diff --git a/firmware/Makefile b/firmware/Makefile
index 8d77fe0..b415906 100644
--- a/firmware/Makefile
+++ b/firmware/Makefile
@@ -60,7 +60,7 @@ EXTRAINCDIRS =
 #  -Wall...:  warning level
 #  -Wa,...:   tell GCC to pass this to the assembler.
 #    -ahlms:  create assembler listing
-CFLAGS  = -O$(OPT)
+CFLAGS  = -O$(OPT) -g
 CFLAGS += -funsigned-char
 CFLAGS += -funsigned-bitfields
 CFLAGS += -fpack-struct
diff --git a/firmware/main.c b/firmware/main.c
index 8bc5d64..58f4174 100644
--- a/firmware/main.c
+++ b/firmware/main.c
@@ -6,11 +6,54 @@
 
 #include "main.h"
 
+volatile uint8_t sample_pending;
 
 //25kHz
-ISR (TIMER0_OVF_vect)
+ISR(TIMER0_OVF_vect)
 {
+	ICR1 = synth.output;
+	sample_pending = 1;
+}
 
+// sample rate is 8M / (5 * 64) = 25000
+
+enum {
+	synth_channel_count = 2
+};
+
+typedef struct {
+	uint16_t phase;
+	uint16_t speed;
+
+} synth_channel_t;
+
+typedef struct {
+	synth_channel_t channels[synth_channel_count];
+	uint16_t output;
+} synth_t;
+
+static synth_t synth;
+
+static void synth_init(void)
+{
+	// some test values
+	synth.channels[0].phase = 0;
+	synth.channels[0].speed = 1153;
+
+	synth.channels[1].phase = 0;
+	synth.channels[1].speed = 23456;
+}
+
+static inline void synth_mix(void)
+{
+	synth.output = 0;
+
+	for (int i = 0; i < synth_channel_count; i++) {
+		synth_channel_t *chan = &synth.channels[i];
+		chan->phase += chan->speed;
+
+		synth.output += (chan->phase >> 16) & 0xff;
+	}
 }
 
 static void init_sampletimer(void)
@@ -18,27 +61,26 @@ static void init_sampletimer(void)
 	// Timer 0
 	//
 	//set timer0 to CTC & prescaler 64 == 125k
-	TCCR0A = (1<<WGM01);
-	TCCR0B = (1<<CS00)|(1<<CS01);
+	TCCR0A = (1 << WGM01);
+	TCCR0B = (1 << CS00) | (1 << CS01);
 	//count up to 5 :
 	OCR0A = 5;
 
 	//enable interrupt
-	TIMSK0 |= (1<<TOIE0);	
+	TIMSK0 |= (1 << TOIE0);
 }
 
-
 static inline void init_pwm(void)
 {
 	//PB1 set to output:
-	DDRB |= (1<<PORTB2);
-	OCR1B = 0xefff; //preselect some default
-	ICR1  = 0xffff; // TOP-wert
+	DDRB |= (1 << PORTB2);
+	OCR1B = 0xefff;		//preselect some default
+	ICR1 = 0xffff;		// TOP-wert
 
-	TCCR1A = (1<<COM1B1) | (1<<WGM11); // only b-chan , fastpwm (mode 14)
-	TCCR1B = (1<<WGM13)|(1<<WGM12) | (1<<CS10); //Fastpwm, no prescale
+	TCCR1A = (1 << COM1B1) | (1 << WGM11);	// only b-chan , fastpwm (mode 14)
+	TCCR1B = (1 << WGM13) | (1 << WGM12) | (1 << CS10);	//Fastpwm, no prescale
 
-	return ;
+	return;
 }
 
 static void init_leds(void)
@@ -52,8 +94,8 @@ static void init_leds(void)
 inline void setleds(uint8_t state)
 {
 	//set leds according to
-	PORTC |= (state | 0b00001111);
-	PORTC &= ~(state | 0b11110000);
+	PORTC |= (state | 0 b00001111);
+	PORTC &= ~(state | 0 b11110000);
 	return;
 }
 
@@ -66,35 +108,41 @@ static void init_motor(void)
 
 static void stupid_pwmtest(void)
 {
-	uint8_t i,t,r;
-		ICR1 = 0xAA00;
-		r=1;
+	uint8_t i, t, r;
+	ICR1 = 0xAA00;
+	t = r = 1;
 	for (;;) {
 
-		t = (r)?(t+1):(t-1);
+		t = (r) ? (t + 1) : (t - 1);
 
 		ICR1 = (t << 7);
-		if(t=0) r ^= 1;
-		for(i=1;i<100;i++) __asm("nop");
+		if (t == 0)
+			r ^= 1;
+		for (i = 1; i < 100; i++)
+			__asm("nop");
 
-
-	
-		
 	}
-	return; //never
+	return;			//never
 }
 
 int main(void)
 {
 
-	//hardware initialisation: 
+	//hardware initialisation:
 	init_leds();
 	init_motor();
 	init_pwm();
+	sample_pending=0;
+	synth_init();
 
-	//just stupid test for now....
-	stupid_pwmtest();
+	sei();
+
+	while(1){
+		while (0 == sample_pending) ;
+		sample_pending = 0;
+		synth_mix();
+	}
 
 	//never get here
-	return;
+	return 0;
 }