diff --git a/src/timer.inc b/src/timer.inc
index c327e5b..db45f4f 100644
--- a/src/timer.inc
+++ b/src/timer.inc
@@ -15,18 +15,31 @@ timer0_set:
 
     ret
 
-; Timer1 will be dynamically changed in main (tempo)
-;timer1_set:
-    ; Load 10011011 to register:
-    ;   CTC1   (Bit 7): Reset timer to 0 when it matches OCR1C
-    ;   COM1A0 (Bit 4): Toggle OC1A (PB1) when it matches OCR1A
-    ;   CS13, CS11, CS10 (Bits 3,1,0): Prescaler 1024 (1011)
-;    ldi     r17,        (1 << CTC1) | (1 << COM1A0) | (1 << CS13) | (1 << CS11) | (1 << CS10)
-;    out     TCCR1,      r16                 ; On T/C Control Register 1, enable: CTC1 COM1A0 CS13 CS11 CS10
+.MACRO SET_BPM target_high, target_low
+    ldi     r20,        \target_high
+    ldi     r19,        \target_low
+.endm
 
-;timer1_use:
-;    ldi     r17,        256                 ; Load counter total  value (ticks)
-;    ldi     r17,        5                   ; Load counter on value (ticks)
-    ; //TODO: change r17 on main and then call timer1_dyn to use the value already on r16 (so its dynamic)
-;    out     OCR1C,      r17                 ; Compare target to current counter (frequency)
-;    out     OCR1A,      r17                 ; Compare target to current counter (phase shift)
+; ISR
+timer1_isr:
+    in      r21,        SREG
+    push    r21
+
+    ldi     r21,        255                         ; Phase increment (quantization)
+    add     r17,        r21                         ; Add low byte to accumulator
+    adc     r18,        r1                          ; Add high byte to accumulator
+
+    cp      r18, r20                                ; Compare HB accumulator vs HB target
+    cpc     r17, r19                                ; Compare LB accumulator vs LB target
+    brlo    .exit                                   ; Exit if accumulator < target
+
+    ;rjump    send_midi (rjump or macro, let's see later...)
+
+    ; To correct any overshoot of acc:
+    sub     r17, r19                                ; Subtract LB target from LB accumulator
+    sbc     r18, r20                                ; Subtract HB target from HB accumulator
+
+.exit:
+    pop     r21
+    out     SREG, r21
+    reti