Datasheet

Figure 15-3. Figure: MCU Start-up, RESET Extended Externally
RESET
TIME-OUT
INTERNAL
RESET
t
TOUT
V
POT
V
RST
V
CC
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System and Reset Characteristics
15.2.2 External Reset
An External Reset is generated by a low level on the RESET pin. Reset pulses longer than the minimum
pulse width (see table in System and Reset Characteristics) 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 15-4. External Reset During Operation
CC
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System and Reset Characteristics
15.2.3 Brown-out Detection
ATmega8A 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 the figure below), the
Brown-out Reset is immediately activated. When V
CC
increases above the trigger level (V
BOT+
in the
figure below), the delay counter starts the MCU after the time-out period t
TOUT
has expired.
AVR 8-Bit Microcontroller
System Control and Reset
© 2017 Microchip Technology Inc.
Datasheet Complete
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