28.9.1 ADC Multiplexer Selection Register

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

Bit 76543210 
 REFS [1:0]ADLAR MUX [3:0] 
Access R/WR/WR/WR/WR/WR/WR/W 
Reset 0000000 

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 into 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 28-4. ADC Voltage Reference Selection
REFS[1:0]Voltage Reference Selection
00AREF, Internal Vref turned OFF
01AVCC with external capacitor at AREF pin
10Reserved
11Internal 1.1V voltage reference with external capacitor at 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 conversions. For a complete description of this bit, refer to ADCL and ADCH.

Bits 3:0 – MUX [3:0] Analog Channel Selection

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 28-3. Input Channel Selection
MUX[3:0]Single Ended Input
0000ADC0
0001ADC1
0010ADC2
0011ADC3
0100ADC4
0101ADC5
0110ADC6
0111ADC7
1000Temperature sensor
1001Reserved
1010Reserved
1011Reserved
1100Reserved
1101Reserved
11101.1V (VBG)
11110V (GND)