Datasheet

Figure 15-3. MCU Start-up, RESET Extended Externally
RESET
TIME-OUT
INTERNAL
RESET
t
TOUT
V
POT
V
RST
V
CC
15.4 External Reset
An external Reset is generated by a low level on the RESET pin. Reset pulses longer than the minimum
pulse width 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 counter starts the MCU after the Time-out period (t
TOUT
) has expired. The external Reset
can be disabled by the RSTDISBL fuse.
Figure 15-4. External Reset During Operation
CC
15.5 Brown-out Detection
The device 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 BOD. 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 the following figure), the
Brown-out Reset is immediately activated. When V
CC
increases above the trigger level (V
BOT+
in the
following figure), 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 longer than
t
BOD
.
ATmega48PA/88PA/168PA
System Control and Reset
© 2018 Microchip Technology Inc.
Datasheet Complete
DS40002011A-page 80