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.
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.