From 69cb10f51321a2443a3314c40fb2b8556aa804c0 Mon Sep 17 00:00:00 2001
From: Lars-Dominik Braun <lars@6xq.net>
Date: Tue, 16 Sep 2014 17:22:55 +0200
Subject: timer: Support timeouts > 8s

---
 timer.c | 48 +++++++++++++++++++++++++++++++++++-------------
 1 file changed, 35 insertions(+), 13 deletions(-)

(limited to 'timer.c')

diff --git a/timer.c b/timer.c
index c4e5704..ce30b60 100644
--- a/timer.c
+++ b/timer.c
@@ -5,19 +5,36 @@
 
 #include "timer.h"
 
-volatile unsigned char hits, maxhits;
+/* useconds per clock tick */
+#define US_PER_TICK 128
+/* max useconds with max OCRA1 */
+#define MAX_US ((uint32_t) 8388480)
+
+/* max/current timer hits */
+volatile uint8_t maxhits, hits;
+/* compare value for n-1, last hits */
+uint16_t count, lastcount;
+/* accumulated time since last timeout/call to timerHit (us) */
+uint32_t time;
 
 ISR(TIMER1_COMPA_vect) {
 	++hits;
+	time += OCR1A * US_PER_TICK;
+	if (hits == maxhits-1) {
+		OCR1A = lastcount;
+	}
 }
 
 /*	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;
+		const uint32_t ret = time;
+		/* reset timer, start again */
 		hits = 0;
+		time = 0;
+		OCR1A = count;
+		TCNT1 = 0;
 		return ret;
 	} else {
 		return 0;
@@ -27,6 +44,9 @@ uint32_t timerHit () {
 /*	Start a timer that fires every t us
  */
 void timerStart (const uint32_t t) {
+	hits = 0;
+	time = 0;
+
 	/* reset timer value */
 	TCNT1 = 0;
 	/* set ctc (compare timer and clear) mode (part 1) */
@@ -35,24 +55,26 @@ void timerStart (const uint32_t t) {
 	TIMSK1 = (1 << OCIE1A);
 	/* set compare value */
 #if F_CPU == 8000000
-	const uint32_t tdiv = t/128;
-	if (tdiv > UINT16_MAX) {
-#warning "fix this"
-		assert (0);
+	/* with divider 1024 each clock tick is 128us with a rounding error of 64us
+	 * per second/4ms per minute, since it’s a 16 bit timer we support up to
+	 * 2**16-1 ticks, which is 8388480us (8s) */
+	if (t >= MAX_US) {
+		maxhits = t/MAX_US+1;
+		count = UINT16_MAX;
+		lastcount = (t % MAX_US) / US_PER_TICK;
+		OCR1A = UINT16_MAX;
 	} 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;
+		const uint16_t tdiv = t / US_PER_TICK;
+		count = tdiv;
+		lastcount = tdiv;
+		OCR1A = tdiv;
 	}
 #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 () {
-- 
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