ATMEGA8U2_14 Datasheet, PDF (50/310 Pages) ATMEL Corporation – 125 Powerful Instructions – Most Single Clock Cycle Execution

ATmega8U2/16U2/32U2
10.2.3
Brown-out Detection
ATmega8U2/16U2/32U2 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+ =
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 10-5), the Brown-out Reset is immediately acti-
vated. 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 lon-
ger than t_BOD given in “System and Reset Characteristics” on page 267.
Figure 10-5. Brown-out Reset During Operation
V_CC
V_BOT-
V_BOT+
RESET
TIME-OUT
t_TOUT
INTERNAL
RESET
10.2.4
Watchdog Reset
When the Watchdog times out, it will generate a short reset pulse of one CK cycle duration. On
the falling edge of this pulse, the delay timer starts counting the Time-out period t_TOUT. Refer to
“Watchdog Timer” on page 51 for details on operation of the Watchdog Timer.
Figure 10-6. Watchdog Reset During Operation
CC
CK
10.2.5
USB Reset
When the USB macro is enabled and configured with the USB reset MCU feature enabled, and
if a valid USB Reset signalling is detected, the microcontroller is reset unless the USB macro
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