Datasheet

39
ATmega8A [DATASHEET]
8159E–AVR–02/2013
Figure 11-5. Brown-out Reset During Operation
11.2.4 Watchdog Reset
When the Watchdog times out, it will generate a short reset pulse of 1 CK cycle duration. On the falling edge of this
pulse, the delay timer starts counting the time-out period t
TOUT
. Refer to page 40 for details on operation of the
Watchdog Timer.
Figure 11-6. Watchdog Reset During Operation
11.3 Internal Voltage Reference
ATmega8A 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 or the ADC. The 2.56V reference to the ADC is generated from the
internal bandgap reference.
11.3.1 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 Table 26-3 on page 228. 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 BODEN Fuse).
2. When the bandgap reference is connected to the Analog Comparator (by setting the ACBG bit in ACSR).
3. When the ADC is enabled.
Thus, when the BOD is not enabled, after setting the ACBG bit or enabling the ADC, the user must always allow
the reference to start up before the output from the Analog Comparator or ADC is used. To reduce power con-
sumption 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.
V
CC
RESET
TIME-OUT
INTERNAL
RESET
V
BOT-
V
BOT+
t
TOUT
CK
CC