ADC Multiplexer Selection Register
Name:
ADMUX
Offset:
0x7C
Reset:
0x00
Access:
-
Bit76543210
REFS [1:0]ADLARMUX [4:0]
AccessR/WR/WR/WR/WR/WR/WR/WR/W
Reset00000000

Bits 7:6 – REFS [1:0]: Reference Selection

Reference Selection

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 Internal 1.1V Voltage Reference with external capacitor at AREF pin
11 Internal 2.56V Voltage Reference with external capacitor at AREF pin
Note: If differential channels are selected, only 2.56V should be used as Internal Voltage Reference.

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 the ADCL and ADCH.

Bits 4:0 – MUX [4:0]: Analog Channel and Gain Selection Bits

Analog Channel and Gain Selection Bits

The value of these bits selects which combination of analog inputs are connected to the ADC. These bits also select the gain for the differential channels. 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 and Gain Selections
MUX[4:0] Single Ended Input Positive Differential Input Negative Differential Input Gain
00000 ADC0 N/A
00001 ADC1
00010 ADC2
00011 ADC3
00100 ADC4
00101 ADC5
00110 ADC6
00111 ADC7
01000 N/A ADC0 ADC0 10x
01001 ADC1 ADC0 10x
01010 ADC0 ADC0 200x
01011 ADC1 ADC0 200x
01100 ADC2 ADC2 10x
01101 ADC3 ADC2 10x
01110 ADC2 ADC2 200x
01111 ADC3 ADC2 200x
10000 N/A ADC0 ADC1 1x
10001 ADC1 ADC1 1x
10010 ADC2 ADC1 1x
10011 ADC3 ADC1 1x
10100 ADC4 ADC1 1x
10101 ADC5 ADC1 1x
10110 ADC6 ADC1 1x
10111 ADC7 ADC1 1x
11000 ADC0 ADC2 1x
11001 ADC1 ADC2 1x
11010 ADC2 ADC2 1x
11011 ADC3 ADC2 1x
11100 ADC4 ADC2 1x
11101 ADC5 ADC2 1x
11110 1.1V (VBG) N/A
11111 0V (GND)