28
ATmega161(L)
1228B–09/01
Figure 26.
MCU Start-up, RESET Controlled Externally
External Reset
An External Reset is generated by a low level on the RESET pin. Reset pulses longer
than 500 ns 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 timer starts the MCU after the Time-out
period (t
TOUT
) has expired.
Figure 27.
External Reset during Operation
Brown-out Detection
ATmega161 has an On-chip Brown-out Detection (BOD) circuit for monitoring the V
CC
level during the operation. The BOD circuit can be enabled/disabled by the fuse
BODEN. When BODEN is enabled (BODEN programmed), and V
CC
decreases to a
value below the trigger level, the Brown-out Reset is immediately activated. When V
CC
increases above the trigger level, the Brown-out Reset is deactivated after a delay. The
delay is defined by the user in the same way as the delay of POR signal in Table 4. The
trigger level for the BOD can be selected by the fuse BODLEVEL to be 2.7V
(BODLEVEL unprogrammed), or 4.0V (BODLEVEL programmed). The trigger level has
a hysteresis of 50 mV to ensure spike-free brown-out detection.
The BOD circuit will only detect a drop in V
CC
if the voltage stays below the trigger level
for longer than 9 µs for trigger level 4.0V, 21 µs for trigger level 2.7V (typical values).
VCC
RESET
TIME-OUT
INTERNAL
RESET
t
TOUT
V
POT
V
RST