Datasheet

52
7674F–AVR–09/09
ATmega164P/324P/644P
Figure 9-2. External Reset During Operation
9.0.5 Brown-out Detection
ATmega164P/324P/644P has an On-chip Brown-out Detection (BOD) circuit for monitoring 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
9-3 on page 52), the Brown-out Reset is immediately activated. When V
CC
increases above the
trigger level (V
BOT+
in Figure 9-3 on page 52), 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 332.
Figure 9-3. Brown-out Reset During Operation
9.0.6 Watchdog Reset
When the Watchdog times out, it will generate a short reset pulse of one CK cycle duration. On
the falling edge of this pulse, the delay timer starts counting the Time-out period t
TOUT
. Refer to
page 60 for details on operation of the Watchdog Timer.
CC
V
CC
RESET
TIME-OUT
INTERNAL
RESET
V
BOT-
V
BOT+
t
TOUT