The Brownout Detector (BOD) monitors the power supply and compares the voltage with two programmable brownout threshold levels: the brownout threshold level defines when to generate a Reset. A voltage level monitor (VLM) monitors the power supply, too, but compares it to a threshold higher than the BOD threshold. The VLM can then generate an interrupt request as an "early warning" when the supply voltage is about to drop below the VLM threshold. The BOD threshold level is expressed in percent above the BOD threshold level.

The BOD is mainly controlled by fuses. The mode used in Standby sleep mode and Power Down sleep mode can be altered in normal program execution. The VLM part of the BOD is controlled by I/O registers as well.

When activated, The BOD can operate in Enabled mode, where the BOD is continuously active, and in Sampled mode, where the BOD is activated briefly at a given period to check the supply voltage level.