166
9151AINDCO07/09
ATA8742
25.10 Register Description
25.10.1 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 25-3 on page 166. 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).
Special care should be taken when changing differential channels. Once a differential channel
has been selected, the stage may take as much as 25 ADC clock cycles to stabilize to the new
value. Thus conversions should not be started within the first 13 clock cycles after selecting a
new differential channel. Alternatively, conversion results obtained within this period should be
discarded.
The same settling time should be observed for the first differential conversion after changing
ADC reference (by changing the REFS1:0 bits in ADMUX).
If channels where differential gain is used ie. the gainstage, using V
CC
or an optional external
AREF higher than (V
CC
- 1V) is not recommended, as this will affect ADC accuracy. It is not
allowed to connect internal voltage reference to AREF pin, if an external voltage is being applied
to it already. Internal voltage reference is connected AREF pin when REFS1:0 is set to value
11.
" 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 25-4 on
page 167 show values for single ended 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 25-4 on page 167 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).
Bit
7
6
5
4
3
2
1
0
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 25-3. Voltage Reference Selections for ADC
REFS1
REFS0
Voltage Reference Selection
0
0
V
CC
used as analog reference, disconnected from PA0 (AREF).
0
1
External Voltage Reference at PA0 (AREF) pin, Internal Voltage Reference
turned off.
1
0
Internal 1.1V Voltage Reference.
1
1
Reserved.
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