Automatic baud rate detection (ABR, autobaud) refers to the process by which a receiving device (such as a

modem A modulator-demodulator or modem is a computer hardware device that converts data from a digital format into a format suitable for an analog transmission medium such as telephone or radio. A modem transmits data by Modulation#Digital modulati ...

) determines the speed, code level, start bit, and

stop bit Asynchronous serial communication is a form of serial communication in which the communicating endpoints' interfaces are not continuously synchronized by a common clock signal. Instead of a common synchronization signal, the data stream contai ...

s of incoming data by examining the first

character Character or Characters may refer to: Arts, entertainment, and media Literature * ''Character'' (novel), a 1936 Dutch novel by Ferdinand Bordewijk * ''Characters'' (Theophrastus), a classical Greek set of character sketches attributed to The ...

, usually a preselected sign-on character (

syncword In computer networks, a syncword, sync character, sync sequence or preamble is used to synchronize a data transmission by indicating the end of header information and the start of data. The syncword is a known sequence of data used to identif ...

) on a

UART A universal asynchronous receiver-transmitter (UART ) is a computer hardware device for asynchronous serial communication in which the data format and transmission speeds are configurable. It sends data bits one by one, from the least significan ...

connection. ABR allows the receiving device to accept data from a variety of transmitting devices operating at different speeds without needing to establish data rates in advance. During the autobaud process, the

baud rate In telecommunication and electronics, baud (; symbol: Bd) is a common unit of measurement of symbol rate, which is one of the components that determine the speed of communication over a data channel. It is the unit for symbol rate or modul ...

of received character stream is determined by examining the received pattern and its timing, and the length of a start bit. These type of baud rate detection mechanism are supported by many hardware chips including processors such a
STM32
MPC8280, MPC8360, and so on. When start bit length is used to determine the baud rate, it requires the character to be odd since UART sends LSB bit first. Often symbols 'a' or 'A' (0x61 or 0x41) are used. For example, the MPC8270 SCC tries to detect the length of the UART start bit for autobaud. Most modems seen in market currently support autobaud. Before receiving any input data, most modems use a default baud rate of 9600 for output. For example, the following modems have been verified for autobaud and default output baud rate 9600: * USRobotics USR5686G 56K Serial Controller Fax modem * Hayes V92 External modem * Microcom DeskPorte 28.8P The baud rate of modems are adjusted automatically after receiving input data by the autobaud process. Many protocols begin each frame with a preamble of alternating 1 and 0 bits that can be used for automatic baud rate detection. For example, the TI PGA460 uses a 'U' ( 0x55 ) sync byte for automatic baud rate detection as well as

frame synchronization In telecommunication, frame synchronization or framing is the process by which, while receiving a stream of framed data, incoming frame alignment signals (i.e., a distinctive bit sequences or syncwords) are identified (that is, distinguished from ...

, and so does the LIN header ( Local Interconnect Network#Header). For example, the UART-based FlexWire protocol begins each frame with a 'U' (0x55)

sync byte In computer networks, a syncword, sync character, sync sequence or preamble is used to synchronize a data transmission by indicating the end of header information and the start of data. The syncword is a known sequence of data used to identif ...

. FlexWire receivers use the sync byte to precisely set their UART bit-clock frequency without a high-precision oscillator. Texas Instruments
"TPS929121-Q1 12-Channel Automotive 40-V High-Side LED Driver With FlexWire"
2021. p. 37 and p. p. 39. For example, the Ethernet preamble contains 56 bits of alternating 1 and 0 bits for synchronizing bit clocks.

References

* "17.2 Autobaud Operation on a UART in MPC8280 PowerQUICC™ II Family Reference Manual" http://www.nxp.com/files/netcomm/doc/ref_manual/MPC8280RM.pdf * "Automatic Baud Rate Detection on the MSP430" https://web.archive.org/web/20161026080239/http://www.ti.com/lit/an/slaa215/slaa215.pdf * "How to implement “auto baud rate detection” feature on Cortex-M3" https://stackoverflow.com/q/38979647 * "mpc8270 SCC2 UART issue" https://community.nxp.com/message/906833 Data transmission Units of measurement Telecommunications techniques {{telecomm-stub

See also

References