Datasheet

43
2570N–AVR–05/11
ATmega325/3250/645/6450
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 301) 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.
Figure 11-4. External Reset During Operation
11.5 Brown-out Detection
Atmel ATmega325/3250/645/6450 has an On-chip Brown-out Detection (BOD) circuit for moni-
toring 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 44), the Brown-out
Reset is immediately activated. When V
CC
increases above the trigger level (V
BOT+
in Figure 11-
5 on page 44), 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 301.
RESET
TIME-OUT
INTERNAL
RESET
t
TOUT
V
POT
V
RST
V
CC
CC