It is possible to select any of the ADC7:0 pins to replace the negative input to the Analog Comparator. The ADC multiplexer is used to select this input, and consequently the ADC must be switched off to utilize this feature. If the Analog Comparator Multiplexer Enable bit (ACME in SFIOR) is set and the ADC is switched off (ADEN in ADCSRA is zero), MUX2:0 in ADMUX select the input pin to replace the negative input to the Analog Comparator, as shown in the table below. If ACME is cleared or ADEN is set, AIN1 is applied to the negative input to the Analog Comparator.
| ACME | ADEN | MUX[2:0] | Analog Comparator Negative Input |
|---|---|---|---|
| 0 | x | xxx | AIN1 |
| 1 | 1 | xxx | AIN1 |
| 1 | 0 | 000 | ADC0 |
| 1 | 0 | 001 | ADC1 |
| 1 | 0 | 010 | ADC2 |
| 1 | 0 | 011 | ADC3 |
| 1 | 0 | 100 | ADC4 |
| 1 | 0 | 101 | ADC5 |
| 1 | 0 | 110 | ADC6 |
| 1 | 0 | 111 | ADC7 |