Datasheet

38
ATmega32A [DATASHEET]
Atmel-8155D-AVR-ATmega32A-Datasheet_02/2014
Figure 11-3. MCU Start-up, RESET Extended Externally
11.2.2 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 299) 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.
Figure 11-4. External Reset During Operation
11.2.3 Brown-out Detection
ATmega32A 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 fuse BODLEVEL to be
2.7V (BODLEVEL unprogrammed), or 4.0V (BODLEVEL programmed). 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.
The BOD circuit can be enabled/disabled by the fuse BODEN. When the BOD is enabled (BODEN programmed),
and V
CC
decreases to a value below the trigger level (V
BOT-
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 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 299.
RESET
TIME-OUT
INTERNAL
RESET
t
TOUT
V
POT
V
RST
V
CC
CC