The Power-on Reset (POR) circuit is a non-configurable circuit that ensures the device is reset when powered on, with what is called a Power-on Reset. During POR, all logic is reset, and all registers are initialized, including registers that are loaded from fuses. This procedure ensures that the MCU starts execution in a known good state.
The POR circuit is always active and works by comparing the supply voltage with fixed threshold levels, either rising or falling. These threshold levels are not configurable, unlike the BOD level. See section Brown-out Detector.
Both POR rising threshold level and POR falling threshold level are specified in the electrical characteristics section of the Relevant Devices data sheet.
When the supply voltage is below the rising threshold, the POR circuit will assert the internal Reset. When the supply voltage rises above the threshold, the device will be allowed to start-up.
In the case of a power dip, the POR circuit compares the supply voltage to a POR falling threshold level. If the supply voltage dips below this threshold, the internal Reset will be reasserted, until the supply voltage again rises above the POR rising threshold level.
If the device is reset by POR, the POR flag in the Reset Flag (RSTCTRL.RSTFR) register is set. After a POR, only the POR flag is set, all other flags are cleared.