Added interrupt-driven non-blocking ADC read. First step in increasing loop frequencies

lib/ADC.cpp

@@ -0,0 +1,46 @@
+#include "ADC.hpp"
+
+#define NUM_PINS 4
+
+const uint8_t adc_pins[] = {0, 1, 4, 5}; // A0, A1, A4, A5
+
+volatile uint16_t adc_results[NUM_PINS];
+volatile uint8_t current_channel_index = 0;
+
+void setupADC() {
+  // 1. Set Reference to AVCC (5V)
+  // REFS0 = 1, REFS1 = 0
+  ADMUX = (1 << REFS0);
+
+  // 2. Set ADC Prescaler to 128 (16MHz / 128 = 125KHz)
+  // Good balance of speed and accuracy. 
+  // Bits: ADPS2, ADPS1, ADPS0
+  ADCSRA |= (1 << ADPS2) | (1 << ADPS1) | (1 << ADPS0);
+
+  // 3. Enable ADC and Enable ADC Interrupt
+  ADCSRA |= (1 << ADEN) | (1 << ADIE);
+
+  // 4. Set initial channel to the first pin in our list
+  ADMUX = (ADMUX & 0xF0) | (adc_pins[0] & 0x0F);
+
+  // 5. Start the first conversion!
+  ADCSRA |= (1 << ADSC); 
+}
+
+ISR(ADC_vect) {
+  // 1. Read the result (must read ADCL first, then ADCH, or just use ADC word)
+  adc_results[current_channel_index] = ADC;
+
+  // 2. Increment to next channel
+  current_channel_index++;
+  if (current_channel_index >= NUM_PINS) {
+    current_channel_index = 0;
+  }
+
+  // 3. Switch MUX to next channel
+  // Clear bottom 4 bits of ADMUX, then OR in the new pin number
+  ADMUX = (ADMUX & 0xF0) | (adc_pins[current_channel_index] & 0x0F);
+
+  // 4. Start next conversion
+  ADCSRA |= (1 << ADSC);
+}