esp8266/esppwm: Fix PWM glitch when setting duty on different channel.

The PWM driver uses a double buffer for the PWM timing array, one in
current use and the other one to update when changing duty parameters.
The issue was that once the duty parameters were changed the updated buffer
was applied immediately without synchronising to the start of the PWM
period.  By moving the buffer toggling/swapping to the interrupt when the
cycle is done there are no more glitches.

ports/esp8266/esppwm.c

@@ -57,6 +57,7 @@ STATIC uint8_t pwm_timer_down = 1;
 STATIC uint8_t pwm_current_channel = 0;
 STATIC uint16_t pwm_gpio = 0;
 STATIC uint8_t pwm_channel_num = 0;
+STATIC volatile uint8_t pwm_toggle_request = 0;
 
 // XXX: 0xffffffff/(80000000/16)=35A
 #define US_TO_RTC_TIMER_TICKS(t)          \
@@ -126,6 +127,9 @@ pwm_start(void) {
 
     LOCK_PWM(critical);   // enter critical
 
+    // if a toggle is pending, we reset it since we're changing the settings again
+    pwm_toggle_request = 0;
+
     struct pwm_single_param *local_single = pwm_single_toggle[pwm_toggle ^ 0x01];
     uint8 *local_channel = &pwm_channel_toggle[pwm_toggle ^ 0x01];
 
@@ -188,14 +192,14 @@ pwm_start(void) {
         // start
         gpio_output_set(local_single[0].gpio_set, local_single[0].gpio_clear, pwm_gpio, 0);
 
+        // do the first toggle because timer has to have a valid set to do it's job
+        pwm_toggle ^= 0x01;
+
         pwm_timer_down = 0;
         RTC_REG_WRITE(FRC1_LOAD_ADDRESS, local_single[0].h_time);
-    }
-
-    if (pwm_toggle == 1) {
-        pwm_toggle = 0;
     } else {
-        pwm_toggle = 1;
+        // request pwm_tim1_intr_handler to swap the timing buffers
+        pwm_toggle_request = 1;
     }
 
     UNLOCK_PWM(critical);   // leave critical
@@ -297,12 +301,18 @@ pwm_get_freq(uint8 channel) {
 STATIC void ICACHE_RAM_ATTR
 pwm_tim1_intr_handler(void *dummy) {
     (void)dummy;
-    uint8 local_toggle = pwm_toggle;                        // pwm_toggle may change outside
+
     RTC_CLR_REG_MASK(FRC1_INT_ADDRESS, FRC1_INT_CLR_MASK);
 
     if (pwm_current_channel >= (*pwm_channel - 1)) {        // *pwm_channel may change outside
-        pwm_single = pwm_single_toggle[local_toggle];
-        pwm_channel = &pwm_channel_toggle[local_toggle];
+
+        if (pwm_toggle_request != 0) {
+            pwm_toggle ^= 1;
+            pwm_toggle_request = 0;
+        }
+
+        pwm_single = pwm_single_toggle[pwm_toggle];
+        pwm_channel = &pwm_channel_toggle[pwm_toggle];
 
         gpio_output_set(pwm_single[*pwm_channel - 1].gpio_set,
             pwm_single[*pwm_channel - 1].gpio_clear,