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ATmega48A/PA/88A/PA/168A/PA/328/P [DATASHEET]
8271E–AVR–07/2012
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 310) 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 Volt-
age – 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, see Table 28-7 on page 287.
Figure 11-4. External Reset During Operation
11.5 Brown-out Detection
ATmega48A/PA/88A/PA/168A/PA/328/P 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 49), the Brown-out
Reset is immediately activated. When V
CC
increases above the trigger level (V
BOT+
in Figure 11-5 on page 49), 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
given
in ”System and Reset Characteristics” on page 310.
CC