diff --git a/grbl/stepper.c b/grbl/stepper.c
index 20b1126..33bb51f 100644
--- a/grbl/stepper.c
+++ b/grbl/stepper.c
@@ -949,7 +949,7 @@ void st_prep_buffer()
     float inv_rate = dt/(last_n_steps_remaining - step_dist_remaining); // Compute adjusted step rate inverse
 
     // Compute CPU cycles per step for the prepped segment.
-    uint32_t cycles = ceil( (TICKS_PER_MICROSECOND*1000000*60)*inv_rate ); // (cycles/step)
+    uint32_t cycles = ceil( float(SystemCoreClock)*60*inv_rate ); // (cycles/step)
 
     #ifdef ADAPTIVE_MULTI_AXIS_STEP_SMOOTHING
       // Compute step timing and multi-axis smoothing level.