ATmega8U2/16U2/32U2
10.2.3
Brown-out Detection
ATmega8U2/16U2/32U2 has an On-chip Brown-out Detection (BOD) circuit for monitoring the
VCC 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 VBOT+
=
V
BOT + VHYST/2 and VBOT- = VBOT - VHYST/2. When the BOD is enabled, and VCC decreases to a
value below the trigger level (VBOT- in Figure 10-5), the Brown-out Reset is immediately acti-
vated. When VCC increases above the trigger level (VBOT+ 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 VCC if the voltage stays below the trigger level for lon-
ger than tBOD given in “System and Reset Characteristics” on page 267.
Figure 10-5. Brown-out Reset During Operation
V
V
BOT+
CC
V
BOT-
RESET
t
TOUT
TIME-OUT
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 tTOUT. 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
50
7799D–AVR–11/10
ATMEL [ ATMEL ]
