Fix autoreload on SAMD21

The issue was that a time.sleep() would set the RTC wake up
further into the future even if we wanted to tick every ms. Ticking
every ms is used to time the autoreload delay and without it,
autoreload doesn't work.

Fixes adafruit#3528

Author: tannewt

ports/atmel-samd/supervisor/port.c

@@ -430,6 +430,33 @@ uint32_t port_get_saved_word(void) {
 static volatile uint64_t overflowed_ticks = 0;
 static volatile bool _ticks_enabled = false;
 
+static uint32_t _get_count(void) {
+    #ifdef SAM_D5X_E5X
+    while ((RTC->MODE0.SYNCBUSY.reg & (RTC_MODE0_SYNCBUSY_COUNTSYNC | RTC_MODE0_SYNCBUSY_COUNT)) != 0) {}
+    #endif
+    #ifdef SAMD21
+    while (RTC->MODE0.STATUS.bit.SYNCBUSY != 0) {}
+    #endif
+
+    return RTC->MODE0.COUNT.reg;
+}
+
+static void _port_interrupt_after_ticks(uint32_t ticks) {
+    uint32_t current_ticks = _get_count();
+    if (ticks > 1 << 28) {
+        // We'll interrupt sooner with an overflow.
+        return;
+    }
+#ifdef SAMD21
+    if (hold_interrupt) {
+        return;
+    }
+#endif
+    RTC->MODE0.COMP[0].reg = current_ticks + (ticks << 4);
+    RTC->MODE0.INTFLAG.reg = RTC_MODE0_INTFLAG_CMP0;
+    RTC->MODE0.INTENSET.reg = RTC_MODE0_INTENSET_CMP0;
+}
+
 void RTC_Handler(void) {
     uint32_t intflag = RTC->MODE0.INTFLAG.reg;
     if (intflag & RTC_MODE0_INTFLAG_OVF) {
@@ -452,7 +479,7 @@ void RTC_Handler(void) {
             supervisor_tick();
             // Check _ticks_enabled again because a tick handler may have turned it off.
             if (_ticks_enabled) {
-                port_interrupt_after_ticks(1);
+                _port_interrupt_after_ticks(1);
             }
         }
         #endif
@@ -462,17 +489,6 @@ void RTC_Handler(void) {
     }
 }
 
-static uint32_t _get_count(void) {
-    #ifdef SAM_D5X_E5X
-    while ((RTC->MODE0.SYNCBUSY.reg & (RTC_MODE0_SYNCBUSY_COUNTSYNC | RTC_MODE0_SYNCBUSY_COUNT)) != 0) {}
-    #endif
-    #ifdef SAMD21
-    while (RTC->MODE0.STATUS.bit.SYNCBUSY != 0) {}
-    #endif
-
-    return RTC->MODE0.COUNT.reg;
-}
-
 uint64_t port_get_raw_ticks(uint8_t* subticks) {
     uint32_t current_ticks = _get_count();
     if (subticks != NULL) {
@@ -490,7 +506,7 @@ void port_enable_tick(void) {
     #endif
     #ifdef SAMD21
     _ticks_enabled = true;
-    port_interrupt_after_ticks(1);
+    _port_interrupt_after_ticks(1);
     #endif
 }
 
@@ -505,20 +521,14 @@ void port_disable_tick(void) {
     #endif
 }
 
+// This is called by sleep, we ignore it when our ticks are enabled because
+// they'll wake us up earlier. If we don't, we'll mess up ticks by overwriting
+// the next RTC wake up time.
 void port_interrupt_after_ticks(uint32_t ticks) {
-    uint32_t current_ticks = _get_count();
-    if (ticks > 1 << 28) {
-        // We'll interrupt sooner with an overflow.
-        return;
-    }
-#ifdef SAMD21
-    if (hold_interrupt) {
+    if (_ticks_enabled) {
         return;
     }
-#endif
-    RTC->MODE0.COMP[0].reg = current_ticks + (ticks << 4);
-    RTC->MODE0.INTFLAG.reg = RTC_MODE0_INTFLAG_CMP0;
-    RTC->MODE0.INTENSET.reg = RTC_MODE0_INTENSET_CMP0;
+    _port_interrupt_after_ticks(ticks);
 }
 
 void port_sleep_until_interrupt(void) {