Datasheet

144
8183F–AVR–06/12
ATtiny24A/44A/84A
16.13 Register Description
16.13.1 ADMUX – ADC Multiplexer Selection Register
• Bits 7:6 – REFS[1:0]: 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 – MUX[5:0]: 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 145 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 145 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 76543210
0x07 (0x27) REFS1 REFS0 MUX5 MUX4 MUX3 MUX2 MUX1 MUX0 ADMUX
Read/Write R/W R/W R/W R/W R/W R/W R/W R/W
Initial Value 0 0 0 0 0 0 0 0
Table 16-3. Voltage Reference Selections for ADC
REFS1 REFS0 Voltage Reference Selection
00V
CC
used as analog reference, disconnected from PA0 (AREF)
0 1 External voltage reference at PA0 (AREF) pin, internal reference turned off
1 0 Internal 1.1V voltage reference
11Reserved