ATMEGA644 datasheet(51/374 Pages) ATMEL

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2593L–AVR–02/07

ATmega644

9.1

Internal Voltage Reference

ATmega644 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.

9.1.1

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 9-4. To save power, the reference is not always turned on. The

reference is on during the following situations:

When the BOD is enabled (by programming the BODLEVEL [2:0] Fuse).

When the bandgap reference is connected to the Analog Comparator (by setting the

ACBG bit in ACSR).

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.

Note:

1. Values are guidelines only. Actual values are TBD.

Table 9-4.

Internal Voltage Reference Characteristics

(1)

Symbol

Parameter

Condition

Min

Typ

Max

Units

VBG

Bandgap reference voltage

VCC = 2.7

TA = 25°C

1.0

1.1

1.2

tBG

Bandgap reference start-up time

VCC = 2.7

TA = 25°C

µs

IBG

Bandgap reference current consumption

CC = 2.7

TA = 25°C

µA

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