1 files changed, 56 insertions, 0 deletions
diff --git a/timer.c b/timer.c
new file mode 100644
index 0000000..5830b38
--- /dev/null
+++ b/timer.c
@@ -0,0 +1,56 @@
+#include <avr/io.h>
+#include <avr/interrupt.h>
+
+#include "timer.h"
+
+volatile unsigned char count = 0;
+volatile unsigned char hit = 0;
+
+ISR(TIMER0_OVF_vect) {
+	/* there seems to be no mode of operation which disconnects pin OC0A _and_
+	 * clears the value */
+	++count;
+	switch (count) {
+		case 1:
+		case 2:
+			TCNT0 = 0;
+			break;
+
+		/* three overflows happened, next one is a little shorter:
+		 * F_CPU/prescaler-3*256=208.5625 -> 256-208=48 -> zero-based=47 */
+		case 3:
+			TCNT0 = 47;
+			break;
+
+		/* one second elapsed */
+		case 4:
+			TCNT0 = 0;
+			count = 0;
+			++hit;
+			break;
+	}
+}
+
+bool timerHit () {
+	bool ret = hit != 0;
+	hit = 0;
+	return ret;
+}
+
+void timerStart () {
+	count = 0;
+	/* reset timer value */
+	TCNT0 = 0;
+	/* set normal mode timer0 */
+	TCCR0A = 0;
+	/* enable overflow interrupt */
+	TIMSK0 = (1 << TOIE0);
+	/* io clock with 1024 prescaler */
+	TCCR0B = (TCCR0B & ~((1 << CS01)) | (1 << CS02) | (1 << CS00));
+}
+
+void timerStop () {
+	/* zero clock source */
+	TCCR0B = 0;
+}
+