ATMEGA8_08 Datasheet, PDF (42/308 Pages) ATMEL Corporation – 8-bit with 8K Bytes In-System Programmable Flash

Internal Voltage
Reference
Voltage Reference
Enable Signals and
Start-up Time
ATmega8 features an internal bandgap reference. This reference is used for Brown-out Detec-
tion, 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.
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 ref-
erence 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 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 Parameter
Min Typ Max Units
V_BG
Bandgap reference voltage
t_BG
Bandgap reference start-up time
I_BG
Bandgap reference current consumption
1.15 1.30 1.40
V
40
70
µs
10
µA
42 ATmega8(L)
2486T–AVR–05/08