Merge pull request #4453 from c1728p9/lp_ticker_fix

Safely initialize RTC on kinetis devices

Author: sg-

targets/TARGET_Freescale/TARGET_MCUXpresso_MCUS/api/lp_ticker.c

@@ -33,16 +33,26 @@ static int lptmr_schedule = 0;
 
 static void rtc_isr(void)
 {
-    RTC_DisableInterrupts(RTC, kRTC_AlarmInterruptEnable);
-    RTC->TAR = 0; /* Write clears the IRQ flag */
-
-    /* Wait subsecond remainder if any */
-    if (lptmr_schedule) {
-        LPTMR_SetTimerPeriod(LPTMR0, lptmr_schedule);
-        LPTMR_EnableInterrupts(LPTMR0, kLPTMR_TimerInterruptEnable);
-        LPTMR_StartTimer(LPTMR0);
-    } else {
-        lp_ticker_irq_handler();
+    uint32_t sr = RTC->SR;
+    if (sr & RTC_SR_TOF_MASK) {
+        // Reset RTC to 0 so it keeps counting
+        RTC_StopTimer(RTC);
+        RTC->TSR = 0;
+        RTC_StartTimer(RTC);
+    } else if (sr & RTC_SR_TAF_MASK) {
+        RTC_DisableInterrupts(RTC, kRTC_AlarmInterruptEnable);
+        RTC->TAR = 0; /* Write clears the IRQ flag */
+
+        /* Wait subsecond remainder if any */
+        if (lptmr_schedule) {
+            LPTMR_SetTimerPeriod(LPTMR0, lptmr_schedule);
+            LPTMR_EnableInterrupts(LPTMR0, kLPTMR_TimerInterruptEnable);
+            LPTMR_StartTimer(LPTMR0);
+        } else {
+            lp_ticker_irq_handler();
+        }
+    } else if (sr & RTC_SR_TIF_MASK) {
+        RTC_DisableInterrupts(RTC, kRTC_TimeOverflowInterruptEnable);
     }
 }
 
@@ -73,6 +83,7 @@ void lp_ticker_init(void)
         RTC_StartTimer(RTC);
     }
 
+    RTC->TAR = 0; /* Write clears the IRQ flag */
     NVIC_ClearPendingIRQ(RTC_IRQn);
     NVIC_SetVector(RTC_IRQn, (uint32_t)rtc_isr);
     NVIC_EnableIRQ(RTC_IRQn);