From b89a708807c72c67b128aa3d684afdbd694f66bc Mon Sep 17 00:00:00 2001
From: Lars-Dominik Braun <lars@6xq.net>
Date: Mon, 24 Feb 2014 15:32:57 +0100
Subject: Add simple timer that counts one second

---
 Makefile |  2 +-
 main.c   | 36 ++++++++++++------------------------
 timer.c  | 56 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 timer.h  | 11 +++++++++++
 4 files changed, 80 insertions(+), 25 deletions(-)
 create mode 100644 timer.c
 create mode 100644 timer.h

diff --git a/Makefile b/Makefile
index d51eefe..ab206ef 100644
--- a/Makefile
+++ b/Makefile
@@ -2,7 +2,7 @@ MCU = atmega88
 
 all: sanduhr.hex
 
-sanduhr.elf: main.c i2c.c i2c.h uart.c uart.h
+sanduhr.elf: main.c i2c.c i2c.h uart.c uart.h timer.c timer.h
 	avr-gcc -std=gnu99 -mmcu=$(MCU) -Os -o $@ $^
 
 sanduhr.hex: sanduhr.elf
diff --git a/main.c b/main.c
index 6bda270..4a65c84 100644
--- a/main.c
+++ b/main.c
@@ -10,6 +10,7 @@
 
 #include "i2c.h"
 #include "uart.h"
+#include "timer.h"
 
 static void ledInit () {
 	/* set led1,led2 to output */
@@ -31,13 +32,6 @@ static void cpuInit () {
 	CLKPR = 0b00000011;
 }
 
-volatile unsigned char count;
-
-ISR(TIMER0_OVF_vect) {
-	++count;
-	//printf ("timer interrupt hit, count now %u\n", count);
-}
-
 #define sleepwhile(cond) \
 	sleep_enable (); \
 	while (cond) { sleep_cpu (); } \
@@ -61,30 +55,24 @@ int main () {
 	sleepwhile (twr.status == TWST_WAIT);
 	printf ("final twi status was %i\n", twr.status);
 
+	timerStart ();
+	unsigned char seconds = 0;
 	while (1) {
 		uint8_t val[6];
+
+		sleepwhile (!timerHit ());
+		++seconds;
+		printf ("running for %u seconds\n", seconds);
+
 		if (!twReadMulti (LIS302DL, 0x28, val, 6)) {
 			printf ("cannot start read\n");
 		}
 		sleepwhile (twr.status == TWST_WAIT);
-		printf ("%i/%i/%i\n", (int8_t) val[1], (int8_t) val[3], (int8_t) val[5]);
-
-		count = 0;
-		/* set normal mode timer0 */
-		TCCR0A = 0;
-		/* io clock with 1024 prescaler */
-		TCCR0B = (TCCR0B & ~((1 << CS01)) | (1 << CS02) | (1 << CS00));
-		/* enable overflow interrupt */
-		TIMSK0 = (1 << TOIE0);
-
-		sleepwhile (count < 3);
-
-		/* stop timer (zero clock source) */
-		TCCR0B = 0;
-
-		/* XXX: why do we need the delay here? */
-		//_delay_ms (250);
+		printf ("%i/%i/%i\n", (int8_t) val[1], (int8_t) val[3],
+				(int8_t) val[5]);
 	}
+	timerStop ();
+
 	/* global interrupt disable */
 	cli ();
 
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;
+}
+
diff --git a/timer.h b/timer.h
new file mode 100644
index 0000000..c1b3511
--- /dev/null
+++ b/timer.h
@@ -0,0 +1,11 @@
+#ifndef TIMER_H
+#define TIMER_H
+
+#include <stdbool.h>
+
+void timerStart ();
+bool timerHit ();
+void timerStop ();
+
+#endif /* TIMER_H */
+
-- 
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