diff --git a/firmware/apps/crazy_synth.c b/firmware/apps/crazy_synth.c
index af132c4..cd79bce 100644
--- a/firmware/apps/crazy_synth.c
+++ b/firmware/apps/crazy_synth.c
@@ -8,8 +8,10 @@
 #include "../lib/freq_table.h"
 #include <lib/bughal.h> 
 
-#include "../lib/apps.h"
+#include <lib/timer.h>
+#include <lib/apps.h>
 
+void tetris_play(void);
 
 static uint16_t	osc0;
 static uint16_t	osc1;
@@ -21,29 +23,17 @@ static uint16_t	speedtime;
 void synth_init(void){
 	init_buzzr();
 
-	cli();
 	osc0 = osc1 = sample = row = 0;
 	speedtime = 3000;
 
-	/* set timer0 to CTC & prescaler 64 → 125kHz increment */
-	TCCR0A = (1 << WGM01);
-	TCCR0B = (1 << CS00) | (1 << CS01);
+	start_timer(tetris_play);
 
-	OCR0A = 4;  /* TOP */
-	TCNT0 = 0;
-
-	/*enable interrupt */
-	TIMSK0 |= (1 << OCIE0A);
-
-	sei();
 	return;
 }
 
 
 void synth_deinit(void) {
-	cli();
-	TIMSK0 = 0;
-	sei();
+	stop_timer();
 	return;
 }
 
@@ -101,8 +91,7 @@ const char music_data[2][SONG_LENGTH] PROGMEM = {
 
 
 
-ISR(TIMER0_COMPA_vect,ISR_NOBLOCK)
-{
+void tetris_play(void) {
 	osc0 += pgm_read_word(&freq_table[ pgm_read_byte(&music_data[0][row])]);
 	osc1 += pgm_read_word(&freq_table[ pgm_read_byte(&music_data[1][row])]);
 	if (++sample == speedtime ) {
diff --git a/firmware/lib/timer.c b/firmware/lib/timer.c
new file mode 100644
index 0000000..973b8b5
--- /dev/null
+++ b/firmware/lib/timer.c
@@ -0,0 +1,40 @@
+#include <avr/io.h>
+#include <avr/interrupt.h>
+#include <stdlib.h>
+
+#include <lib/timer.h>
+
+timer_cb cur_cb;
+
+void start_timer(timer_cb cb) {
+	cur_cb = cb;
+
+	cli();
+
+	/* set timer0 to CTC & prescaler 64 → 125kHz increment */
+	TCCR0A = (1 << WGM01);
+	TCCR0B = (1 << CS00) | (1 << CS01);
+
+	OCR0A = 4;  /* TOP */
+	TCNT0 = 0;
+
+	/*enable interrupt */
+	TIMSK0 |= (1 << OCIE0A);
+
+	sei();
+}
+
+void stop_timer(void) {
+	cli();
+	TIMSK0 = 0;
+	sei();
+
+	cur_cb = NULL;
+}
+
+ISR(TIMER0_COMPA_vect,ISR_NOBLOCK) {
+	if(cur_cb != NULL) {
+		cur_cb();
+	}
+}
+
diff --git a/firmware/lib/timer.h b/firmware/lib/timer.h
new file mode 100644
index 0000000..900cc84
--- /dev/null
+++ b/firmware/lib/timer.h
@@ -0,0 +1,10 @@
+#ifndef TIMER_H
+#define TIMER_H 
+
+typedef void (*timer_cb)(void);
+
+void start_timer(timer_cb cb);
+
+void stop_timer(void);
+
+#endif /* TIMER_H */