down
Resistors ECN_: Released in May 2002
* _Interface Associations ECN_: Released in May 2003.
New standard descriptor was added that allows associating multiple
interfaces with a single device function.
* _Rounded Chamfer ECN_: Released in October 2003.
A recommended, backward compatible change to Mini-B plugs that
results in longer lasting connectors.
Unicode ECN_: Released in February 2005.
This ECN specifies that strings are encoded using
Unicode , but did not specify the encoding. * _Inter-Chip
USB Supplement_: Released in March 2006
* _On-The-Go Supplement 1.3_: Released in December 2006.
USB On-The-Go makes it possible for two
USB devices to communicate
with each other without requiring a separate
USB host. In practice,
one of the
USB devices acts as a host for the other device.
* _Battery Charging Specification 1.1_: Released in March 2007 and
updated on 15 April 2009.
Adds support for dedicated chargers (power supplies with USB
connectors), host chargers (
USB hosts that can act as chargers) and
the No Dead Battery provision, which allows devices to temporarily
draw 100 mA current after they have been attached. If a
USB device is
connected to a dedicated charger, maximum current drawn by the device
may be as high as 1.8 A. (Note that this document is not distributed
USB 2.0 specification package, only
USB 3.0 and
USB Cables and Connectors Specification 1.01_: Released in
* _Link Power Management Addendum ECN_: Released in July 2007.
This adds _sleep_, a new power state between enabled and suspended
states. Device in this state is not required to reduce its power
consumption. However, switching between enabled and sleep states is
much faster than switching between enabled and suspended states, which
allows devices to sleep while idle.
* _Battery Charging Specification 1.2_: Released in December 2010.
Several changes and increasing limits including allowing 1.5 A on
charging ports for unconfigured devices, allowing High Speed
communication while having a current up to 1.5 A and allowing a
maximum current of 5 A.
USB 3.0 The
USB 3.0 specification was released on 12 November 2008, with its
management transferring from
USB 3.0 Promoter Group to the USB
Implementers Forum (USB-IF), and announced on 17 November 2008 at the
USB Developers Conference.
USB 3.0 defines a new _SuperSpeed_ transfer mode, with associated new
backward compatible plugs, receptacles, and cables.
and receptacles are identified with a distinct logo and blue inserts
in standard format receptacles.
SuperSpeed mode provides a data signaling rate of 5.0 Gbit/s.
However, due to the overhead incurred by
8b/10b encoding , the payload
throughput is actually 4 Gbit/s, and the specification considers it
reasonable to achieve only around 3.2 Gbit/s (0.4 GB/s or 400 MB/s).
However, this should increase with future hardware advances.
Communication is full-duplex in
SuperSpeed transfer mode; earlier
modes are half-duplex, arbitrated by the host.
Low-power and high-power devices remain operational with this
standard, but devices using
SuperSpeed can take advantage of increased
available current of between 150 mA and 900 mA, respectively.
Additionally, there is a Battery Charging Specification (Version 1.2
– December 2010), which increases the power handling capability to
1.5 A but does _not_ allow concurrent data transmission. The Battery
Charging Specification requires that the physical ports themselves be
capable of handling 5 A of current but limits the maximum current
drawn to 1.5 A.
A January 2013 press release, from the
USB group revealed plans to
USB 3.0 to 10 Gbit/s. The group ended up creating a new USB
USB 3.1, which was released on 31 July 2013, replacing
USB 3.0 standard. The
USB 3.1 specification takes over the
USB 3.0's _
SuperSpeed USB_ transfer rate, also referred to as
USB 3.1 Gen 1_, and introduces a faster transfer rate called
USB 10 Gbps_, also referred to as _
USB 3.1 Gen 2,_
putting it on par with a single first-generation Thunderbolt channel.
The new mode's logo features a caption stylized as _SUPERSPEED+_. The
USB 3.1 Gen 2 standard increases the data signaling rate to 10 Gbit/s,
double that of
SuperSpeed USB, and reduces line encoding overhead to
just 3% by changing the encoding scheme to
128b/132b . The first USB
3.1 Gen 2 implementation demonstrated transfer speeds of 7.2 Gbit/s.
USB 3.1 standard is backward compatible with
USB 3.0 and
The design architecture of
USB is asymmetrical in its topology,
consisting of a host , a multitude of downstream
USB ports, and
multiple peripheral devices connected in a tiered-star topology .
USB hubs may be included in the tiers, allowing branching
into a tree structure with up to five tier levels. A
USB host may
implement multiple host controllers and each host controller may
provide one or more
USB ports. Up to 127 devices, including hub
devices if present, may be connected to a single host controller.
USB devices are linked in series through hubs. One hub—built into
the host controller—is the root hub.
USB device may consist of several logical sub-devices that
are referred to as _device functions_. A single device may provide
several functions, for example, a webcam (video device function) with
a built-in microphone (audio device function). This kind of device is
called a _composite device_. An alternative to this is _compound
device ,_ in which the host assigns each logical device a distinctive
address and all logical devices connect to a built-in hub that
connects to the physical
USB endpoints actually reside
on the connected device: the channels to the host are referred to as
USB device communication is based on _pipes_ (logical channels). A
pipe is a connection from the host controller to a logical entity,
found on a device, and named an _endpoint _. Because pipes correspond
1-to-1 to endpoints, the terms are sometimes used interchangeably. A
USB device could have up to 32 endpoints (16 IN, 16 OUT), though it is
rare to have so many. An endpoint is defined and numbered by the
device during initialization (the period after physical connection
called "enumeration") and so is relatively permanent, whereas a pipe
may be opened and closed.
There are two types of pipe: stream and message. A message pipe is
bi-directional and is used for _control_ transfers. Message pipes are
typically used for short, simple commands to the device, and a status
response, used, for example, by the bus control pipe number 0. A
stream pipe is a uni-directional pipe connected to a uni-directional
endpoint that transfers data using an _isochronous_, _interrupt_, or
_bulk_ transfer: Isochronous transfers At some guaranteed data rate
(often, but not necessarily, as fast as possible) but with possible
data loss (e.g., realtime audio or video) Interrupt transfers Devices
that need guaranteed quick responses (bounded latency) (e.g., pointing
devices and keyboards) Bulk transfers Large sporadic transfers using
all remaining available bandwidth, but with no guarantees on bandwidth
or latency (e.g., file transfers)
An endpoint of a pipe is addressable with a tuple _(device_address,
endpoint_number)_ as specified in a TOKEN packet that the host sends
when it wants to start a data transfer session. If the direction of
the data transfer is from the host to the endpoint, an OUT packet (a
specialization of a TOKEN packet) having the desired device address
and endpoint number is sent by the host. If the direction of the data
transfer is from the device to the host, the host sends an IN packet
instead. If the destination endpoint is a uni-directional endpoint
whose manufacturer's designated direction does not match the TOKEN
packet (e.g. the manufacturer's designated direction is IN while the
TOKEN packet is an OUT packet), the TOKEN packet is ignored.
Otherwise, it is accepted and the data transaction can start. A
bi-directional endpoint, on the other hand, accepts both IN and OUT
USB 3.0 standard A sockets (left) and two
sockets (right) on a computer's front panel
Endpoints are grouped into _interfaces_ and each interface is
associated with a single device function. An exception to this is
endpoint zero, which is used for device configuration and is not
associated with any interface. A single device function composed of
independently controlled interfaces is called a _composite device_. A
composite device only has a single device address because the host
only assigns a device address to a function.
USB device is first connected to a
USB host, the
enumeration process is started. The enumeration starts by sending a
reset signal to the
USB device. The data rate of the
USB device is
determined during the reset signaling. After reset, the
information is read by the host and the device is assigned a unique
7-bit address. If the device is supported by the host, the device
drivers needed for communicating with the device are loaded and the
device is set to a configured state. If the
USB host is restarted, the
enumeration process is repeated for all connected devices.
The host controller directs traffic flow to devices, so no
can transfer any data on the bus without an explicit request from the
host controller. In
USB 2.0, the host controller polls the bus for
traffic, usually in a round-robin fashion. The throughput of each USB
port is determined by the slower speed of either the
USB port or the
USB device connected to the port.
USB 2.0 hubs contain devices called transaction
translators that convert between high-speed
USB 2.0 buses and full and
low speed buses. When a high-speed
USB 2.0 hub is plugged into a
USB host or hub, it operates in high-speed mode. The USB
hub then uses either one transaction translator per hub to create a
full/low-speed bus routed to all full and low speed devices on the
hub, or uses one transaction translator per port to create an isolated
full/low-speed bus per port on the hub.
Because there are two separate controllers in each
USB 3.0 host, USB
3.0 devices transmit and receive at
USB 3.0 data rates regardless of
USB 2.0 or earlier devices connected to that host. Operating data
rates for earlier devices are set in the legacy manner.
The functionality of a
USB device is defined by a class code sent to
USB host. This allows the host to load software modules for the
device and to support new devices from different manufacturers.
Device classes include:
EXAMPLES, OR EXCEPTION
Device class is unspecified, interface descriptors are used to
determine needed drivers
Speaker , microphone , sound card ,
Communications and CDC Control
Ethernet adapter ,
RS232 serial adapter .
Used together with class 0Ah _(below)_
Human interface device (HID)
Keyboard , mouse , joystick
Physical Interface Device (PID)
Force feedback joystick
Webcam , scanner
Laser printer , inkjet printer , CNC machine
Mass storage (MSC or UMS)
USB flash drive
USB flash drive , memory card reader , digital audio player ,
digital camera , external drive
Full bandwidth hub
Used together with class 02h _(above)_
USB smart card reader
Personal healthcare device class (PHDC)
Pulse monitor (watch)
Webcam , TV
USB Type-C alternate modes supported by device
USB compliance testing device
IrDA Bridge, Test "> A flash drive , a typical
device Circuit board from a
USB 3.0 external 2.5-inch
enclosure See also:
USB mass storage device class ,
Disk enclosure ,
External hard disk drive
USB implements connections to storage devices using a set of
standards called the
USB mass storage device class (MSC or UMS). This
was at first intended for traditional magnetic and optical drives and
has been extended to support flash drives . It has also been extended
to support a wide variety of novel devices as many systems can be
controlled with the familiar metaphor of file manipulation within
directories. The process of making a novel device look like a familiar
device is also known as extension. The ability to boot a write-locked
SD card with a
USB adapter is particularly advantageous for
maintaining the integrity and non-corruptible, pristine state of the
Though most computers since mid-2004 can boot from
USB mass storage
USB is not intended as a primary bus for a computer's
internal storage. Buses such as
Parallel ATA (PATA or IDE), Serial ATA
SCSI fulfill that role in PC class computers. However, USB
has one important advantage, in that it is possible to install and
remove devices without rebooting the computer (hot-swapping ), making
it useful for mobile peripherals, including drives of various kinds
SCSI devices may or may not support hot-swapping).
Firstly conceived and still used today for optical storage devices
DVD drives, etc.), several manufacturers offer external
USB hard disk drives , or empty enclosures for disk drives.
These offer performance comparable to internal drives, limited by the
current number and types of attached
USB devices, and by the upper
limit of the
USB interface (in practice about 30 MB/s for
USB 2.0 and
potentially 400 MB/s or more for
USB 3.0). These external drives
typically include a "translating device" that bridges between a
drive's interface to a
USB interface port. Functionally, the drive
appears to the user much like an internal drive. Other competing
standards for external drive connectivity include e
SATA , ExpressCard
FireWire (IEEE 1394), and most recently Thunderbolt .
Another use for
USB mass storage devices is the portable execution of
software applications (such as web browsers and
VoIP clients) with no
need to install them on the host computer.
MEDIA TRANSFER PROTOCOL
Picture Transfer Protocol
Media Transfer Protocol (MTP) was designed by
Microsoft to give
higher-level access to a device's filesystem than
USB mass storage, at
the level of files rather than disk blocks. It also has optional DRM
features. MTP was designed for use with portable media players , but
it has since been adopted as the primary storage access protocol of
the Android operating system from the version 4.1 Jelly Bean as well
as Windows Phone 8 (Windows Phone 7 devices had used the Zune protocol
which was an evolution of MTP). The primary reason for this is that
MTP does not require exclusive access to the storage device the way
UMS does, alleviating potential problems should an Android program
request the storage while it is attached to a computer. The main
drawback is that MTP is not as well supported outside of Windows
HUMAN INTERFACE DEVICES
USB human interface device class
Joysticks, keypads, tablets and other human-interface devices (HIDs)
are also progressively migrating from MIDI, and PC game port
connectors to USB.
USB mice and keyboards can usually be used with older computers that
have PS/2 connectors with the aid of a small USB-to-PS/2 adapter. For
mice and keyboards with dual-protocol support, an adaptor that
contains no logic circuitry may be used: the hardware in the USB
keyboard or mouse is designed to detect whether it is connected to a
USB or PS/2 port, and communicate using the appropriate protocol.
Converters also exist that connect PS/2 keyboards and mice (usually
one of each) to a
USB port. These devices present two HID endpoints
to the system and use a microcontroller to perform bidirectional data
translation between the two standards.
DEVICE FIRMWARE UPGRADE
Firmware Upgrade_ (DFU) is a vendor- and device-independent
mechanism for upgrading the firmware of
USB devices with improved
versions provided by their manufacturers, offering (for example) a way
for firmware bugfixes to be deployed. During the firmware upgrade
USB devices change their operating mode effectively
becoming a PROM programmer. Any class of
USB device can implement this
capability by following the official DFU specifications.
In addition to its intended legitimate purposes, DFU can also be
exploited by uploading maliciously crafted firmware that causes USB
devices to spoof various other device types; one such exploiting
approach is known as
Type-A plug and, as part of a non-standard cable, receptacle
The connectors the
USB committee specifies support a number of USB's
underlying goals, and reflect lessons learned from the many connectors
the computer industry has used.
Receptacles And Plugs
The connector mounted on the host or device is called the
_receptacle_, and the connector attached to the cable is called the
_plug_. The official
USB specification documents also periodically
define the term _male_ to represent the plug, and _female_ to
represent the receptacle.
Usability And Orientation
USB extension cable
By design, it is difficult to insert a
USB plug into its receptacle
USB specification states that the required
must be embossed on the "topside" of the
USB plug, which "...provides
easy user recognition and facilitates alignment during the mating
process." The specification also shows that the "recommended"
"Manufacturer's logo" ("engraved" on the diagram but not specified in
the text) is on the opposite side of the
USB icon. The specification
further states, "The
USB Icon is also located adjacent to each
receptacle. Receptacles should be oriented to allow the icon on the
plug to be visible during the mating process." However, the
specification does not consider the height of the device compared to
the eye level height of the user, so the side of the cable that is
"visible" when mated to a computer on a desk can depend on whether the
user is standing or kneeling.
While connector interfaces can be designed to allow plugging with
either orientation, the original design omitted such functionality to
decrease manufacturing costs. The reversible type-C plug is an
addition to the
USB 3.1 specification comparable in size to the
Only moderate force is needed to insert or remove a
USB cable. USB
cables and small
USB devices are held in place by the gripping force
from the receptacle (without need of the screws, clips, or thumb-turns
other connectors have required).
The standard connectors were deliberately intended to enforce the
directed topology of a
USB network: type-A receptacles on host devices
that supply power and type-B receptacles on target devices that draw
power. This prevents users from accidentally connecting two
supplies to each other, which could lead to short circuits and
dangerously high currents, circuit failures, or even fire.
not support cyclic networks and the standard connectors from
USB devices are themselves incompatible.
However, some of this directed topology is lost with the advent of
USB connections (such as
USB On-The-Go in smartphones,
Wi-Fi routers), which require A-to-A, B-to-B, and
sometimes Y/splitter cables. See the
USB On-The-Go connectors section
below for a more detailed summary description.
The standard connectors were designed to be more robust than many
past connectors. This is because
USB is hot-pluggable , and the
connectors would be used more frequently, and perhaps with less care,
than previous connectors.
USB has a minimum rated lifetime of 1,500 cycles of
insertion and removal, the mini-
USB receptacle increases this to
5,000 cycles, and the newer Micro-
USB-C receptacles are both
designed for a minimum rated lifetime of 10,000 cycles of insertion
and removal. To accomplish this, a locking device was added and the
leaf-spring was moved from the jack to the plug, so that the
most-stressed part is on the cable side of the connection. This change
was made so that the connector on the less expensive cable would bear
the most wear.
In standard USB, the electrical contacts in a
USB connector are
protected by an adjacent plastic tongue, and the entire connecting
assembly is usually protected by an enclosing metal sheath.
USB connectors, the construction always ensures that the
external sheath on the plug makes contact with its counterpart in the
receptacle before any of the four connectors within make electrical
contact. The external metallic sheath is typically connected to system
ground, thus dissipating damaging static charges. This enclosure
design also provides a degree of protection from electromagnetic
interference to the
USB signal while it travels through the mated
connector pair (the only location when the otherwise twisted data pair
travels in parallel). In addition, because of the required sizes of
the power and common connections, they are made after the system
ground but before the data connections. This type of staged make-break
timing allows for electrically safe hot-swapping.
USB standard specifies relatively loose tolerances for compliant
USB connectors to minimize physical incompatibilities in connectors
from different vendors. To address a weakness present in some other
connector standards, the
USB specification also defines limits to the
size of a connecting device in the area around its plug. This was done
to prevent a device from blocking adjacent ports due to the size of
the cable strain relief mechanism (usually molding integral with the
cable outer insulation) at the connector. Compliant devices must
either fit within the size restrictions or support a compliant
extension cable that does.
USB cables have only plugs on their ends, while hosts and
devices have only receptacles. Hosts almost universally have Type-A
receptacles, while devices have one or another Type-B variety. Type-A
plugs mate only with Type-A receptacles, and the same applies to their
Type-B counterparts; they are deliberately physically incompatible.
However, an extension to the
USB standard specification called USB
On-The-Go (OTG) allows a single port to act as either a host or a
device, which is selectable by the end of the cable that plugs into
the receptacle on the OTG-enabled unit. Even after the cable is hooked
up and the units are communicating, the two units may "swap" ends
under program control. This capability is meant for units such as PDAs
in which the
USB link might connect to a PC's host port as a device in
one instance, yet connect as a host itself to a keyboard and mouse
device in another instance.
USB connectors along a centimeter ruler for scale (1 cm
is 3⁄8 in). From left to right:
* Micro-B plug
* Mini-B plug
* type-A receptacle
* type-A plug
* type-B plug
* ^ The VBUS supply from a low-powered hub port may drop to 4.40 V.
* ^ UC-E6 is a proprietary non-
* ^ Inverted, so the contacts are visible.
There are several types of
USB connector, including some that have
been added while the specification progressed. The original USB
specification detailed standard-A and standard-B plugs and
receptacles; the B connector was necessary so that cabling could be
plug ended at both ends and still prevent users from connecting one
computer receptacle to another. The first engineering change notice to
USB 2.0 specification added Mini-B plugs and receptacles.
The data pins in the standard plugs are actually recessed in the plug
compared to the outside power pins. This permits the power pins to
connect first, preventing data errors by allowing the device to power
up first and then establish the data connection. Also, some devices
operate in different modes depending on whether the data connection is
To reliably enable a charge-only feature, modern
peripherals now include charging cables that provide power connections
to the host port but no data connections, and both home and vehicle
charging docks are available that supply power from a converter device
and do not include a host device and data pins, allowing any capable
USB device to charge or operate from a standard
In a charge-only cable, the data wires are shorted at the device end.
These wires are usually green and white. If these wires are left
as-is, the device will often reject the charger as unsuitable.
Pin configuration of the type-A and type-B
viewed from the mating (male) end of plugs
The type-A plug has an elongated rectangular cross-section, inserts
into a type-A receptacle on a _downstream port_ on a
USB host or hub,
and carries both power and data. Captive cables on
USB devices, such
as keyboards or mice, will be terminated with a type-A plug.
The type-B plug has a near square cross-section with the top exterior
corners beveled. As part of a removable cable, it inserts into an
_upstream port_ on a device, such as a printer. On some devices, the
type-B receptacle has no data connections, being used solely for
accepting power from the upstream device. This two-connector-type
scheme (A/B) prevents a user from accidentally creating a loop.
The spring contacts in the connectors eventually relax and wear out
with repeated cycles of plugging and unplugging. The lifetime of a
type-A plug is approximately 1,500 connect/disconnect cycles.
The maximum allowed cross-section of the _overmold boot_ (which is
part of the connector used for its handling) is 16 by 8 mm (0.63 by
0.31 in) for the standard-A plug type, while for the type-B it is 11.5
by 10.5 mm (0.45 by 0.41 in).
Mini-A (left) and Mini-B (right) plugs
For smaller devices such as digital cameras , smartphones , and
tablet computers , various smaller connectors have been used – the
USB-standard first introduced the Mini-
USB connectors were introduced together with
USB 2.0 in April
2000 – however the Mini-A connector and the Mini-AB receptacle
connector are deprecated (i.e. de-certified, but standardized) since
May 2007. Mini-B connectors are still supported, but are not
On-The-Go-compliant; the Mini-B
USB connector was standard for
transferring data to and from the early smartphones and PDAs. Both
Mini-A and Mini-B plugs are approximately 3 by 7 mm (0.12 by 0.28 in).
Micro-A plug Micro-B plug
USB connectors, which were announced by the USB-IF on 4 January
2007, have a similar width to Mini-USB, but approximately half the
thickness, enabling their integration into thinner portable devices.
The Micro-A connector is 6.85 by 1.8 mm (0.270 by 0.071 in) with a
maximum overmold boot size of 11.7 by 8.5 mm (0.46 by 0.33 in), while
the Micro-B connector is 6.85 by 1.8 mm (0.270 by 0.071 in) with a
maximum overmold size of 10.6 by 8.5 mm (0.42 by 0.33 in).
The thinner Micro-
USB connectors were introduced to replace the Mini
connectors in devices manufactured since May 2007, including
smartphones , personal digital assistants , and cameras. While some
of the devices and cables still use the older Mini variant, the newer
Micro connectors are widely adopted, and as of December 2010 they are
the most widely used.
The Micro plug design is rated for at least 10,000 connect-disconnect
cycles, which is more than the Mini plug design. The Micro connector
is also designed to reduce the mechanical wear on the device; instead
the easier-to-replace cable is designed to bear the mechanical wear of
connection and disconnection. The _Universal Serial Bus Micro-USB
Cables and Connectors Specification_ details the mechanical
characteristics of Micro-A plugs , Micro-AB receptacles (which accept
both Micro-A and Micro-B plugs), and Micro-B plugs and receptacles,
along with a standard-A receptacle to Micro-A plug adapter.
USB was endorsed as the standard connector for data and power
on mobile devices by the cellular phone carrier group Open Mobile
Terminal Platform (OMTP) in 2007.
USB was embraced as the "Universal Charging Solution" by the
International Telecommunication Union (ITU) in October 2009.
In Europe, micro-
USB became the defined common external power supply
(EPS) for use with smartphones sold in the EU, 14 of the world's
largest mobile phone manufacturers signed the EU's common EPS
Memorandum of Understanding (MoU). Apple , one of the original MoU
signers, makes Micro-
USB adapters available – as permitted in the
Common EPS MoU – for its iPhones equipped with Apple's proprietary
30-pin dock connector or (later) Lightning connector . according to
the CEN , CENELEC and
Non-standard reversible micro-B plug connector
A reversible micro connector plug that can be connected to existing
Micro-B sockets has been developed by Winner Gear, crowdfunded on
Indiegogo , with no functional enhancement to the USB. And there are
many USB-A to reversible Micro-B cable manufacturer offerings, as well
USB On-The-Go (OTG) to reversible Micro-B cables.
USB 3.0 Connectors And Backward Compatibility
USB 3.0 Micro-B
SuperSpeed plug See also:
USB 3.0 §
USB 3.0 introduced Type-A
SuperSpeed plugs and receptacles as well as
SuperSpeed plugs and receptacles. The 3.0
receptacles are backward-compatible with the corresponding pre-3.0
USB 3.0 and
USB 1.0 Type-A plugs and receptacles are designed to
interoperate. In order to achieve
USB 3.1 Gen 2), 5 extra pins are added to the unused
area of the original 4 pin
USB 1.0 design, making
USB 3.0 Type-A plugs
and receptacles backward compatible to those of
USB 1.0. USB
USB 2.0 vs
On the device side, a modified Micro-B plug (Micro-B SuperSpeed) is
used to cater for the five additional pins required to achieve the USB
3.0 features (
USB Type-C plug can also be used). The
USB 3.0 Micro-B
plug effectively consists of a standard
USB 2.0 Micro-B cable plug,
with an additional 5 pins plug "stacked" to the side of it. In this
way, cables with smaller 5 pin
USB 2.0 Micro-B plugs can be plugged
into devices with 10 contact
USB 3.0 Micro-B receptacles and achieve
USB cables exist with various combinations of plugs on each end of
the cable, as displayed below in the _
USB cables matrix_.
B type plug
USB On-The-Go Connectors
USB On-The-Go (OTG) introduces the concept of a device performing
both master and slave roles. All current OTG devices are required to
have one, and only one,
USB connector: a Micro-AB receptacle. (In the
past, before the development of Micro-USB, On-The-Go devices used
The Micro-AB receptacle is capable of accepting both Micro-A and
Micro-B plugs, attached to any of the legal cables and adapters as
defined in revision 1.01 of the Micro-
To enable Type-AB receptacles to distinguish which end of a cable is
plugged in, plugs have an "ID" pin in addition to the four contacts
found in standard-size
USB connectors. This ID pin is connected to GND
in Type-A plugs, and left unconnected in Type-B plugs. Typically, a
pull-up resistor in the device is used to detect the presence or
absence of an ID connection.
The OTG device with the A-plug inserted is called the A-device and is
responsible for powering the
USB interface when required, and by
default assumes the role of host. The OTG device with the B-plug
inserted is called the B-device and by default assumes the role of
peripheral. An OTG device with no plug inserted defaults to acting as
a B-device. If an application on the B-device requires the role of
host, then the Host Negotiation Protocol (HNP) is used to temporarily
transfer the host role to the B-device.
OTG devices attached either to a peripheral-only B-device or a
standard/embedded host have their role fixed by the cable, since in
these scenarios it is only possible to attach the cable one way.
USB-C port on an Apple
Developed at roughly the same time as the
USB 3.1 specification, but
distinct from it, the
USB Type-C Specification 1.0 was finalized in
August 2014 and defines a new small reversible-plug connector for USB
devices. The Type-C plug connects to both hosts and devices,
replacing various Type-A and Type-B connectors and cables with a
standard meant to be future-proof.
The 24-pin double-sided connector provides four power-ground pairs,
two differential pairs for
USB 2.0 data bus (though only one pair is
implemented in a Type-C cable), four pairs for
SuperSpeed data bus
(only two pairs are used in
USB 3.1 mode), two "sideband use" pins,
VCONN +5 V power for active cables, and a configuration pin for cable
orientation detection and dedicated biphase mark code (BMC)
configuration data channel. Type-A and Type-B adaptors and cables
are required for older devices to plug into Type-C hosts. Adapters and
cables with a Type-C receptacle are not allowed.
USB 3.1 Type-C cables are electronically marked cables
that contain a full set of wires and a chip with an ID function based
on the configuration data channel and vendor-defined messages (VDMs)
USB Power Delivery 2.0 specification.
USB Type-C devices also
support power currents of 1.5 A and 3.0 A over the 5 V power bus in
addition to baseline 900 mA; devices can either negotiate increased
USB current through the configuration line, or they can support the
full Power Delivery specification using both BMC-coded configuration
line and legacy BFSK-coded VBUS line.
Alternate Mode dedicates some of the physical wires in the USB-C
cable for direct device-to-host transmission of alternate data
protocols. The four high-speed lanes, two sideband pins,
and—for dock, detachable device and permanent cable
USB 2.0 pins and one configuration pin
can be used for Alternate Mode transmission. The modes are configured
using VDMs through the configuration channel.
Host And Device Interface Receptacles
USB plugs fit one receptacle with notable exceptions for USB
On-The-Go "AB" support and the general backward compatibility of USB
3.0 as shown.
USB connectors mating matrix (images not to scale)
SuperSpeed Only non-
USB cables matrix
PLUGS, EACH END
Non-standard Existing for specific proprietary purposes , and in
most cases not inter-operable with USB-IF compliant equipment. In
addition to the above cable assemblies comprising two plugs, an
"adapter" cable with a Micro-A plug and a standard-A receptacle is
USB specifications. Other combinations of connectors
are not compliant. There do exist A-to-A assemblies, referred to as
cables (such as the
Easy Transfer Cable
Easy Transfer Cable ); however, these have a pair
USB devices in the middle, making them more than just cables.
Deprecated Some older devices and cables with Mini-A connectors have
been certified by USB-IF. The Mini-A connector is obsolete: no new
Mini-A connectors and neither Mini-A nor Mini-AB receptacles will be
certified. Note: Mini-B is not deprecated, but less and less used
since the arrival of Micro-B.
USB 3.0 § Pinouts
USB is a serial bus, using four shielded wires for the
variant: two for power (VBUS and GND), and two for differential data
signals (labelled as D+ and D− in pinouts ). Non-Return-to-Zero
Inverted (NRZI) encoding scheme is used for transferring data, with a
sync field to synchronize the host and receiver clocks. D+ and D−
signals are transmitted on a differential pair , providing half-duplex
data transfers for
USB 2.0. Mini and micro connectors have their GND
connections moved from pin #4 to pin #5, while their pin #4 serves as
an ID pin for the On-The-Go host/client identification.
USB 3.0 provides two additional differential pairs (four wires,
SSTx+, SSTx−, SSRx+ and SSRx−), providing full-duplex data
transfers at _SuperSpeed_, which makes it similar to
Serial ATA or
PCI Express . STANDARD, MINI-, AND MICRO-
(not to scale). White areas are empty. The receptacles are pictured
USB logo to the top, looking into the open end; note this means
the pin order is mirrored from plug to socket. MICRO-B
* Power (VBUS, 5 V)
* Data− (D−)
* Data+ (D+)
* ID (On-The-Go)
SuperSpeed transmit− (SSTx−)
SuperSpeed transmit+ (SSTx+)
SuperSpeed receive− (SSRx−)
SuperSpeed receive+ (SSRx+)
Type-A and -B pinout
Mini/Micro-A and -B pinout
On-The-Go ID distinguishes cable ends:
* "A" plug (host): connected to GND
* "B" plug (device): not connected
Proprietary Connectors And Formats
Manufacturers of personal electronic devices might not include a USB
standard connector on their product for technical or marketing
reasons. Some manufacturers provide proprietary cables that permit
their devices to physically connect to a
USB standard port. Full
functionality of proprietary ports and cables with
USB standard ports
is not assured; for example, some devices only use the
for battery charging and do not implement any data transfer functions.
USB port and connector
Nokia Pop-Port connector
An Apple Lightning -to-
USB adapter and captive
An orange charge-only
USB port on a front panel
USB 3.0 switch
with card reader. A blue Standard-A
USB connector on a Sagemcom
F@ST 3864OP ADSL modem router without
USB 3.0 contacts fitted.
USB AC adaptor with a green
USB connector supporting
Black or white
Ports & plugs
Type-A or type-B
Blue (Pantone 300C)
Ports & plugs
Type-A or type-B, SuperSpeed
Ports & plugs
Type-A or type-B, SuperSpeed+
Ports for example,
USB 3.0 specification mandates appropriate
color-coding while it only recommends blue inserts for standard-A USB
3.0 connectors and plugs.
USB twisted pair, where the Data+_ and _Data−_ conductors
are twisted together in a double helix . The wires are enclosed in a
further layer of shielding.
The D± signals used by low, full, and high speed are carried over a
twisted pair (typically, unshielded) to reduce noise and crosstalk .
SuperSpeed uses separate transmit and receive differential pairs ,
which additionally require shielding (typically, shielded twisted pair
but twinax is also mentioned by the specification). Thus, to support
SuperSpeed data transmission, cables contain twice as many wires and
are thus larger in diameter.
USB 1.1 standard specifies that a standard cable can have a
maximum length of 3 metres (9 ft 10 in) with devices operating at full
speed (12 Mbit/s), and a maximum length of 5 metres (16 ft 5 in) with
devices operating at low speed (1.5 Mbit/s).
USB 2.0 provides for a maximum cable length of 5 metres (16 ft 5 in)
for devices running at high speed (480 Mbit/s). The primary reason for
this limit is the maximum allowed round-trip delay of about 1.5 μs.
USB host commands are unanswered by the
USB device within the
allowed time, the host considers the command lost. When adding USB
device response time, delays from the maximum number of hubs added to
the delays from connecting cables, the maximum acceptable delay per
cable amounts to 26 ns. The
USB 2.0 specification requires that cable
delay be less than 5.2 ns per meter (1.6 ns/ft, 192000 km/s) which is
close to the maximum achievable transmission speed for standard copper
USB 3.0 standard does not directly specify a maximum cable
length, requiring only that all cables meet an electrical
specification: for copper cabling with AWG 26 wires the maximum
practical length is 3 meters (9.8 ft).
USB power standards
USB 3.0) device
USB 3.0) device
Battery Charging (BC) 1.2
Power Delivery micro-format
Power Delivery standard format or Type-C
* ^ Up to five unit loads; with non-
SuperSpeed devices, one unit
load is 100 mA.
* ^ Up to six unit loads; with
SuperSpeed devices, one unit load is
* ^ _A_ _B_ _C_ Either
SuperSpeed or non-SuperSpeed.
* ^ Requires PD 5 A cable.
USB 3.0 cable; with such a cable, a device can draw
power from two
USB ports simultaneously
USB supplies bus power across VBUS and GND at a nominal voltage 5 V
± 5%, at supply, to power
USB devices. Power is sourced solely from
upstream devices or hosts, and is consumed solely by downstream
USB provides for various voltage drops and losses in
providing bus power. As such, the voltage at the hub port is specified
to be in the range 7000500000000000000♠5.00+0.25
−0.60 V by
USB 2.0, and 7000500000000000000♠5.00+0.25
−0.55 V by
USB 3.0. It is specified that devices' configuration
and low-power functions must operate down to 4.40 V at the hub port by
USB 2.0 and that devices' configuration, low-power, and high-power
functions must operate down to 4.00 V at the device port by
There are limits on the power a device may draw, stated in terms of a
_unit load_, which is 100 mA, or 150 mA for
SuperSpeed devices. There
are low-power and high-power devices. Low-power devices may draw at
most 1 unit load, and all devices must act as low-power devices when,
starting out as, unconfigured. High-power devices draw at least 1 unit
load and at most 5 unit loads (500 mA), or 6 unit loads (900 mA) for
SuperSpeed devices. A high-powered device must be configured, and may
only draw as much power as specified in its configuration. I.e.,
the maximum power may not be available.
A bus-powered hub is a high-power device providing low-power ports.
It draws 1 unit load for the hub controller and 1 unit load for each
of at most 4 ports. The hub may also have some non-removable functions
in place of ports. A self-powered hub is a device that provides
high-power ports. Optionally, the hub controller draw power for its
operation as a low-power device, but all high-power ports draw from
the hub's self-power.
Where devices (for example, high-speed disk drives) require more
power than a high-power device can draw, they function erratically,
if at all, from bus power of a single port.
USB provides for these
devices as being self-powered. However, such devices may come with a
Y-shaped cable that has 2
USB plugs (1 for power and data, the other
for only power), so as to draw power as 2 devices. Such a cable is
non-standard, with the
USB compliance specification stating that "use
of a 'Y' cable (a cable with two A-plugs) is prohibited on any USB
peripheral", meaning that "if a
USB peripheral requires more power
than allowed by the
USB specification to which it is designed, then it
must be self-powered."
USB BATTERY CHARGING
A small device that provides voltage and current readouts for
devices charged over USB. This
USB power meter additionally
provides a charge readout (in mAh) and data logging.
USB Battery Charging defines a new port type, the _charging port_, as
opposed to the _standard downstream port_ (SDP) of the base
specification. Charging ports are divided into 2 further types: the
_charging downstream port_ (CDP), which has data signals, and the
_dedicated charging port_ (DCP), which does not. Dedicated charging
ports can be found on
USB power adapters that convert utility power or
another power source (e.g., a car's electrical system) to run attached
devices and battery packs. On a host (such as a laptop computer) with
both standard and charging
USB ports, the charging ports should be
labeled as such.
The charging device identifies the type of port through non-data
signalling on the D+ and D− signals immediately after attach. A DCP
simply has to place a resistance not exceeding 200 Ω across the D+
and D− signals.
Per the base specification, any device attached to an SDP must
initially be a low-power device, with high-power mode contingent on
USB configuration by the host. Charging ports, however, can
immediately supply between 0.5 and 1.5A of current. The charging port
may apply current limiting or shut down completely, but must not apply
limiting below 0.5A, and must not shut down below 1.5A or before the
voltage drops to 2V.
These bus power currents being much higher than cables were designed
for, though not unsafe, cause a larger voltage between the ends of the
ground signal, significantly reducing noise margins causing problems
with High Speed signalling. Battery Charging 1.1 specifies that
charging devices must dynamically limit bus power current draw during
High Speed signalling; 1.2 simply specifies that charging devices and
ports must be designed to tolerate the higher ground voltage
difference in High Speed signalling.
Revision 1.2 of the specification was released in 2010. Several
changes are made and limits are increased including allowing 1.5 A on
charging downstream ports for unconfigured devices, allowing High
Speed communication while having a current up to 1.5 A, and allowing a
maximum current of 5 A. Also, support is removed for charging port
detection via resistive mechanisms.
Before the battery charging specification was defined, there was no
standardized way for the portable device to inquire how much current
was available. For example, Apple's iPod and iPhone chargers indicate
the available current by voltages on the D− and D+ lines. When D+ =
D− = 2.0 V, the device may pull up to 500 mA. When D+ = 2.0 V and
D− = 2.8 V, the device may pull up to 1 A of current. When D+ = 2.8
V and D− = 2.0 V, the device may pull up to 2 A of current.
Accessory Charging Adaptors (ACA)
Portable devices having an On The Go port may want to charge and
USB peripheral at the same time, but having only a single port
(both due to On The Go and space requirement) prevents this.
_Accessory charging adapters (ACA)_ are devices which allow a charging
power to be injected into an On The Go connection between host and
ACAs have three ports: the OTG port for the portable device, which is
required to have a Micro-A plug on a captive cable; the accessory
port, which is required to have a Micro-AB or type-A receptacle; and
the charging port, which is required to have a Micro-B receptacle, or
type-A plug or charger on a captive cable. The ID pin of the OTG port
is not connected within plug as usual, but to the ACA itself, where
signals outside the OTG floating and ground states are used for ACA
detection and state signalling. The charging port does not pass data,
but does use the D± signals for charging port detection. The
accessory port acts as any other port. When appropriately signalled by
the ACA, the portable device can charge from the bus power as if there
were a charging port present; any OTG signals over bus power are
instead passed to the portable device via the ID signal. Bus power is
also provided to the accessory port from the charging port
POWER DELIVERY (PD)
List of 60W/100W USB chargeable laptops
USB PD rev. 1 source profiles
2.0 A, 10 W
1.5 A, 18 W
3.0 A, 36 W
3.0 A, 60 W
5.0 A, 60 W
5.0 A, 100 W
* ^ Default start-up profile
USB PD rev. 2 source power rules
power (W) CURRENT, AT: (A)
(15 W) 1.7–3.0
(27 W) 1.8–3.0
(45 W) 2.25–3.0
In July 2012, the
USB Promoters Group announced the finalization of
USB Power Delivery (PD) specification, an extension that specifies
using certified _PD aware_
USB cables with standard
USB Type-A and
Type-B connectors to deliver increased power (more than 7.5 W) to
devices with larger power demand. Devices can request higher currents
and supply voltages from compliant hosts – up to 2 A at 5 V (for a
power consumption of up to 10 W), and optionally up to 3 A or 5 A at
either 12 V (36 W or 60 W) or 20 V (60 W or 100 W). In all cases,
both host-to-device and device-to-host configurations are supported.
The intent is to permit uniformly charging laptops, tablets,
USB-powered disks and similarly higher-power consumer electronics, as
a natural extension of existing European and Chinese mobile telephone
charging standards. This may also affect the way electric power used
for small devices is transmitted and used in both residential and
The Power Delivery specification defines six fixed power profiles for
the power sources. PD-aware devices implement a flexible power
management scheme by interfacing with the power source through a
bidirectional data channel and requesting a certain level of
electrical power, variable up to 5 A and 20 V depending on supported
profile. The power configuration protocol uses a 24 MHz
transmission channel on the VBUS line.
USB Power Delivery revision 2.0 specification has been released
as part of the
USB 3.1 suite. It covers the Type-C cable and
connector with four power/ground pairs and a separate configuration
channel, which now hosts a
DC coupled low-frequency BMC -coded data
channel that reduces the possibilities for RF interference . Power
Delivery protocols have been updated to facilitate Type-C features
such as cable ID function, Alternate Mode negotiation, increased VBUS
currents, and VCONN-powered accessories.
USB Power Delivery Revision 2.0 Version 1.2, the six fixed
power profiles for power sources have been deprecated.
USB PD Power
Rules replace power profiles, defining four normative voltage levels
at 5V, 9V, 15V, and 20V. Instead of six fixed profiles, power supplies
may support any maximum source output power from 0.5W to 100W.
USB Power Delivery 3.0 specification defines new power rules
based on supplied wattage. Programmable power supply protocol allows
granular control over VBUS power in 10 mV steps to facilitate constant
current or constant voltage charging. Revision 3.0 also adds extended
configuration messages, fast role swap, and deprecates the BFSK
As of April 2016 , there are silicon controllers available from
several sources (TI, Cypress) and several others. Power supplies
bundled with Type-C based laptops from Apple, Google, HP, Dell, and
USB PD. In addition, accessories from third party
vendors including Anker,
Belkin , iVoler and Innergie support USB
PD 2.0 at multiple voltages. There are several PD aware projects such
as the USB-PD Sniffer that are PD aware. ASUS also make a fully Power
Delivery compliant adapter card the
USB 3.1 UPD PANEL
USB port denoting sleep-and-charge
USB ports can be used to charge electronic devices
even when the computer is switched off. Normally, when a computer is
powered off the
USB ports are powered down, preventing phones and
other devices from charging. Sleep-and-charge
USB ports remain powered
even when the computer is off. On laptops, charging devices from the
USB port when it is not being powered from AC drains the laptop
battery faster; most laptops have a facility to stop charging if their
own battery charge level gets too low. This feature has also been
implemented on some laptop docking stations allowing device charging
even when no laptop is present.
USB ports may be found colored differently than
regular ports, mostly red or yellow, though that is not always the
On Dell and Toshiba laptops, the port is marked with the standard USB
symbol with an added lightning bolt icon on the right side. Dell calls
this feature _PowerShare_, while Toshiba calls it _USB
Acer Inc. and
Packard Bell laptops,
USB ports are marked with a non-standard symbol (the
letters _USB_ over a drawing of a battery); the feature is simply
called _Power-off USB_. On some laptops such as Dell and Apple
MacBook models, it is possible to plug a device in, close the laptop
(putting it into sleep mode) and have the device continue to charge.
MOBILE DEVICE CHARGER STANDARDS
USB interface is commonly found on chargers for mobile
phones Australian and New Zealand power socket with USB
As of 14 June 2007 , all new mobile phones applying for a license in
China are required to use a
USB port as a power port for battery
charging. This was the first standard to use the convention of
shorting D+ and D−.
OMTP/GSMA Universal Charging Solution
In September 2007, the
Open Mobile Terminal Platform group (a forum
of mobile network operators and manufacturers such as
Nokia , Samsung
Sony Ericsson and LG ) announced that its members had
agreed on Micro-
USB as the future common connector for mobile devices.
GSM Association (GSMA) followed suit on 17 February 2009,
and on 22 April 2009, this was further endorsed by the CTIA – The
Wireless Association , with the International Telecommunication Union
(ITU) announcing on 22 October 2009 that it had also embraced the
Universal Charging Solution as its "energy-efficient
one-charger-fits-all new mobile phone solution," and added: "Based on
USB interface, UCS chargers will also include a 4-star or
higher efficiency rating—up to three times more energy-efficient
than an unrated charger."
Smartphone Power Supply Standard
Common external power supply
In June 2009, many of the world's largest mobile phone manufacturers
signed an EC -sponsored Memorandum of Understanding (MoU), agreeing to
make most data-enabled mobile phones marketed in the European Union
compatible with a common External Power Supply (common EPS). The EU's
common EPS specification (EN 62684:2010) references the
Charging standard and is similar to the GSMA/OMTP and Chinese charging
solutions. In January 2011, the International Electrotechnical
Commission (IEC) released its version of the (EU's) common EPS
standard as IEC 62684:2011.
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USB-powered mini fans
USB vacuum cleaner novelty device
USB devices require more power than is permitted by the
specifications for a single port. This is common for external hard and
optical disc drives , and generally for devices with motors or lamps .
Such devices can use an external power supply , which is allowed by
the standard, or use a dual-input
USB cable, one input of which is
used for power and data transfer, the other solely for power, which
makes the device a non-standard
USB device. Some
USB ports and
external hubs can, in practice, supply more power to
USB devices than
required by the specification but a standard-compliant device may not
depend on this.
In addition to limiting the total average power used by the device,
USB specification limits the inrush current (i.e., the current
used to charge decoupling and filter capacitors ) when the device is
first connected. Otherwise, connecting a device could cause problems
with the host's internal power.
USB devices are also required to
automatically enter ultra low-power suspend mode when the
USB host is
suspended. Nevertheless, many
USB host interfaces do not cut off the
power supply to
USB devices when they are suspended.
USB devices use the 5 V power supply without
participating in a proper
USB network, which negotiates power draw
with the host interface. These are usually called _
USB decorations _.
Examples include USB-powered keyboard lights, fans, mug coolers and
heaters, battery chargers, miniature vacuum cleaners , and even
miniature lava lamps . In most cases, these items contain no digital
circuitry, and thus are not standard compliant
USB devices. This may
cause problems with some computers, such as drawing too much current
and damaging circuitry. Prior to the Battery Charging Specification,
USB specification required that devices connect in a low-power
mode (100 mA maximum) and communicate their current requirements to
the host, which then permits the device to switch into high-power
Some devices, when plugged into charging ports, draw even more power
(10 watts at 2.1 amperes) than the Battery Charging Specification
allows — The iPad is one such device. Barnes this is 70% of the
total available bus bandwidth. For
USB 3.0, typical write speed is
70–90 MB/s, while read speed is 90–110 MB/s. Mask tests, also
known as eye diagram tests , are used to determine the quality of a
signal in the time domain. They are defined in the referenced document
as part of the electrical test description for the high-speed (HS)
mode at 480 Mbit/s.
According to a USB-IF chairman, "at least 10 to 15 percent of the
stated peak 60 MB/s (480 Mbit/s) of Hi-Speed
USB goes to
overhead—the communication protocol between the card and the
peripheral. Overhead is a component of all connectivity standards".
Tables illustrating the transfer limits are shown in Chapter 5 of the
For isochronous devices like audio streams, the bandwidth is
constant, and reserved exclusively for a given device. The bus
bandwidth therefore only has an effect on the number of channels that
can be sent at a time, not the "speed" or latency of the transmission.
* LOW-SPEED (LS) rate of 1.5
Mbit/s is defined by
USB 1.0. It is
very similar to full-bandwidth operation except each bit takes 8 times
as long to transmit. It is intended primarily to save cost in
low-bandwidth human interface devices (HID) such as keyboards, mice,
* FULL-SPEED (FS) rate of 12
Mbit/s is the basic
USB data rate
USB 1.0. All
USB hubs can operate at this speed.
* HIGH-SPEED (HS) rate of 480
Mbit/s was introduced in 2001. All
hi-speed devices are capable of falling back to full-bandwidth
operation if necessary; i.e., they are backward compatible with USB
1.1. Connectors are identical for
USB 2.0 and
* SUPERSPEED (SS) rate of 5.0 Gbit/s. The written
specification was released by
Intel and its partners in August 2008.
USB 3.0 controller chips were sampled by
NEC in May 2009,
and the first products using the
USB 3.0 specification arrived in
USB 3.0 connectors are generally backward compatible,
but include new wiring and full duplex operation.
For low-speed (1.5 Mbit/s) and full-speed (12 Mbit/s) devices the
shortest time for a transaction in one direction is 1 ms. High-speed
(480 Mbit/s) uses transactions within each micro frame (125 µs)
where using 1-byte interrupt packet results in a minimal response time
of 940 ns. 4-byte interrupt packet results in 984 ns.
USB signals are transmitted using differential signalling on a
twisted-pair data cable with 90 Ω ± 15% characteristic impedance .
* LOW-SPEED (LS) and FULL-SPEED (FS) modes use a single data pair,
labelled D+ and D−, in half-duplex . Transmitted signal levels are
0.0–0.3 V for logical low, and 2.8–3.6 V for logical high level.
The signal lines are not terminated .
* HIGH-SPEED (HS) mode uses the same wire pair, but with different
electrical conventions. Lower signal voltages of −10 to 10 mV for
low and 360 to 440 mV for logical high level, and termination of 45 Ω
to ground, or 90 Ω differential to match the data cable impedance.
* SUPERSPEED (SS) adds two additional pairs of shielded twisted wire
(and new, mostly compatible expanded connectors). These are dedicated
SuperSpeed operation. The half-duplex lines are still
used for configuration.
USB connection is always between a host or hub at the _A_ connector
end, and a device or hub's "upstream" port at the other end.
Originally, this was a _B_ connector, preventing erroneous loop
connections, but additional upstream connectors were specified, and
some cable vendors designed and sold cables that permitted erroneous
connections (and potential damage to circuitry).
are not as fool-proof or as simple as originally intended.
The host includes 15 kΩ pull-down resistors on each data line. When
no device is connected, this pulls both data lines low into the
so-called _single-ended zero_ state (SE0 in the
and indicates a reset or disconnected connection.
Line Transition State
The following terminology is used to assist in the technical
USB PHY signalling.
LINE TRANSITION STATE
USB 1.x Low Speed
(1.5 kΩ pullup on D−)
USB 1.x Full Speed
(1.5 kΩ pullup on D+)
Same as Idle line state
This is present during a transmission line transition.
Alternatively, it is waiting for a new packet.
Inverse of J state
This is present during a transmission line transition.
Both D+ and D− is low. This may indicate an end of packet signal
or a detached
This is an illegal state and should never occur. This is seen as an
* The idle line state is when the device is connected to the host
with a pullup on either D+ and D−, with transmitter output on both
host and device is set to high impedance (hi-Z) (disconnected output).
USB device pulls one of the data lines high with a 1.5 kΩ
resistor. This overpowers one of the pull-down resistors in the host
and leaves the data lines in an idle state called _J_.
USB 1.x, the choice of data line indicates what signal rates
the device is capable of:
* full-bandwidth devices pull D+ high,
* low-bandwidth devices pull D− high.
* The _K_ state has just the opposite polarity to the _J_ state.
Line State (covering
USB 1.x And 2.x)
USB 1.X LOW SPEED
USB 1.X FULL SPEED
USB 2.X HIGH SPEED
No device detected. Both lines are pulled down by 15 kΩ pull-down
resistors on the host side.
SE0 >= 2us
SE0 >= 2us
SE0 >= 2us
USB device pullups on D+ or D- will wakes the host from detached line
This will start the
USB enumeration process. THIS SETS THE IDLE
STATE. D- is pulled up by 1.5 kΩ device side
D+ is pulled up by 1.5 kΩ device side
Special Chirping Sequence
Idle / J
Host and Device Transmitter at Hi-Z.
Sensing line state in case of Detached state. Same as Detached or
Same as Detached or Connect state
Start of a Packet line transition pattern
Line Transitions: KJKJKJKK
Line Transitions: KJKJKJKK
15 KJ pairs followed by 2 K’s, for a total of 32 symbols.
End of Packet line transition pattern
Line Transitions: SE0 + SE0 + J
Line Transitions: SE0 + SE0 + J
USB device to a known initial state
SE0 >= 2.5ms
SE0 >= 2.5ms
Power down the device, such that it would only consume 0.5 mA from
Exits this state only after a resume or reset signal is received.
To avoid this state a SOF packet (high speed) or a Keep Alive (low
speed) signal is given. J >= 3ms
J >= 3ms
Host wants to wake device up
K >= 20ms then EOP pattern
K >= 20ms then EOP pattern
device wants to wake up.
(Must be in idle for at least 5ms) device drives K >= 1ms
host then sends a resume signal device drives K >= 1ms
host then sends a resume signal
(Low Speed) Host wants to tell low speed device to stay awake
EOP pattern once every millisecond
USB data is transmitted by toggling the data lines between the J
state and the opposite K state.
USB encodes data using the NRZI line
* 0 bit is transmitted by toggling the data lines from J to K or
* 1 bit is transmitted by leaving the data lines as-is.
To ensure that there is enough signal transitions for clock recovery
to occur in the bitstream , bit stuffing techniques is applied to the
data stream. This is via inserting extra 0 bit into the data stream
after any appearance of six consecutive 1 bits (Thus ensuring that
there is a 0 bit to cause a transmission state transition). Seven
consecutive received 1 bits is always an error. For
additional data transmission encoding was included to deal with the
higher speed rate that was required by the newer standard.
Transmission Example On A
USB 1.1 Full Speed Device
Example of a Negative Acknowledge packet transmitted by
full-speed device when there is no more data to read. It consists of
the following fields: clock synchronization byte, type of packet and
end of packet. Data packets would have more information between the
type of packet and end of packet.
* SYNCHRONISATION PATTERN: A
USB packet begins with an 8-bit
synchronization sequence, 00000001₂. That is, after the initial idle
state J, the data lines toggle KJKJKJKK. The final 1 bit (repeated K
state) marks the end of the sync pattern and the beginning of the USB
frame. For high bandwidth USB, the packet begins with a 32-bit
* END OF PACKET (EOP): is indicated by the transmitter driving 2 bit
times of SE0 (D+ and D− both below max.) and 1 bit time of J state.
After this, the transmitter ceases to drive the D+/D− lines and the
aforementioned pull up resistors hold it in the J (idle) state.
Sometimes skew due to hubs can add as much as one bit time before the
SE0 of the end of packet. This extra bit can also result in a "bit
stuff violation" if the six bits before it in the CRC are 1s. This bit
should be ignored by receiver.
* BUS RESET: A
USB bus is reset using a prolonged (10 to 20
milliseconds) SE0 signal.
USB 2.0 SPEED NEGOTIATION
USB 2.0 devices use a special protocol during reset, called
_chirping_, to negotiate the high bandwidth mode with the host/hub. A
device that is
USB 2.0 High Speed capable first connects as an Full
Speed device (D+ pulled high), but upon receiving a
USB RESET (both D+
and D− driven LOW by host for 10 to 20 ms) it pulls the D− line
high, known as chirp K. This indicates to the host that the device is
high bandwidth. If the host/hub is also HS capable, it chirps (returns
alternating J and K states on D− and D+ lines) letting the device
know that the hub operates at high bandwidth. The device has to
receive at least three sets of KJ chirps before it changes to high
bandwidth terminations and begins high bandwidth signaling. Because
USB 3.0 uses wiring separate and additional to that used by
USB 1.x, such bandwidth negotiation is not required.
Clock tolerance is 480.00±0.24 Mbit/s, 12.00±0.03 Mbit/s,
Though high bandwidth devices are commonly referred to as "
and advertised as "up to 480 Mbit/s," not all
USB 2.0 devices are high
bandwidth. The USB-IF certifies devices and provides licenses to use
special marketing logos for either "basic bandwidth" (low and full) or
high bandwidth after passing a compliance test and paying a licensing
fee. All devices are tested according to the latest specification, so
recently compliant low bandwidth devices are also 2.0 devices.
USB 3 uses tinned copper stranded AWG-28 cables with
7001900000000000000♠90±7 Ω impedance for its high-speed
differential pairs and linear feedback shift register and 8b/10b
encoding sent with a voltage of 1 V nominal with a 100 mV receiver
threshold; the receiver uses equalization. SSC clock and 300 ppm
precision is used. Packet headers are protected with CRC-16, while
data payload is protected with CRC-32. Power up to 3.6 W may be used.
One unit load in superspeed mode is equal to 150 mA.
USB communication, data is transmitted as packets . Initially,
all packets are sent from the host, via the root hub and possibly more
hubs, to devices. Some of those packets direct a device to send some
packets in reply.
After the sync field, all packets are made of 8-bit bytes,
transmitted least-significant bit first . The first byte is a packet
identifier (PID) byte. The PID is actually 4 bits; the byte consists
of the 4-bit PID followed by its bitwise complement. This redundancy
helps detect errors. (Note also that a PID byte contains at most four
consecutive 1 bits, and thus never needs bit-stuffing, even when
combined with the final 1 bit in the sync byte. However, trailing 1
bits in the PID may require bit-stuffing within the first few bits of
USB PID bytes
(msb -first) Transmitted byte
(lsb -first) NAME
USB 2.0) split transaction
Check if endpoint can accept data (
Split transaction error (
Data packet accepted
Data packet not accepted; please retransmit
Data not ready yet (
Transfer impossible; do error recovery
Address for host-to-device transfer
Address for device-to-host transfer
Start of frame marker (sent each ms)
Address for host-to-device control transfer
Even-numbered data packet
Odd-numbered data packet
Data packet for high-bandwidth isochronous transfer (
Data packet for high-bandwidth isochronous transfer (
Packets come in three basic types, each with a different format and
CRC (cyclic redundancy check ):
KJ KJ KJ KK
Handshake packets consist of only a single PID byte, and are
generally sent in response to data packets. Error detection is
provided by transmitting four bits that represent the packet type
twice, in a single PID byte using complemented form. Three basic types
are _ACK_, indicating that data was successfully received, _NAK_,
indicating that the data cannot be received and should be retried, and
_STALL_, indicating that the device has an error condition and cannot
transfer data until some corrective action (such as device
USB 2.0 added two additional handshake packets: _NYET_ and _ERR_.
NYET indicates that a split transaction is not yet complete, while ERR
handshake indicates that a split transaction failed. A second use for
a NYET packet is to tell the host that the device has accepted a data
packet, but cannot accept any more due to full buffers. This allows a
host to switch to sending small PING tokens to inquire about the
device's readiness, rather than sending an entire unwanted DATA packet
just to elicit a NAK.
The only handshake packet the
USB host may generate is ACK. If it is
not ready to receive data, it should not instruct a device to send.
Token packets consist of a PID byte followed by two payload bytes: 11
bits of address and a five-bit CRC. Tokens are only sent by the host,
never a device. Below are tokens present from
* _IN_ and _OUT_ tokens contain a seven-bit device number and
four-bit function number (for multifunction devices) and command the
device to transmit DATAx packets, or receive the following DATAx
* IN token expects a response from a device. The response may be a
NAK or STALL response, or a DATAX frame. In the latter case, the host
issues an ACK handshake if appropriate.
* OUT token is followed immediately by a DATAX frame. The device
responds with ACK, NAK, NYET, or STALL, as appropriate.
* _SETUP_ operates much like an OUT token, but is used for initial
device setup. It is followed by an eight-byte DATA0 frame with a
* SOF (START OF FRAME) Every millisecond (12000 full-bandwidth bit
USB host transmits a special _SOF_ (start of frame) token,
containing an 11-bit incrementing frame number in place of a device
address. This is used to synchronize isochronous and interrupt data
USB 2.0 devices receive seven additional SOF
tokens per frame, each introducing a 125 µs "microframe" (60000
high-bandwidth bit times each).
USB 2.0 also added a _PING_ Token and _a larger three-byte SPLIT
* _PING_ asks a device if it is ready to receive an OUT/DATA packet
pair. PING is usually sent by a host when polling a device that most
recently responded with NAK or NYET. This avoids the need to send a
large data packet to a device that the host suspects to be unwilling
to accept it. The device responds with ACK, NAK or STALL, as
* _SPLIT_ is used to perform split transactions. Rather than tie up
USB bus sending data to a slower
USB device, the
nearest high-bandwidth capable hub receives a SPLIT token followed by
one or two
USB packets at high bandwidth, performs the data transfer
at full or low bandwidth, and provides the response at high bandwidth
when prompted by a second SPLIT token. It contains a seven-bit hub
number, 12 bits of control flags, and a five-bit CRC.
OUT, IN, SETUP And PING Token Packets
KJ KJ KJ KK
* ADDR: Address of
USB device (maximum of 127 devices)
* ENDP: Select endpoint hardware source/sink buffer on device. ( E.g.
PID OUT would be for sending data from host source buffer into the USB
device sink buffer. )
* By default, all
USB devices must at least support endpoint buffer
0 (EP0). This is since EP0 is used for device control and status
information during enumeration and normal operation.
SOF : Start-of-Frame
KJ KJ KJ KK
XXXX XXXX XXX
SE0 SE0 J
* Frame Number: This is a frame number that is incremented by the
host periodically to allows endpoints to identify the start of the
frame (or microframe) and synchronize internal endpoint clocks to the
SSPLIT And CSPLIT: Start-Split Transaction And Complete Split
0 = SSPLIT
1 = CSPLIT
KJ KJ KJ KK
SE0 SE0 J
* S/C: Start Complete
* 0 = SSPLIT : Start Split Transaction
* 1 = CSPLIT : Complete Split Transaction
* S : 1 = Low Speed, 0 = High Speed
* E : End of full speed payload
* U : U bit is reserved/unused and must be reset to zero (0B)
* EP : End Point Type ( 00 = Control ) ( 01 = Isochronous ) ( 10 =
bulk ) ( 11 = interrupt )
KJ KJ KJ KK
XXXX XXXX XXXX XXXX
SE0 SE0 J
A data packet consists of the PID followed by 0–1,024 bytes of data
payload (up to 1,024 bytes for high-speed devices, up to 64 bytes for
full-speed devices, and at most eight bytes for low-speed devices),
and a 16-bit CRC.
There are two basic forms of data packet, _DATA0_ and _DATA1_. A data
packet must always be preceded by an address token, and is usually
followed by a handshake token from the receiver back to the
transmitter. The two packet types provide the 1-bit sequence number
required by stop-and-wait ARQ . If a
USB host does not receive a
response (such as an ACK) for data it has transmitted, it does not
know if the data was received or not; the data might have been lost in
transit, or it might have been received but the handshake response was
To solve this problem, the device keeps track of the type of DATAx
packet it last accepted. If it receives another DATAx packet of the
same type, it is acknowledged but ignored as a duplicate. Only a DATAx
packet of the opposite type is actually received.
If the data is corrupted while transmitted or received, the CRC check
fails. When this happens, the receiver does not generate an ACK, which
makes the sender resend the packet.
When a device is reset with a SETUP packet, it expects an 8-byte
DATA0 packet next.
USB 2.0 added _DATA2_ and _MDATA_ packet types as well. They are used
only by high-bandwidth devices doing high-bandwidth isochronous
transfers that must transfer more than 1024 bits per 125 µs micro
frame (8,192 kbit/s).
PRE PACKET (TELLS HUBS TO TEMPORARILY SWITCH TO LOW SPEED MODE)
A hub is able to support Low-bandwidth devices mixed with other speed
device via a special PID value, _PRE_. This is required as a
functions as a very simple repeater, broadcasting the host message to
all connected devices regardless if the packet was for it or not. This
means in a mixed speed environment, there is a potential danger that a
low speed will misinterpret a high or full speed signal from the host.
To eliminate this danger, if a
USB hub detects a mix of highspeed or
full speed and low speed devices, it will by default disable
communication to low speed device unless requested to switch to low
speed mode. On reception of a PRE packet however, it will temporarily
re-enable the output port to all low speed devices, to allow the host
to send a single low speed packet to low speed devices. After the low
speed packet is sent, an end of packet (EOP) signal will tell the hub
to disable all outputs to low speed devices again.
Since all PID bytes include four 0 bits, they leave the bus in the
full-bandwidth K state, which is the same as the low-bandwidth J
state. It is followed by a brief pause, during which hubs enable their
low-bandwidth outputs, already idling in the J state. Then a
low-bandwidth packet follows, beginning with a sync sequence and PID
byte, and ending with a brief period of SE0. Full-bandwidth devices
other than hubs can simply ignore the PRE packet and its low-bandwidth
contents, until the final SE0 indicates that a new packet follows.
FULL SPEED PRE PREAMBLE
Hub Setup Enable Output
To Low Speed Devices LOW SPEED PACKET EXAMPLE
Hub Disable Output
To Low Speed Devices
KJ KJ KJ KK
KJ KJ KJ KK
OUT TRANSACTION ( 3 PACKETS TOTAL )
Tell device on
to start listening for incoming data packet on endpoint
USB device the data that you want to send to it
Device tells the host that it has successfully received and loaded
the data payload to buffer EPx
IN TRANSACTION ( 3 PACKETS TOTAL )
Tell device on
to send any data that it has on its endpoint buffer
EPx Device checks its EPx endpoint buffer and sends the requested
data to host.
Host lets device know that it has successfully received the payload
and have loaded the payload into its EPx buffer.
This is used for device enumeration and connection management and
informs the device that the host would like to start a Control
SETUP TRANSACTION (3 PACKETS TOTAL)
Tell device on
to start setup mode and be ready for a data packet Send to Device
the 8 bytes long setup packet
Device Acknowledge reception of SETUP data and updates its setup
* Depending on the setup packet, an optional data packet from device
to host or host to device may occur.
DATA PHASE TRANSFER DIRECTION
Numbers of Bytes expected to be transferred in the data stage
This is a parameter value. Depends on bRequest.
Typically used for specifying endpoint or interface. This is a
parameter value. Depends on bRequest
This is the setup request command
0 = Host to Device
1 = Device to Host 0 = Standard,
1 = Class, 2 = Vendor, 3 = Reserved 0 = Device,
1 = Interface, 2 = Endpoint, 3 = Other, 4 to 31 = Reserved
CONTROL TRANSFER EXCHANGE
The control transfer exchange consist of three distinct stages.
* Setup Stage: This is the setup command sent by the host to the
* Data Stage (Optional): The device may optionally send data in
response to a setup request.
* Status Stage: Dummy IN or OUT transaction. Which is probably for
indicating the end of a control transfer exchange.
What this allows the host to do, is to perform bus management action
like enumerating new
USB devices via retrieving the new device DEVICE
DESCRIPTORS. Retrieval of the device descriptors would especially
allow for determining the
USB Class, VID and PID, which are often used
for determining the correct
USB driver for the device.
Also after the device descriptor is retrieved. The host will perform
another control transfer exchange, but instead to set the address of
USB device to a new ADDRx .
USB Device Working Group has laid out specifications for audio
USB technology wasn't designed with audio
streaming in mind, specific standards have been developed and
implemented for audio class uses.
The DWG distinguishes two audio device modes specifications: Audio
1.0 specification and Audio 2.0 specification. Three types of devices
USB headphone devices
USB microphone devices
USB headset devices
Three levels of synchronisation were defined: asynchronous,
synchronous, and adaptive.
COMPARISONS WITH OTHER CONNECTION METHODS
A variety of
USB cables for sale in Hong Kong
USB was considered a complement to
IEEE 1394 (FireWire)
technology, which was designed as a high-bandwidth serial bus that
efficiently interconnects peripherals such as disk drives, audio
interfaces, and video equipment. In the initial design,
at a far lower data rate and used less sophisticated hardware. It was
suitable for small peripherals such as keyboards and pointing devices.
The most significant technical differences between
FireWire and USB
USB networks use a tiered-star topology, while
IEEE 1394 networks
use a tree topology.
USB 1.0, 1.1 and 2.0 use a "speak-when-spoken-to" protocol,
meaning that each peripheral communicates with the host when the host
specifically requests it to.
USB 3.0 allows for device-initiated
communications towards the host. A
FireWire device can communicate
with any other node at any time, subject to network conditions.
USB network relies on a single host at the top of the tree to
control the network. All communications are between the host and one
peripheral. In a
FireWire network, any capable node can control the
USB runs with a 5 V power line, while
FireWire in current
implementations supplies 12 V and theoretically can supply up to 30 V.
USB hub ports can provide from the typical 500 mA/2.5 W
of current, only 100 mA from non-hub ports.
USB 3.0 and
supply 1.8 A/9.0 W (for dedicated battery charging, 1.5 A/7.5 W Full
bandwidth or 900 mA/4.5 W High Bandwidth), while
FireWire can in
theory supply up to 60 watts of power, although 10 to 20 watts is more
These and other differences reflect the differing design goals of the
USB was designed for simplicity and low cost, while
FireWire was designed for high performance, particularly in
time-sensitive applications such as audio and video. Although similar
in theoretical maximum transfer rate,
FireWire 400 is faster than USB
2.0 Hi-Bandwidth in real-use, especially in high-bandwidth use such
as external hard drives. The newer
FireWire 800 standard is twice
as fast as
FireWire 400 and faster than
USB 2.0 Hi-Bandwidth both
theoretically and practically. However, Firewire's speed advantages
rely on low-level techniques such as direct memory access (DMA), which
in turn have created opportunities for security exploits such as the
DMA attack .
The chipset and drivers used to implement
FireWire have a
crucial impact on how much of the bandwidth prescribed by the
specification is achieved in the real world, along with compatibility
The IEEE 802.3af
Power over Ethernet (PoE) standard specifies a more
elaborate power negotiation scheme than powered USB. It operates at 48
V DC and can supply more power (up to 12.95 W, PoE+ 25.5 W) over a
cable up to 100 meters compared to
USB 2.0, which provides 2.5 W with
a maximum cable length of 5 meters. This has made PoE popular for VoIP
telephones, security cameras , wireless access points and other
networked devices within buildings. However,
USB is cheaper than PoE
provided that the distance is short, and power demand is low.
Ethernet standards require electrical isolation between the networked
device (computer, phone, etc.) and the network cable up to 1500 V AC
or 2250 V DC for 60 seconds.
USB has no such requirement as it was
designed for peripherals closely associated with a host computer, and
in fact it connects the peripheral and host grounds. This gives
Ethernet a significant safety advantage over
USB with peripherals such
as cable and DSL modems connected to external wiring that can assume
hazardous voltages under certain fault conditions.
Digital musical instruments are another example where
competitive for low-cost devices. However
Power over Ethernet and the
MIDI plug standard have an advantage in high-end devices that may have
USB can cause ground loop problems between equipment,
because it connects ground references on both transceivers. By
MIDI plug standard and
Ethernet have built-in isolation
to 500V or more.
SATA connector is a more robust
SATA connector, intended for
connection to external hard drives and SSDs. eSATA's transfer rate (up
to 6 Gbit/s) is similar to that of
USB 3.0 (up to 5 Gbit/s on current
devices; 10 Gbit/s speeds via
USB 3.1, announced on 31 July 2013). A
device connected by e
SATA appears as an ordinary
SATA device, giving
both full performance and full compatibility associated with internal
SATA does not supply power to external devices. This is an
increasing disadvantage compared to USB. Even though
USB 3.0's 4.5 W
is sometimes insufficient to power external hard drives, technology is
advancing and external drives gradually need less power, diminishing
SATA advantage. eSATAp (power over eSATA; aka ESATA/USB) is a
connector introduced in 2009 that supplies power to attached devices
using a new, backward compatible, connector. On a notebook eSATAp
usually supplies only 5 V to power a 2.5-inch HDD/SSD; on a desktop
workstation it can additionally supply 12 V to power larger devices
including 3.5-inch HDD/SSD and 5.25-inch optical drives.
eSATAp support can be added to a desktop machine in the form of a
bracket connecting to motherboard SATA, power, and
eSATA, like USB, supports hot plugging , although this might be
limited by OS drivers and device firmware.
PCI Express and
Mini DisplayPort into a new
serial data interface. Original Thunderbolt implementations have two
channels, each with a transfer speed of 10 Gbit/s, resulting in an
aggregate unidirectional bandwidth of 20 Gbit/s.
Thunderbolt 2 uses link aggregation to combine the two 10 Gbit/s
channels into one bi-directional 20 Gbit/s channel.
Thunderbolt 3 is announced to use
USB Type-C connectors.
Thunderbolt 3 has one 40 Gbit/s channel.
Various protocol converters are available that convert
signals to and from other communications standards.
USB Implementers Forum is working on a wireless networking
standard based on the
USB is a
cable-replacement technology, and uses ultra-wideband wireless
technology for data rates of up to 480 Mbit/s.
USB 2.0 High-Speed Inter-Chip (HSIC) is a chip-to-chip variant of USB
2.0 that eliminates the conventional analog transceivers found in
normal USB. It was adopted as a standard by the
USB Implementers Forum
in 2007. The HSIC physical layer uses about 50% less power and 75%
less board area compared to traditional
USB 2.0. HSIC uses two signals
at 1.2 V and has a throughput of 480 Mbit/s. Maximum PCB trace length
for HSIC is 10 cm. It does not have low enough latency to support RAM
memory sharing between two chips.
USB 3.0 successor of HSIC is called
SuperSpeed Inter-Chip (SSIC).
* Computing portal
* Electronics portal
Easy Transfer Cable
Easy Transfer Cable
Extensible Host Controller Interface (XHCI)
List of device bit rates#Peripheral
Media Transfer Protocol
Mobile High-Definition Link
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* "Universal Host