Datasheet

43
7701F–AVR–10/12
Atmel ATtiny24/44/84 [Preliminary]
Figure 9-4. External Reset During Operation
9.5 Brown-out Detection
ATtiny24/44/84 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 Fig-
ure 9-5 on page 43), the Brown-out Reset is immediately activated. When V
CC
increases
above the trigger level (V
BOT+
in Figure 9-5 on page 43), 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 Characterizations” on page 181.
Figure 9-5. Brown-out Reset During Operation
9.6 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
. See
“Watchdog Timer” on page 44 for details on operation of the Watchdog Timer.
CC
V
CC
RESET
TIME-OUT
INTERNAL
RESET
V
BOT-
V
BOT+
t
TOUT