Datasheet

47
2545T–AVR–05/11
ATmega48/88/168
Figure 11-3. MCU start-up, RESET extended externally.
11.4 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 307) 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 coun-
ter starts the MCU after the time-out period – t
TOUT
has expired. The external reset can be
disabled by the RSTDISBL fuse, see Table 28-6 on page 287.
Figure 11-4. External reset during operation.
11.5 Brown-out detection
The Atmel ATmega48/88/168 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 on page 48), the brown-out
reset is immediately activated. When V
CC
increases above the trigger level (V
BOT+
in Figure 11-5
on page 48), 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 307.
RESET
TIME-OUT
INTERNAL
RESET
t
TOUT
V
POT
V
RST
V
CC
CC