ATMega chip full datasheet - UCSD Department of Physics

ATmega48A/PA/88A/PA/168A/PA/328/PFigure 11-3.MCU Start-up, RESET Extended ExternallyV CCV POTRESETV RSTTIME-OUTt TOUTINTERNALRESET11.4 External ResetAn External Reset is generated by a low level on the RESET pin. Reset pulses longer than theminimum pulse width (see ”System and Reset Characteristics” on page 324) will generate areset, even if the clock is not running. Shorter pulses are not guaranteed to generate a reset.When the applied signal reaches the Reset Threshold Voltage – V RST – on its positive edge, thedelay counter starts the MCU after the Time-out period – t TOUT – has expired. The External Resetcan be disabled by the RSTDISBL fuse, see Table 28-7 on page 299.Figure 11-4.External Reset During OperationCC11.5 Brown-out DetectionATmega48A/PA/88A/PA/168A/PA/328/P has an On-chip Brown-out Detection (BOD) circuit formonitoring the V CC level during operation by comparing it to a fixed trigger level. The trigger levelfor the BOD can be selected by the BODLEVEL Fuses. The trigger level has a hysteresis toensure spike free Brown-out Detection. The hysteresis on the detection level should be interpretedas V BOT+ = V BOT + V HYST /2 and V BOT- = V BOT - V HYST /2.When the BOD is enabled, and V CCdecreases to a value below the trigger level (V BOT- in Figure 11-5 on page 51), the Brown-outReset is immediately activated. When V CC increases above the trigger level (V BOT+ in Figure 11-5 on page 51), 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 longerthan t BOD given in ”System and Reset Characteristics” on page 324.8271D–AVR–05/1150