Datasheet

61
ATmega640/1280/1281/2560/2561
2549K–AVR–01/07
Watchdog Reset When the Watchdog times out, it will generate a short reset pulse of one CK cycle dura-
tion. On the falling edge of this pulse, the delay timer starts counting the Time-out period
t
TOUT
. See “Watchdog Timer” on page 53. for details on operation of the Watchdog
Timer.
Figure 29. Watchdog Reset During Operation
Internal Voltage
Reference
ATmega640/1280/1281/2560/2561 features an internal bandgap reference. This refer-
ence is used for Brown-out Detection, and it can be used as an input to the Analog
Comparator or the ADC.
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 29. 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).
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 Com-
parator or ADC 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.
Note: 1. Values are guidelines only. Actual values are TBD.
CK
CC
Table 29. Internal Voltage Reference Characteristics
(1)
Symbol Parameter Condition Min Typ Max Units
V
BG
Bandgap reference voltage TBD TBD 1.1 TBD V
t
BG
Bandgap reference start-up time TBD 40 70 µs
I
BG
Bandgap reference current
consumption
TBD 10 TBD µA