A power-on reset (PoR) 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.
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 voltages 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.
On an IBM mainframes, 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.