WIP: Add a TIMER1 configuration routine in avr.h

The function hasn't been tested on real hardware.

lib/c/avr.h

@@ -34,6 +34,11 @@
 #ifdef __AVR__
 #include <util/atomic.h>
 #include <avr/interrupt.h>
+#include <avr/io.h>
+
+#ifndef F_CPU
+#error "F_CPU must be defined"
+#endif
 
 void run_timers();
 
@@ -41,6 +46,19 @@ ISR(TIMER0_OVF_vect) {
   run_timers();
 }
 
+static inline void set_ocr1a(uint16_t value) {
+  OCR1AH = value >> 8;
+  OCR1AL = value;
+}
+
+/* Source: https://adnbr.co.uk/articles/counting-milliseconds */
+void init_timer1() {
+  TCCR1B |= _BV(WGM12) | _BV(CS11);
+  set_ocr1a((F_CPU / 1000) / 8);
+  TIMSK1 |= _BV(OCIE1A);
+  sei();
+}
+
 static inline void atomic_memcpy(void *dest, const void *src, size_t size) {
   ATOMIC_BLOCK(ATOMIC_FORCEON) {
     memcpy(dest, src, size);