ATmega8U2/16U2/32U2
that remains enabled. This allows the device to stay attached to the bus during and after the
reset, while enhancing firmware reliability.
Figure 10-7. USB Reset During Operation
CC
End of Reset
tUSBRSTMIN
DP
USB Traffic
USB Traffic
DM
10.3 Internal Voltage Reference
ATmega8U2/16U2/32U2 features an internal bandgap reference. This reference is used for
Brown-out Detection, and it can be used as an input to the Analog Comparator.
10.3.1
Voltage Reference Enable Signals and Start-up Time
The voltage reference has a start-up time that may influence the way it should be used. The
start-up time is given in “System and Reset Characteristics” on page 267. To save power, the
reference is not always turned on. The reference is on during the following situations:
1. When the BOD is enabled (by programming the BODLEVEL [2..0] Fuse).
2. When the bandgap reference is connected to the Analog Comparator (by setting the
ACBG bit in ACSR).
Thus, when the BOD is not enabled, after setting the ACBG bit, the user must always allow the
reference to start up before the output from the Analog Comparator is used. To reduce power
consumption in Power-down mode, the user can avoid the three conditions above to ensure that
the reference is turned off before entering Power-down mode.
10.4 Watchdog Timer
10.4.1
Features
• Clocked from separate On-chip Oscillator
• 3 Operating modes
– Interrupt
– System Reset
– Interrupt and System ResetSelectable Time-out period from 16ms to 8s
• Possible Hardware fuse Watchdog always on (WDTON) for fail-safe mode
• Early warning after one Time-Out period reached, programmable Reset (see operating modes)
after 2 Time-Out periods reached.
10.4.2
Overview
ATmega8U2/16U2/32U2 has an Enhanced Watchdog Timer (WDT). The WDT is a timer count-
ing cycles of a separate on-chip 128 kHz oscillator. The WDT gives a early warning interrupt
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ATMEL [ ATMEL ]
