diff --git a/ports/stm32/machine_bitstream.c b/ports/stm32/machine_bitstream.c
index 6cc367937..3e1a7d6f1 100644
--- a/ports/stm32/machine_bitstream.c
+++ b/ports/stm32/machine_bitstream.c
@@ -146,7 +146,7 @@ void machine_bitstream_high_low(mp_hal_pin_obj_t pin, uint32_t *timing_ns, const
         ".done:\n"
         :
         : "m" (len), "m" (buf), "m" (timing_ns), "m" (bsrr), "m" (high_mask), "m" (low_mask)
-        : "r0", "r1", "r2", "r3", "r7", "r8"
+        : "r0", "r1", "r2", "r3", "r4", "r5", "r6", "r7", "r8"
         );
 
     MICROPY_END_ATOMIC_SECTION(atomic_state);