From bdd5ae4dd66941e27128f3ad45d7452c33af716b Mon Sep 17 00:00:00 2001
From: Lars-Dominik Braun <lars@6xq.net>
Date: Tue, 19 Aug 2014 15:50:26 +0200
Subject: timer: Support arbitrary timer values

FIXME: large values do not work yet.
---
 timer.c | 42 +++++++++++++++++++++++++++++-------------
 timer.h |  6 +++---
 2 files changed, 32 insertions(+), 16 deletions(-)

diff --git a/timer.c b/timer.c
index d2634f2..c4e5704 100644
--- a/timer.c
+++ b/timer.c
@@ -5,24 +5,28 @@
 
 #include "timer.h"
 
-/* counts one second, off by +1s per 2m
- */
-
-volatile uint16_t count = 0;
-volatile unsigned char hit = 0;
+volatile unsigned char hits, maxhits;
 
 ISR(TIMER1_COMPA_vect) {
-	++hit;
+	++hits;
 }
 
-bool timerHit () {
-	bool ret = hit != 0;
-	hit = 0;
-	return ret;
+/*	Check if timer was hit, return time since last restart or 0 if not hit yet
+ */
+uint32_t timerHit () {
+	if (hits >= maxhits) {
+		const uint32_t ret = TCNT1 * (uint32_t) 128 +
+				(uint32_t) hits * OCR1A * (uint32_t) 128;
+		hits = 0;
+		return ret;
+	} else {
+		return 0;
+	}
 }
 
-void timerStart () {
-	count = 0;
+/*	Start a timer that fires every t us
+ */
+void timerStart (const uint32_t t) {
 	/* reset timer value */
 	TCNT1 = 0;
 	/* set ctc (compare timer and clear) mode (part 1) */
@@ -31,12 +35,24 @@ void timerStart () {
 	TIMSK1 = (1 << OCIE1A);
 	/* set compare value */
 #if F_CPU == 8000000
-	OCR1A = 7812;
+	const uint32_t tdiv = t/128;
+	if (tdiv > UINT16_MAX) {
+#warning "fix this"
+		assert (0);
+	} else {
+		/* with divider 1024 each clock tick is 128us with a rounding error of
+		 * 64us per second/4ms per minute, since its a 16 bit timer we support
+		 * up to 2**16-1 ticks, which is 8388480us (8s) */
+		OCR1A = tdiv;
+		maxhits = 1;
+	}
 #else
 #error "cpu speed not supported"
 #endif
 	/* io clock with 1024 prescaler; ctc (part 2) */
 	TCCR1B = (1 << CS12) | (1 << CS10) | (1 << WGM12);
+
+	hits = 0;
 }
 
 void timerStop () {
diff --git a/timer.h b/timer.h
index c1b3511..6284be7 100644
--- a/timer.h
+++ b/timer.h
@@ -1,10 +1,10 @@
 #ifndef TIMER_H
 #define TIMER_H
 
-#include <stdbool.h>
+#include <stdint.h>
 
-void timerStart ();
-bool timerHit ();
+void timerStart (const uint32_t t);
+uint32_t timerHit ();
 void timerStop ();
 
 #endif /* TIMER_H */
-- 
cgit v1.2.3