Internal Voltage
Reference
ATmega8 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.
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 16. 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 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.
Table 16. Internal Voltage Reference Characteristics
Symbol
VBG
Parameter
Min
Typ
1.23
40
Max
1.35
70
Units
V
Bandgap reference voltage
Bandgap reference start-up time
Bandgap reference current consumption
1.15
tBG
µs
IBG
10
µA
40
ATmega8(L)
2486M–AVR–12/03
ATMEL [ ATMEL ]
