diff --git a/ports/stm32f4/common-hal/pulseio/PWMOut.c b/ports/stm32f4/common-hal/pulseio/PWMOut.c
index 0a5d0cc857..000325be30 100644
--- a/ports/stm32f4/common-hal/pulseio/PWMOut.c
+++ b/ports/stm32f4/common-hal/pulseio/PWMOut.c
@@ -68,8 +68,9 @@ STATIC uint32_t timer_get_source_freq(uint32_t tim_id) {
 }
 
 STATIC uint32_t timer_get_internal_duty(uint16_t duty, uint32_t period) {
-    //duty cycle is duty/0xFFFF fraction x (number of pulses per period)
-    return (duty*period) / ((1 << 16) - 1);
+    //duty cycle is (0xFFFF - duty)/0xFFFF fraction x (number of pulses per period)
+    //Note that pulses are inverted, so duty cycle is inverted
+    return ((0xFFFF - duty)*period) / ((1 << 16) - 1);
 }
 
 STATIC void timer_get_optimal_divisors(uint32_t*period, uint32_t*prescaler,