Datasheet

59
ATmega640/V-1280/V-1281/V-2560/V-2561/V [DATASHEET]
2549Q–AVR–02/2014
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 360) 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 Volt-
age – 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 monitoring the AV
CC
level
during operation by comparing it to a fixed trigger level. The trigger level for the BOD can be selected by the BOD-
LEVEL 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 AV
CC
decreases to a value below the trigger level (V
BOT-
in Figure 12-5 on page
60), the Brown-out Reset is immediately activated. When AV
CC
increases above the trigger level (V
BOT+
in Figure
12-5 on page 60), the delay counter starts the MCU after the Time-out period t
TOUT
has expired.
The BOD circuit will only detect a drop in AV
CC
if the voltage stays below the trigger level for longer than t
BOD
given
in “System and Reset Characteristics” on page 360.
RESET
TIME-OUT
INTERNAL
RESET
t
TOUT
V
POT
V
RST
V
CC
CC