15.2.3 Brown-out Detection

ATmega8A has an On-chip Brown-out Detection (BOD) circuit for monitoring the V_CC level during operation by comparing it to a fixed trigger level. The trigger level for the BOD can be selected by the fuse BODLEVEL to be 2.7V (BODLEVEL unprogrammed), or 4.0V (BODLEVEL programmed). The trigger level has a hysteresis to ensure spike free Brown-out Detection. The hysteresis on the detection level should be interpreted as V_BOT+ = V_BOT + V_HYST/2 and V_BOT- = V_BOT - V_HYST/2.

The BOD circuit can be enabled/disabled by the fuse BODEN. When the BOD is enabled (BODEN programmed), and V_CC decreases to a value below the trigger level (V_BOT- in the figure below), the Brown-out Reset is immediately activated. When V_CC increases above the trigger level (V_BOT+ in the figure below), the delay counter starts the MCU after the time-out period t_TOUT has expired.

The BOD circuit will only detect a drop in V_CC if the voltage stays below the trigger level for longer than t_BOD given in the table in System and Reset Characteristics.