case 5: // gain stage offset: 1.1V, ADC1,1, gain 20
ADMUX = _BV(REFS1) | _BV(MUX3) | _BV(MUX2) | _BV(MUX0);
sum_shift = 0; // 1 measurement
+ break;
case 6: // buttons: 1.1V, ADC3, single-ended
PORTA |= _BV(PA3); // +5V to the voltage splitter
ADMUX = _BV(REFS1) | _BV(MUX1) | _BV(MUX0);