Move source code to /firmware and fix build paths

firmware/atmega/source/adc.c

@@ -0,0 +1,27 @@
+#include <stdint.h>
+#include <avr/io.h>
+#include <avr/interrupt.h>
+
+void adcInit(void) {
+    // Set reference voltage to AVcc (REFS0 = 1, REFS1 = 0)
+    ADMUX = (1 << REFS0);
+
+    // Enable ADC and set prescaler to 128 (for 16 MHz clock, gives 125 kHz ADC clock)
+    ADCSRA = (1 << ADEN) | (1 << ADPS2) | (1 << ADPS1) | (1 << ADPS0);
+}
+
+uint16_t adcRead(uint8_t channel) {
+    // Select ADC pin (A0-A7) by setting MUX bits in ADMUX
+    // Pins A6 and A7 would make an 8 sensor array possible (for large vases).
+    // Achievable only when using the 328p as a DIP, through port C (PC6, PC7)
+    ADMUX = (ADMUX & 0xF8) | (channel & 0x07);
+
+    // Start conversion
+    ADCSRA |= (1 << ADSC);
+
+    // Wait for conversion to complete (ADSC becomes '0' when done)
+    while (ADCSRA & (1 << ADSC));
+
+    // Read ADC (reading order: ADCL --> ADCH)
+    return ADC;
+}