Added ReadReq() which ensures the CLOCK register synchronised.

Reading the CLOCK elements when it is not synchronised can cause
the bus to stall. This can block interrupts. This code currently increases
the cost of calling getEpoch() from 6ms to about 35ms.

Todo: Change getEpoch() to single read from the CLOCK register. This
will reduce the cost back to ~6ms.

Author: GabrielNotman

src/RTCZero.cpp

@@ -121,31 +121,37 @@ void RTCZero::standbyMode()
 
 uint8_t RTCZero::getSeconds()
 {
+  RTCreadRequest();
   return RTC->MODE2.CLOCK.bit.SECOND;
 }
 
 uint8_t RTCZero::getMinutes()
 {
+  RTCreadRequest();
   return RTC->MODE2.CLOCK.bit.MINUTE;
 }
 
 uint8_t RTCZero::getHours()
 {
+  RTCreadRequest();
   return RTC->MODE2.CLOCK.bit.HOUR;
 }
 
 uint8_t RTCZero::getDay()
 {
+  RTCreadRequest();
   return RTC->MODE2.CLOCK.bit.DAY;
 }
 
 uint8_t RTCZero::getMonth()
 {
+  RTCreadRequest();
   return RTC->MODE2.CLOCK.bit.MONTH;
 }
 
 uint8_t RTCZero::getYear()
 {
+  RTCreadRequest();
   return RTC->MODE2.CLOCK.bit.YEAR;
 }
 
@@ -357,6 +363,13 @@ void RTCZero::config32kOSC()
                          SYSCTRL_XOSC32K_ENABLE;
 }
 
+/* Synchronise the CLOCK register for reading*/
+inline void RTCZero::RTCreadRequest() {
+  RTC->MODE2.READREQ.reg |= RTC_READREQ_RREQ;
+  while (RTCisSyncing())
+    ;
+}
+
 /* Wait for sync in write operations */
 inline bool RTCZero::RTCisSyncing()
 {

src/RTCZero.h

@@ -100,6 +100,7 @@ class RTCZero {
 
 private:
   void config32kOSC(void);
+  void RTCreadRequest();
   bool RTCisSyncing(void);
   void RTCdisable();
   void RTCenable();