ports/stm: round-to-nearest TIM6 period in audioio

Chris Nourse · claude · Chris Nourse · commit d112d01ed552 · 2026-05-02T01:52:05.000-07:00
Truncating the divisor biased the realised sample rate slightly fast
(e.g. 84 MHz / 44100 = 1904.76 truncated to 1904 yields 44117.6 Hz,
~0.7 cents sharp). Round to nearest so the rate is always the closest
achievable, not the next one above.

Co-Authored-By: Claude Opus 4.7 &lt;noreply@anthropic.com&gt;
diff --git a/ports/stm/common-hal/audioio/AudioOut.c b/ports/stm/common-hal/audioio/AudioOut.c
@@ -443,7 +443,9 @@ void common_hal_audioio_audioout_play(audioio_audioout_obj_t *self,
     __HAL_RCC_TIM6_CLK_ENABLE();
 
     uint32_t tim6_clk = get_tim6_freq();
-    uint32_t period = (tim6_clk / sample_rate);
+    // Round to nearest, not truncate, so the realised sample rate is the
+    // closest TIM6 division to the requested rate.
+    uint32_t period = (tim6_clk + sample_rate / 2) / sample_rate;
     if (period < 2) {
         period = 2;
     }
