Datasheet

61
2549O–AVR–05/12
ATmega640/1280/1281/2560/2561
Figure 12-3. MCU Start-up, RESET Extended Externally
12.2.2 External Reset
An External Reset is generated by a low level on the RESET
pin. Reset pulses longer than the
minimum pulse width (see “System and Reset Characteristics” on page 372) 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 12-4. External Reset During Operation
12.2.3 Brown-out Detection
ATmega640/1280/1281/2560/2561 has an On-chip Brown-out Detection (BOD) circuit for moni-
toring 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 BODLEVEL Fuses. 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.
When the BOD is enabled, and V
CC
decreases to a value below the trigger level (V
BOT-
in Figure
12-5 on page 62), the Brown-out Reset is immediately activated. When V
CC
increases above the
trigger level (V
BOT+
in Figure 12-5 on page 62), 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 lon-
ger than t
BOD
given in “System and Reset Characteristics” on page 372.
RESET
TIME-OUT
INTERNAL
RESET
t
TOUT
V
POT
V
RST
V
CC
CC