Arduino - reading multiple pots asynchronously with a free-running ADC

gistfile1.cpp

#include "avr/interrupt.h"

uint8_t pots[4] = { 0, 0, 0, 0 };
uint16_t samples[4] = { 0, 0, 0, 0 };
const uint8_t nextpot[4] = { _BV(ADLAR) | 0b0001, _BV(ADLAR) | 0b0010, _BV(ADLAR) | 0b0011, _BV(ADLAR) | 0b0000 };

void setup() {
	Serial.begin(9600);
	ADMUX = _BV(ADLAR) | _BV(MUX1) | _BV(MUX0);
	ADCSRA = _BV(ADEN) | _BV(ADSC) | _BV(ADATE) | _BV(ADIE) | _BV(ADPS0)| _BV(ADPS1) | _BV(ADPS2);
	ADCSRB = 0; // free-running - all ADTS bits cleared
}

void loop() {
	char buf[150];
	snprintf(buf,149,"cutoff:%-3d/%-6d resonance:%-3d/%-6d envelope:%-3d/%-6d decay:%-3d/%-6d",
		pots[0], samples[0], pots[1], samples[1], pots[2], samples[2], pots[3], samples[3]);
	Serial.println(buf);
	delay(1000);
}

ISR(ADC_vect) {
	uint8_t reading = ADCH << 4;
	static uint8_t first = 1;
	if (first) {
		first = 0;
		return;
	}
	uint8_t mux;
	mux = ADMUX & 0b00000111;
	ADMUX = nextpot[mux];
	pots[mux] = reading;
	samples[mux]++;
}