Microcontroller Supervisory IC Application Note

A voltage detector is an IC that compares a sensed voltage with a preset internal reference voltage and outputs a reset signal once the sensed voltage drops below the reference voltage. However, it is also used to generate a power on reset signal during the start up sequence.
The voltage supplied to the CPU becomes unstable momentarily at the power-on even with a regulator. The power-on reset function continuously resets the CPU till the supplied voltage stabilizes since the voltage outside of the CPU's operating voltage range may cause a CPU runaway. The duration of the power-on reset should be set individually by the output delay function since the duration required for stabilizing the voltage varies by circuit.