implemented timeslot queue to fix #2 but sound is still a bit strange

firmware/main.c

@@ -111,6 +111,7 @@ int main(void)
 	for(;;) /* ever */  {
 
 		//do something
+		synth_poll();
 		if (10 == 1){
 			uint16_t out = darkval;
 			USART0_put_uint16(out);
@@ -125,11 +126,6 @@ int main(void)
 }
 
 
-ISR(TIMER0_COMPA_vect)
-{
-	/* calculate next analog sample value in synth mixer:*/
-	OCR1B = synth_mix();
-}
 
 ISR(TIMER2_COMPA_vect)
 {

firmware/synth.c

@@ -2,6 +2,7 @@
 #include "synth.h"
 #include "freq_table.h"
 #include <avr/pgmspace.h>
+#include <avr/interrupt.h>
 
 // sample rate is 8M / (3 * 64)
 
@@ -177,12 +178,27 @@ static int8_t row;
 static int8_t seq;
 
 
+/* PROTOTYPES */
+uint16_t synth_mix(void);
+
+static uint16_t timeslots[SYNTH_BUFSIZE];
+static uint8_t timeslots_write; // current write head 
+static uint8_t timeslots_read; // current read head 
+
+void enqueue_timeslot(uint16_t synthval);
+uint16_t dequeue_timeslot(void);
+uint8_t timeslots_fill(void);
+
+
 void synth_init(void)
 {
 	sample = 0;
 	tick = 0;
 	row = 0;
 	seq = 0;
+	//prefill timeslot buffer
+	enqueue_timeslot(synth_mix());
+	enqueue_timeslot(synth_mix());
 }
 
 uint16_t synth_mix(void)
@@ -270,3 +286,41 @@ uint16_t synth_mix(void)
 	return output;
 }
 
+
+/* fill all the timeslots */
+void synth_poll(void) {
+	/* refill timeslots queue */
+	while (timeslots_fill() < (SYNTH_BUFSIZE-1))
+		enqueue_timeslot(synth_mix());
+}
+
+/* timeslot queue handling */
+void enqueue_timeslot(uint16_t synthval) {
+	timeslots[timeslots_write] = synthval;
+	timeslots_write++;
+	if (timeslots_write >= SYNTH_BUFSIZE)
+		timeslots_write = 0;
+}
+
+uint16_t dequeue_timeslot() {
+	uint16_t t = timeslots[timeslots_read];
+	timeslots_read++;
+	if (timeslots_read >= SYNTH_BUFSIZE) timeslots_read =0;
+	return t;
+}
+
+uint8_t timeslots_fill() {
+	if (timeslots_write >= timeslots_read)
+		return timeslots_write - timeslots_read;
+	else
+		return SYNTH_BUFSIZE - (timeslots_read - timeslots_write);
+}
+
+
+ISR(TIMER0_COMPA_vect)
+{
+	/* calculate next analog sample value in synth mixer:*/
+	OCR1B = dequeue_timeslot();
+}
+
+

firmware/synth.h

@@ -1,3 +1,8 @@
+#ifndef SYNTH_H
+#define SYNTH_H
+
+#define SYNTH_BUFSIZE 2
+
 enum {  WAVE_OFF, WAVE_PULSE, WAVE_SAW, WAVE_NOISE };
 
 typedef struct {
@@ -23,5 +28,6 @@ typedef struct {
 
 
 void synth_init(void);
-uint16_t synth_mix(void);
+void synth_poll(void);
 
+#endif