ADMUX – ADC Multiplexer Selection Register

When using the I/O specific commands IN and OUT, the I/O addresses 0x00 - 0x3F must be used. When addressing I/O Registers as data space using LD and ST instructions, 0x20 must be added to these offset addresses.

Name:
ADMUX
Offset:
0x07
Reset:
0x00
Access:
When addressing I/O Registers as data space the offset address is 0x27
Bit76543210
REFSn[1:0]ADLARMUXn[3:0]
AccessR/WR/WR/WR/WR/WR/WR/W
Reset0000000

Bits 7:6 – REFSn: Reference Selection [n = 1:0]

Reference Selection [n = 1:0]

These bits select the voltage reference for the ADC. 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.

Table 1. ADC Voltage Reference Selection
REFS[1:0] Voltage Reference Selection
00 AREF, Internal Vref turned off
01 AVCC with external capacitor at AREF pin
10 Reserved
11 Internal 2.56V Voltage Reference with external capacitor at AREF pin

Bit 5 – ADLAR: ADC Left Adjust Result

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 conversions. For a complete description of this bit, see ADCL and ADCH.

Bits 3:0 – MUXn: Analog Channel Selection [n = 3:0]

Analog Channel Selection [n = 3:0]

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

Table 2. Input Channel Selection
MUX[3:0] Single Ended Input
0000 ADC0
0001 ADC1
0010 ADC2
0011 ADC3
0100 ADC4
0101 ADC5
0110 ADC6
0111 ADC7
1000 Reserved
1001 Reserved
1010 Reserved
1011 Reserved
1100 Reserved
1101 Reserved
1110 1.30V (VBG)
1111 0V (GND)