Datasheet

50
8284D–AVR–6/11
ATmega169A/PA/329A/PA/3290A/PA/649A/P/6490A/P
11.2.3 Brown-out Detection
ATmega169A/169PA/329A/329PA/3290A/3290PA/649A/649P/6490A/6490P 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 11-5), the Brown-out Reset is immediately activated. When V
CC
increases above the
trigger level (V
BOT+
in Figure 11-5), 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 353.
Figure 11-5. Brown-out Reset During Operation
11.2.4 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
”Watchdog Timer” on page 51 for details on operation of the Watchdog Timer.
Figure 11-6. Watchdog Reset During Operation
V
CC
RESET
TIME-OUT
INTERNAL
RESET
V
BOT-
V
BOT+
t
TOUT
CK
CC