27.8.1 ADC Multiplexer Selection Register

Name: ADMUX
Offset: 0x7C
Reset: 0x00
Property: -

Bit 76543210 
 REFS [1:0]ADLARMUX [4:0] 
Access R/WR/WR/WR/WR/WR/WR/WR/W 
Reset 00000000 

Bits 7:6 – REFS [1:0] 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 27-3. ADC Voltage Reference Selection
REFS[1:0]Voltage Reference Selection
00AREF, Internal Vref turned off
01AVCC with external capacitor at AREF pin
10Internal 1.1V Voltage Reference with external capacitor at AREF pin
11Internal 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

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

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 27-4. Input Channel and Gain Selections
MUX[4:0]Single Ended InputPositive Differential InputNegative Differential InputGain
00000ADC0N/A
00001ADC1
00010ADC2
00011ADC3
00100ADC4
00101ADC5
00110ADC6
00111ADC7
01000N/AADC0ADC010x
01001ADC1ADC010x
01010ADC0ADC0200x
01011ADC1ADC0200x
01100ADC2ADC210x
01101ADC3ADC210x
01110ADC2ADC2200x
01111ADC3ADC2200x
10000N/AADC0ADC11x
10001ADC1ADC11x
10010ADC2ADC11x
10011ADC3ADC11x
10100ADC4ADC11x
10101ADC5ADC11x
10110ADC6ADC11x
10111ADC7ADC11x
11000ADC0ADC21x
11001ADC1ADC21x
11010ADC2ADC21x
11011ADC3ADC21x
11100ADC4ADC21x
11101ADC5ADC21x
111101.1V (VBG)N/A
111110V (GND)