Power-on reset
Encyclopedia
A power-on reset generator is a microcontroller
or microprocessor
peripheral
that generates a reset
signal when power is applied to the device
. It ensures that the device starts operating in a known state
.
A simple PoR is composed by an RC device that charges with the rising of the supply voltage. A schmitt trigger is used so that the rising charged voltage of the RC network generates an impulse. This impulse is generated based on the two threshold voltage
s of the schmitt trigger. When the input voltage at the schmitt trigger coming from the RC network reaches the first threshold voltage the output of the schmitt trigger switches so that it generates the first edge of the input. The charging of the RC network should be long enough so that the PoR can reset all the internal circuits before the charging voltage reaches the other threshold voltage of the schmitt trigger and the output to switch back.
One of the issues with using RC network to generate 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.
Microcontroller
A microcontroller is a small computer on a single integrated circuit containing a processor core, memory, and programmable input/output peripherals. Program memory in the form of NOR flash or OTP ROM is also often included on chip, as well as a typically small amount of RAM...
or microprocessor
Microprocessor
A microprocessor incorporates the functions of a computer's central processing unit on a single integrated circuit, or at most a few integrated circuits. It is a multipurpose, programmable device that accepts digital data as input, processes it according to instructions stored in its memory, and...
peripheral
Peripheral
A peripheral is a device attached to a host computer, but not part of it, and is more or less dependent on the host. It expands the host's capabilities, but does not form part of the core computer architecture....
that generates a reset
Reset
Reset may refer to:* Ramsey RESET test , a general specification test for the linear regression model* Reset , to clear any pending errors or events and bring a system to normal condition or initial state...
signal when power is applied to the device
Computer hardware
Personal computer hardware are component devices which are typically installed into or peripheral to a computer case to create a personal computer upon which system software is installed including a firmware interface such as a BIOS and an operating system which supports application software that...
. It ensures that the device starts operating in a known state
State (computer science)
In computer science and automata theory, a state is a unique configuration of information in a program or machine. It is a concept that occasionally extends into some forms of systems programming such as lexers and parsers....
.
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 is composed by an RC device that charges with the rising of the supply voltage. A schmitt trigger is used so that the rising charged voltage of the RC network generates an impulse. This impulse is generated based on the two threshold voltage
Threshold voltage
The threshold voltage of a MOSFET is usually defined as the gate voltage where an inversion layer forms at the interface between the insulating layer and the substrate of the transistor. The purpose of the inversion layer's forming is to allow the flow of electrons through the gate-source junction...
s of the schmitt trigger. When the input voltage at the schmitt trigger coming from the RC network reaches the first threshold voltage the output of the schmitt trigger switches so that it generates the first edge of the input. The charging of the RC network should be long enough so that the PoR can reset all the internal circuits before the charging voltage reaches the other threshold voltage of the schmitt trigger and the output to switch back.
One of the issues with using RC network to generate 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.