Merge pull request adafruit#9447 from dhalbert/9.1.x-revert-9343-code

revert code changes from PR#9343

Author: dhalbert

ports/atmel-samd/common-hal/pwmio/PWMOut.c

@@ -249,34 +249,41 @@ extern void common_hal_pwmio_pwmout_set_duty_cycle(pwmio_pwmout_obj_t *self, uin
     // Track it here so that if frequency is changed we can use this value to recalculate the
     // proper duty cycle.
     // See https://github.com/adafruit/circuitpython/issues/2086 for more details
-
     self->duty_cycle = duty;
+
     const pin_timer_t *t = self->timer;
     if (t->is_tc) {
         uint16_t adjusted_duty = tc_periods[t->index] * duty / 0xffff;
-        if (adjusted_duty == 0 && duty != 0) {
-            adjusted_duty = 1; // prevent rounding down to 0
-        }
         #ifdef SAMD21
         tc_insts[t->index]->COUNT16.CC[t->wave_output].reg = adjusted_duty;
         #endif
         #ifdef SAM_D5X_E5X
         Tc *tc = tc_insts[t->index];
+        while (tc->COUNT16.SYNCBUSY.bit.CC1 != 0) {
+        }
         tc->COUNT16.CCBUF[1].reg = adjusted_duty;
         #endif
     } else {
         uint32_t adjusted_duty = ((uint64_t)tcc_periods[t->index]) * duty / 0xffff;
-        if (adjusted_duty == 0 && duty != 0) {
-            adjusted_duty = 1; // prevent rounding down to 0
-        }
         uint8_t channel = tcc_channel(t);
         Tcc *tcc = tcc_insts[t->index];
+
+        // Write into the CC buffer register, which will be transferred to the
+        // CC register on an UPDATE (when period is finished).
+        // Do clock domain syncing as necessary.
+
+        while (tcc->SYNCBUSY.reg != 0) {
+        }
+
+        // Lock out double-buffering while updating the CCB value.
+        tcc->CTRLBSET.bit.LUPD = 1;
         #ifdef SAMD21
         tcc->CCB[channel].reg = adjusted_duty;
         #endif
         #ifdef SAM_D5X_E5X
         tcc->CCBUF[channel].reg = adjusted_duty;
         #endif
+        tcc->CTRLBCLR.bit.LUPD = 1;
     }
 }