ATMEGA64_08 Datasheet, PDF (54/393 Pages) ATMEL Corporation – 8-bit Microcontroller with 64K Bytes In-System Programmable Flash

External Reset
An External Reset is generated by a low level on the RESET pin. Reset pulses longer than the
minimum pulse width (see Table 19) will generate a reset, 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, the delay counter starts the MCU after the
Time-out period t_TOUT has expired.
Figure 25. External Reset during Operation
CC
Brown-out Detection
ATmega64 has an On-chip Brown-out Detection (BOD) circuit for monitoring the V_CC level dur-
ing 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 Figure
26), the Brown-out Reset is immediately activated. When V_CC increases above the trigger level
(V_BOT+ in Figure 26), 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 Table 19.
Figure 26. Borwn-out Reset During Operation
V_CC
V_BOT-
V_BOT+
RESET
TIME-OUT
t_TOUT
INTERNAL
RESET
54 ATmega64(L)
2490N–AVR–05/08