262
8025I–AVR–02/09
ATmega48P/88P/168P/328P
21.9
Register Description
21.9.1
ADMUX – ADC Multiplexer Selection Register
• Bit 7:6 – REFS1:0: Reference Selection Bits
These bits select the voltage reference for the ADC, as shown in
. If these bits are
changed during a conversion, the change will not go in effect until this conversion is complete
(ADIF in ADCSRA is set). The internal voltage reference options may not be used if an external
reference voltage is being applied to the AREF pin.
•
Bit 5 – ADLAR: ADC Left Adjust Result
The ADLAR bit affects the presentation of the ADC conversion result in the ADC Data Register.
Write one to ADLAR to left adjust the result. Otherwise, the result is right adjusted. Changing the
ADLAR bit will affect the ADC Data Register immediately, regardless of any ongoing conver-
sions. For a complete description of this bit, see
”ADCL and ADCH – The ADC Data Register” on
.
• Bit 4 – Res: Reserved Bit
This bit is an unused bit in the ATmega48P/88P/168P/328P, and will always read as zero.
• Bits 3:0 – MUX3:0: Analog Channel Selection Bits
The value of these bits selects which analog inputs are connected to the ADC. See
for details.
If these bits are changed during a conversion, the change will not go in effect until this
conversion is complete (ADIF in ADCSRA is set).
Bit
7
6
5
4
3
2
1
0
REFS1
REFS0
ADLAR
–
MUX3
MUX2
MUX1
MUX0
ADMUX
Read/Write
R/W
R/W
R/W
R
R/W
R/W
R/W
R/W
Initial Value
0
0
0
0
0
0
0
0
Table 21-3.
Voltage Reference Selections for ADC
REFS1
REFS0
Voltage Reference Selection
0
0
AREF, Internal V
ref
turned off
0
1
AV
CC
with external capacitor at AREF pin
1
0
Reserved
1
1
Internal 1.1V Voltage Reference with external capacitor at AREF pin