diff --git a/compiler/obc/c/async_avr.ml b/compiler/obc/c/async_avr.ml
index eac9c4e..8044e15 100644
--- a/compiler/obc/c/async_avr.ml
+++ b/compiler/obc/c/async_avr.ml
@@ -52,10 +52,23 @@ struct
 
   let fold_lcm l = List.fold_left lcm (List.hd l) (List.tl l)
 
+  let count_of_ack base = function
+    | Timer_ms ms -> ms / base
+
+  let incr_mod name modulo =
+    let one_const = Cconst (Ccint 1) in
+    let modulo_const = Cconst (Ccint modulo) in
+    let incr = Cbop ("+", Cvar name, one_const) in
+    Caffect (CLvar name, Cbop ("%", incr, modulo_const))
+
   let call_step_async base tick_var (od, ack) =
     let step = (cname_of_qn od.o_class) ^ "_async_step" in
     let global = async_global_var_name od in
-    Csexpr (Cfun_call (step, [Caddrof (Cvar global)]))
+    let call = Csexpr (Cfun_call (step, [Caddrof (Cvar global)])) in
+    let zero = Cconst (Ccint 0) in
+    let timer = Cconst (Ccint (count_of_ack base ack)) in
+    let cond = Cbop ("==", Cbop ("%", tick_var, timer), zero) in
+    Cif (cond, [call], [])
 
   let decls_and_defs objs =
     let trans = List.map
@@ -68,13 +81,16 @@ struct
       (fun (_, ack) -> match ack with Timer_ms ms -> ms)
       trans
     in
-    let tick = "mod_ticks" in
     let body = match timers with
       | [] -> []
       | _ ->
          let base_timer = fold_gcd timers in
          let max_timer = fold_lcm timers in
-         List.map (call_step_async base_timer (Cvar tick)) trans
+         let steps =
+           List.map (call_step_async base_timer (Cvar "tick")) trans
+         in
+         let incr = incr_mod "tick" (max_timer / base_timer) in
+         steps @ [incr]
     in
     (* run_timers is declared in avr.h (because of the ISR macro which
      * I don't know how to generate here) *)
@@ -85,7 +101,7 @@ struct
         f_args = [];
         f_body = {
           var_decls = [
-            mk_vardecl_val ~static:true tick Cty_int (Cconst (Ccint 0))
+            mk_vardecl_val ~static:true "tick" Cty_int (Cconst (Ccint 0))
           ];
           block_body = body
         }