diff --git a/src/bpm.inc b/src/bpm.inc
index e89b801..d49b5f7 100644
--- a/src/bpm.inc
+++ b/src/bpm.inc
@@ -2,3 +2,23 @@
     ldi     r20,        \target_high
     ldi     r19,        \target_low
 .endm
+
+ticks_to_bpm:
+    ret
+
+tap_tempo_isr:
+    ; Timer not started yet (first button press after expiration)
+    lds     r16,    timer0_overflow_count
+    cp      r16,    r0
+    breq    start_timer
+
+    ; Timer started
+    ; wait for next press
+    ; better to update at each press, unless expiration
+    rcall   ticks_to_bpm
+
+    reti
+
+start_timer:
+    rcall   timer0_start
+    reti