ATtiny44 datasheet(145/236 Pages) ATMEL

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145

8006H–AVR–10/09

ATtiny24/44/84

16.13 Register Description

16.13.1

ADMUX – ADC Multiplexer Selection Register

• Bit 7:6 – REFS1:REFS0: Reference Selection Bits

These bits select the voltage reference for the ADC, as shown in Table 16-3.

If these bits are changed during a conversion, the change will not go in effect until this

conversion is complete (ADIF in ADCSR is set). Also note, that when these bits are changed,

the next conversion will take 25 ADC clock cycles.

Special care should be taken when changing differential channels. Once a differential channel

has been selected the input stage may take a while to stabilize. It is therefore recommended to

force the ADC to perform a long conversion when changing multiplexer or voltage reference set-

tings. This can be done by first turning off the ADC, then changing reference settings and then

turn on the ADC. Alternatively, the first conversion results after changing reference settings

should be discarded.

It is not recommended to use an external AREF higher than (V

CC - 1V) for channels with differ-

ential gain, as this will affect ADC accuracy.

Internal voltage reference options may not be used if an external voltage is being applied to the

AREF pin.

• Bits 5:0 – MUX5:0: Analog Channel and Gain Selection Bits

The value of these bits selects which combination of analog inputs are connected to the ADC. In

case of differential input, gain selection is also made with these bits. Selections on Table 16-4 on

page 146 show values for single endid channels and where the differential channels as well as

the offset calibration selections are located. Selecting the single-ended channel ADC8 enables

the temperature measurement. See Table 16-4 on page 146 for details. If these bits are

changed during a conversion, the change will not go into effect until this conversion is complete

(ADIF in ADCSRA is set).

Special care should be taken when changing differential channels. Once a differential channel

has been selected the input stage may take a while to stabilize. It is therefore recommended to

force the ADC to perform a long conversion when changing multiplexer settings. This can be

Bit

765

432

0x07 (0x27)

REFS1

REFS0

MUX5

MUX4

MUX3

MUX2

MUX1

MUX0

ADMUX

Read/Write

R/W

Initial Value

Table 16-3.

Voltage Reference Selections for ADC

REFS1

REFS0

Voltage Reference Selection

CC used as analog reference, disconnected from PA0 (AREF)

External voltage reference at PA0 (AREF) pin, internal reference turned off

Internal 1.1V voltage reference

Reserved

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