ATmega640/1280/1281/2560/2561
Figure 26. MCU Start-up, RESET Tied to VCC
VPOT
VCC
VRST
RESET
tTOUT
TIME-OUT
INTERNAL
RESET
Figure 27. MCU Start-up, RESET Extended Externally
VPOT
VCC
VRST
RESET
tTOUT
TIME-OUT
INTERNAL
RESET
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 23) 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 – VRST – on its positive edge, the delay
counter starts the MCU after the Time-out period – tTOUT – has expired.
Figure 28. External Reset During Operation
CC
Brown-out Detection
ATmega640/1280/1281/2560/2561 has an On-chip Brown-out Detection (BOD) circuit
for monitoring the VCC level during operation by comparing it to a fixed trigger level. The
trigger level for the BOD can be selected by the BODLEVEL Fuses. The trigger level
59
2549A–AVR–03/05
ATMEL [ ATMEL ]
