
43
2552K–AVR–04/11
ATmega329/3290/649/6490
Figure 10-3. MCU Start-up, RESET Extended Externally
10.4
External Reset
An External Reset is generated by a low level on the RESET pin. Reset pulses longer than the
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 10-4. External Reset During Operation
10.5
Brown-out Detection
ATmega329/3290/649/6490 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+ = VBOT + VHYST/2 and VBOT- = VBOT - VHYST/2.
When the BOD is enabled, and V
CC decreases to a value below the trigger level (VBOT- in Figure 10-5), the Brown-out Reset is immediately activated. When V
CC increases above the trigger
level (V
BOT+ in Figure 10-5), the delay counter starts the MCU after the Time-out period tTOUT 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
RESET
TIME-OUT
INTERNAL
RESET
t
TOUT
V
POT
V
RST
VCC
CC