Datasheet

ATmega48A/PA/88A/PA/168A/PA/328/P
 2018 Microchip Technology Inc. Data Sheet Complete DS40002061A-page 58
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 314) will generate a reset, even if the clock is not
running. Shorter pulses are not ensured 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, see Table 28-7 on page 291.
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 59), the Brown-out Reset is immediately activated. When V
CC
increases above the trigger level (V
BOT+
in
Figure 11-5 on page 59), 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 314.
RESET
TIME-OUT
INTERNAL
RESET
t
TOUT
V
POT
V
RST
V
CC
CC