What is a power on reset?

Voltage Detectors (Reset ICs)Function

In addition to outputting a reset signal at a reduction of supply voltage, Reset IC is usable to "Power-on Reset" by monitoring that the supply voltage is over than the minimum operating voltage of the system.

After power-up, the power-on reset function works to put the system to a reset state until the initialization of internal registers is completed after the power supply voltage and the internal clock of microcontroller became a stable state.