A power-on reset (PoR, POR) generator is a
microcontroller
A microcontroller (MC, uC, or μC) or microcontroller unit (MCU) is a small computer on a single integrated circuit. A microcontroller contains one or more CPUs (processor cores) along with memory and programmable input/output peripherals. Pro ...
or
microprocessor
A microprocessor is a computer processor (computing), processor for which the data processing logic and control is included on a single integrated circuit (IC), or a small number of ICs. The microprocessor contains the arithmetic, logic, a ...
peripheral
A peripheral device, or simply peripheral, is an auxiliary hardware device that a computer uses to transfer information externally. A peripheral is a hardware component that is accessible to and controlled by a computer but is not a core compo ...
that generates a
reset
Reset may refer to:
Film and television
* Reset (2015 film), ''Reset'' (2015 film), a 2015 French documentary film
* Reset (2017 film), ''Reset'' (2017 film), a 2017 Chinese film
* Reset (2024 film), ''Reset'' (2024 film), a 2024 Sri Lankan sh ...
signal when power is applied to the
device. It ensures that the device starts operating in a known
state
State most commonly refers to:
* State (polity), a centralized political organization that regulates law and society within a territory
**Sovereign state, a sovereign polity in international law, commonly referred to as a country
**Nation state, a ...
.
PoR generator
In
VLSI devices, the power-on reset (PoR) is an electronic device incorporated into the integrated circuit that detects the power applied to the chip and generates a reset impulse that goes to the entire circuit placing it into a known state.
A simple PoR uses the charging of a capacitor, in series with a resistor, to measure a time period during which the rest of the circuit is held in a reset state. A
Schmitt trigger
In electronics, a Schmitt trigger is a comparator circuit with hysteresis implemented by applying positive feedback to the noninverting input of a comparator or differential amplifier. It is an passivity (engineering), active circuit which con ...
may be used to deassert the reset signal cleanly, once the rising voltage of the
RC network passes the
threshold voltage
The threshold voltage, commonly abbreviated as Vth or VGS(th), of a field-effect transistor (FET) is the minimum gate-to-source voltage (VGS) that is needed to create a conducting path between the source and drain terminals. It is an important s ...
of the Schmitt trigger. The resistor and capacitor values should be determined so that the charging of the RC network takes long enough that the supply voltage will have stabilised by the time the threshold is reached.
One of the issues with using an RC network to generate a PoR pulse is the sensitivity of the R and C values to the power-supply ramp characteristics. When the power supply ramp is rapid, the R and C values can be calculated so that the time to reach the switching threshold of the Schmitt trigger is enough to apply a long enough reset pulse. When the power-supply ramp itself is slow, the RC network tends to get charged up along with the power-supply ramp up. So when the input Schmitt stage is all powered up and ready, the input voltage from the RC network would already have crossed the Schmitt trigger point. This means that there might not be a reset pulse supplied to the core of the VLSI.
Power-on reset on IBM mainframes
On an IBM mainframe, a power-on reset (POR) is a sequence of actions that the processor performs either due to a POR request from the operator or as part of turning on power. The operator requests a POR for configuration changes that cannot be recognized by a simple
System Reset.
See also
*
Low-voltage detect
*
Oscillator start-up timer
*
Power-up timer (PWRT),
similar to a
watchdog timer
A watchdog timer (WDT, or simply a ''watchdog''), sometimes called a ''computer operating properly timer'' (''COP timer''), is an electronic or software timer that is used to detect and recover from computer malfunctions. Watchdog timers are wide ...
References
{{DEFAULTSORT:Power-On Reset
Embedded systems