Handle quadrature encoder input

It is now possible to act on the input from the encoder, no effort to
debounce has been made yet.

firmware/src/main.c

@@ -1,6 +1,7 @@
 #define	F_CPU   8000000UL
 
 #include <avr/io.h>
+#include <avr/interrupt.h>
 #include <util/delay.h>
 #include <stdbool.h>
 
@@ -86,6 +87,26 @@ void lcd_splash() {
     }
 }
 
+ISR(PCINT0_vect) {
+    cli();
+
+    while (~PINB & (1 << PB7));
+
+    // if PB6 Low -> CW
+    if (PINB & (1 << PB6)) {
+        OCR0B++;
+        if (OCR0B > 100)
+            OCR0B = 100;
+    // if PB6 High -> CCW
+    } else {
+        OCR0B--;
+        if (OCR0B > 100)
+            OCR0B = 0;
+    }
+
+    sei();
+}
+
 int main(void) {
     // FastPWM: 1.25kHz
     TCCR0A = (1 << WGM01) | (1 << WGM00) | (1 << COM0B1);
@@ -94,8 +115,12 @@ int main(void) {
     OCR0B = 10;
     DDRD |= (1 << PD5);
 
-    bool reverse = 0;
+    // Encoder setup
+    PORTB |= (1 << PB7) | (1 << PB6);
+    PCICR |= (1 << PCIE0);
+    PCMSK0 |= (1 << PCINT7);
 
+    // SPI setup
     spi_init();
     lcd_init();
     lcd_splash();