diff --git a/README.adoc b/README.adoc
index c7bc164..f7dd6f5 100644
--- a/README.adoc
+++ b/README.adoc
@@ -1,6 +1,9 @@
 = RTC Library for Arduino =
 
-The RTC library enables an Arduino Zero or MKR1000 board to take control of the internal RTC.
+The RTC library enables a supported board to take control of the internal RTC.
+Supported boards are:
+SAMD21-based boards (Arduino Zero, MKR1000, Adafruit Feather M0, Sparkfun SAMD21 Mini Breakout, etc),
+SAMD51-based boards (Adafruit Grand Central M4, SparkFun SAMD51 Thing Plus, etc).
 
 For more information about this library please visit us at
 http://arduino.cc/en/Reference/RTC
diff --git a/library.properties b/library.properties
index df69458..8b37c6c 100644
--- a/library.properties
+++ b/library.properties
@@ -2,8 +2,8 @@ name=RTCZero
 version=1.6.0
 author=Arduino
 maintainer=Arduino <info@arduino.cc>
-sentence=Allows to use the RTC functionalities. For Arduino Zero, MKRZero and MKR1000 only.
-paragraph=With this library you can use the RTC peripheral of an Arduino Zero or MKR1000 in order to program actions related to date and time.
+sentence=Allows to use the RTC functionalities. For Arduino Zero, MKRZero, MKR1000 (SAMD21), and SAMD51 boards.
+paragraph=With this library you can use the RTC peripheral of a supported board in order to program actions related to date and time. Supported boards are: SAMD21-based boards (Arduino Zero, MKR1000, Adafruit Feather M0, Sparkfun SAMD21 Mini Breakout, etc), SAMD51-based boards (Adafruit Grand Central M4, SparkFun SAMD51 Thing Plus, etc).
 category=Timing
 url=http://www.arduino.cc/en/Reference/RTC
 architectures=samd
diff --git a/src/RTCZero.cpp b/src/RTCZero.cpp
index a7c334c..166dad9 100644
--- a/src/RTCZero.cpp
+++ b/src/RTCZero.cpp
@@ -41,47 +41,37 @@ RTCZero::RTCZero()
 
 void RTCZero::begin(bool resetTime)
 {
-  uint16_t tmp_reg = 0;
-  
+#ifdef __SAMD51__
+  MCLK->APBAMASK.reg |= MCLK_APBAMASK_RTC;
+#else // SAMD21
   PM->APBAMASK.reg |= PM_APBAMASK_RTC; // turn on digital interface clock
+#endif
+
   config32kOSC();
 
   // If the RTC is in clock mode and the reset was
   // not due to POR or BOD, preserve the clock time
   // POR causes a reset anyway, BOD behaviour is?
   bool validTime = false;
-  RTC_MODE2_CLOCK_Type oldTime;
+  RTC_MODE2_CLOCK_Type oldTime; // same for SAMD21 and SAMD51
 
-  if ((!resetTime) && (PM->RCAUSE.reg & (PM_RCAUSE_SYST | PM_RCAUSE_WDT | PM_RCAUSE_EXT))) {
-    if (RTC->MODE2.CTRL.reg & RTC_MODE2_CTRL_MODE_CLOCK) {
-      validTime = true;
-      oldTime.reg = RTC->MODE2.CLOCK.reg;
-    }
+  if ((!resetTime) && isHotReset() && RTCisClock()) {
+    validTime = true;
+    oldTime.reg = RTC->MODE2.CLOCK.reg;
   }
 
-  // Setup clock GCLK2 with OSC32K divided by 32
   configureClock();
 
   RTCdisable();
 
   RTCreset();
 
-  tmp_reg |= RTC_MODE2_CTRL_MODE_CLOCK; // set clock operating mode
-  tmp_reg |= RTC_MODE2_CTRL_PRESCALER_DIV1024; // set prescaler to 1024 for MODE2
-  tmp_reg &= ~RTC_MODE2_CTRL_MATCHCLR; // disable clear on match
-  
-  //According to the datasheet RTC_MODE2_CTRL_CLKREP = 0 for 24h
-  tmp_reg &= ~RTC_MODE2_CTRL_CLKREP; // 24h time representation
-
-  RTC->MODE2.READREQ.reg &= ~RTC_READREQ_RCONT; // disable continuously mode
-
-  RTC->MODE2.CTRL.reg = tmp_reg;
-  while (RTCisSyncing())
-    ;
+  RTCconfigureRegs();
 
   NVIC_EnableIRQ(RTC_IRQn); // enable RTC interrupt 
   NVIC_SetPriority(RTC_IRQn, 0x00);
 
+  // same for SAMD21 and SAMD51
   RTC->MODE2.INTENSET.reg |= RTC_MODE2_INTENSET_ALARM0; // enable alarm interrupt
   RTC->MODE2.Mode2Alarm[0].MASK.bit.SEL = MATCH_OFF; // default alarm match is off (disabled)
   
@@ -93,9 +83,10 @@ void RTCZero::begin(bool resetTime)
 
   // If desired and valid, restore the time value, else use first valid time value
   if ((!resetTime) && (validTime) && (oldTime.reg != 0L)) {
-    RTC->MODE2.CLOCK.reg = oldTime.reg;
+    RTC->MODE2.CLOCK.reg = oldTime.reg; // same for SAMD21 and SAMD51
   }
   else {
+    // same for SAMD21 and SAMD51
     RTC->MODE2.CLOCK.reg = RTC_MODE2_CLOCK_YEAR(DEFAULT_YEAR - 2000) | RTC_MODE2_CLOCK_MONTH(DEFAULT_MONTH) 
         | RTC_MODE2_CLOCK_DAY(DEFAULT_DAY) | RTC_MODE2_CLOCK_HOUR(DEFAULT_HOUR) 
         | RTC_MODE2_CLOCK_MINUTE(DEFAULT_MINUTE) | RTC_MODE2_CLOCK_SECOND(DEFAULT_SECOND);
@@ -112,13 +103,14 @@ void RTC_Handler(void)
     RTC_callBack();
   }
 
+  // same for SAMD21 and SAMD51
   RTC->MODE2.INTFLAG.reg = RTC_MODE2_INTFLAG_ALARM0; // must clear flag at end
 }
 
 void RTCZero::enableAlarm(Alarm_Match match)
 {
   if (_configured) {
-    RTC->MODE2.Mode2Alarm[0].MASK.bit.SEL = match;
+    RTC->MODE2.Mode2Alarm[0].MASK.bit.SEL = match; // same for SAMD21 and SAMD51
     while (RTCisSyncing())
       ;
   }
@@ -127,7 +119,7 @@ void RTCZero::enableAlarm(Alarm_Match match)
 void RTCZero::disableAlarm()
 {
   if (_configured) {
-    RTC->MODE2.Mode2Alarm[0].MASK.bit.SEL = 0x00;
+    RTC->MODE2.Mode2Alarm[0].MASK.bit.SEL = 0x00; // same for SAMD21 and SAMD51
     while (RTCisSyncing())
       ;
   }
@@ -147,6 +139,7 @@ void RTCZero::standbyMode()
 {
   // Entering standby mode when connected
   // via the native USB port causes issues.
+  // same for SAMD21 and SAMD51
   SCB->SCR |= SCB_SCR_SLEEPDEEP_Msk;
   __DSB();
   __WFI();
@@ -159,67 +152,67 @@ void RTCZero::standbyMode()
 uint8_t RTCZero::getSeconds()
 {
   RTCreadRequest();
-  return RTC->MODE2.CLOCK.bit.SECOND;
+  return RTC->MODE2.CLOCK.bit.SECOND; // same for SAMD21 and SAMD51
 }
 
 uint8_t RTCZero::getMinutes()
 {
   RTCreadRequest();
-  return RTC->MODE2.CLOCK.bit.MINUTE;
+  return RTC->MODE2.CLOCK.bit.MINUTE; // same for SAMD21 and SAMD51
 }
 
 uint8_t RTCZero::getHours()
 {
   RTCreadRequest();
-  return RTC->MODE2.CLOCK.bit.HOUR;
+  return RTC->MODE2.CLOCK.bit.HOUR; // same for SAMD21 and SAMD51
 }
 
 uint8_t RTCZero::getDay()
 {
   RTCreadRequest();
-  return RTC->MODE2.CLOCK.bit.DAY;
+  return RTC->MODE2.CLOCK.bit.DAY; // same for SAMD21 and SAMD51
 }
 
 uint8_t RTCZero::getMonth()
 {
   RTCreadRequest();
-  return RTC->MODE2.CLOCK.bit.MONTH;
+  return RTC->MODE2.CLOCK.bit.MONTH; // same for SAMD21 and SAMD51
 }
 
 uint8_t RTCZero::getYear()
 {
   RTCreadRequest();
-  return RTC->MODE2.CLOCK.bit.YEAR;
+  return RTC->MODE2.CLOCK.bit.YEAR; // same for SAMD21 and SAMD51
 }
 
 uint8_t RTCZero::getAlarmSeconds()
 {
-  return RTC->MODE2.Mode2Alarm[0].ALARM.bit.SECOND;
+  return RTC->MODE2.Mode2Alarm[0].ALARM.bit.SECOND; // same for SAMD21 and SAMD51
 }
 
 uint8_t RTCZero::getAlarmMinutes()
 {
-  return RTC->MODE2.Mode2Alarm[0].ALARM.bit.MINUTE;
+  return RTC->MODE2.Mode2Alarm[0].ALARM.bit.MINUTE; // same for SAMD21 and SAMD51
 }
 
 uint8_t RTCZero::getAlarmHours()
 {
-  return RTC->MODE2.Mode2Alarm[0].ALARM.bit.HOUR;
+  return RTC->MODE2.Mode2Alarm[0].ALARM.bit.HOUR; // same for SAMD21 and SAMD51
 }
 
 uint8_t RTCZero::getAlarmDay()
 {
-  return RTC->MODE2.Mode2Alarm[0].ALARM.bit.DAY;
+  return RTC->MODE2.Mode2Alarm[0].ALARM.bit.DAY; // same for SAMD21 and SAMD51
 }
 
 uint8_t RTCZero::getAlarmMonth()
 {
-  return RTC->MODE2.Mode2Alarm[0].ALARM.bit.MONTH;
+  return RTC->MODE2.Mode2Alarm[0].ALARM.bit.MONTH; // same for SAMD21 and SAMD51
 }
 
 uint8_t RTCZero::getAlarmYear()
 {
-  return RTC->MODE2.Mode2Alarm[0].ALARM.bit.YEAR;
+  return RTC->MODE2.Mode2Alarm[0].ALARM.bit.YEAR; // same for SAMD21 and SAMD51
 }
 
 /*
@@ -229,7 +222,7 @@ uint8_t RTCZero::getAlarmYear()
 void RTCZero::setSeconds(uint8_t seconds)
 {
   if (_configured) {
-    RTC->MODE2.CLOCK.bit.SECOND = seconds;
+    RTC->MODE2.CLOCK.bit.SECOND = seconds; // same for SAMD21 and SAMD51
     while (RTCisSyncing())
       ;
   }
@@ -238,7 +231,7 @@ void RTCZero::setSeconds(uint8_t seconds)
 void RTCZero::setMinutes(uint8_t minutes)
 {
   if (_configured) {
-    RTC->MODE2.CLOCK.bit.MINUTE = minutes;
+    RTC->MODE2.CLOCK.bit.MINUTE = minutes; // same for SAMD21 and SAMD51
     while (RTCisSyncing())
       ;
   }
@@ -247,7 +240,7 @@ void RTCZero::setMinutes(uint8_t minutes)
 void RTCZero::setHours(uint8_t hours)
 {
   if (_configured) {
-    RTC->MODE2.CLOCK.bit.HOUR = hours;
+    RTC->MODE2.CLOCK.bit.HOUR = hours; // same for SAMD21 and SAMD51
     while (RTCisSyncing())
       ;
   }
@@ -265,7 +258,7 @@ void RTCZero::setTime(uint8_t hours, uint8_t minutes, uint8_t seconds)
 void RTCZero::setDay(uint8_t day)
 {
   if (_configured) {
-    RTC->MODE2.CLOCK.bit.DAY = day;
+    RTC->MODE2.CLOCK.bit.DAY = day; // same for SAMD21 and SAMD51
     while (RTCisSyncing())
       ;
   }
@@ -274,7 +267,7 @@ void RTCZero::setDay(uint8_t day)
 void RTCZero::setMonth(uint8_t month)
 {
   if (_configured) {
-    RTC->MODE2.CLOCK.bit.MONTH = month;
+    RTC->MODE2.CLOCK.bit.MONTH = month; // same for SAMD21 and SAMD51
     while (RTCisSyncing())
       ;
   }
@@ -283,7 +276,7 @@ void RTCZero::setMonth(uint8_t month)
 void RTCZero::setYear(uint8_t year)
 {
   if (_configured) {
-    RTC->MODE2.CLOCK.bit.YEAR = year;
+    RTC->MODE2.CLOCK.bit.YEAR = year; // same for SAMD21 and SAMD51
     while (RTCisSyncing())
       ;
   }
@@ -301,7 +294,7 @@ void RTCZero::setDate(uint8_t day, uint8_t month, uint8_t year)
 void RTCZero::setAlarmSeconds(uint8_t seconds)
 {
   if (_configured) {
-    RTC->MODE2.Mode2Alarm[0].ALARM.bit.SECOND = seconds;
+    RTC->MODE2.Mode2Alarm[0].ALARM.bit.SECOND = seconds; // same for SAMD21 and SAMD51
     while (RTCisSyncing())
       ;
   }
@@ -310,7 +303,7 @@ void RTCZero::setAlarmSeconds(uint8_t seconds)
 void RTCZero::setAlarmMinutes(uint8_t minutes)
 {
   if (_configured) {
-    RTC->MODE2.Mode2Alarm[0].ALARM.bit.MINUTE = minutes;
+    RTC->MODE2.Mode2Alarm[0].ALARM.bit.MINUTE = minutes; // same for SAMD21 and SAMD51
     while (RTCisSyncing())
       ;
   }
@@ -319,7 +312,7 @@ void RTCZero::setAlarmMinutes(uint8_t minutes)
 void RTCZero::setAlarmHours(uint8_t hours)
 {
   if (_configured) {
-    RTC->MODE2.Mode2Alarm[0].ALARM.bit.HOUR = hours;
+    RTC->MODE2.Mode2Alarm[0].ALARM.bit.HOUR = hours; // same for SAMD21 and SAMD51
     while (RTCisSyncing())
       ;
   }
@@ -337,7 +330,7 @@ void RTCZero::setAlarmTime(uint8_t hours, uint8_t minutes, uint8_t seconds)
 void RTCZero::setAlarmDay(uint8_t day)
 {
   if (_configured) {
-    RTC->MODE2.Mode2Alarm[0].ALARM.bit.DAY = day;
+    RTC->MODE2.Mode2Alarm[0].ALARM.bit.DAY = day; // same for SAMD21 and SAMD51
     while (RTCisSyncing())
       ;
   }
@@ -346,7 +339,7 @@ void RTCZero::setAlarmDay(uint8_t day)
 void RTCZero::setAlarmMonth(uint8_t month)
 {
   if (_configured) {
-    RTC->MODE2.Mode2Alarm[0].ALARM.bit.MONTH = month;
+    RTC->MODE2.Mode2Alarm[0].ALARM.bit.MONTH = month; // same for SAMD21 and SAMD51
     while (RTCisSyncing())
       ;
   }
@@ -355,7 +348,7 @@ void RTCZero::setAlarmMonth(uint8_t month)
 void RTCZero::setAlarmYear(uint8_t year)
 {
   if (_configured) {
-    RTC->MODE2.Mode2Alarm[0].ALARM.bit.YEAR = year;
+    RTC->MODE2.Mode2Alarm[0].ALARM.bit.YEAR = year; // same for SAMD21 and SAMD51
     while (RTCisSyncing())
       ;
   }
@@ -374,7 +367,7 @@ time_t RTCZero::getEpoch()
 {
   RTCreadRequest();
   RTC_MODE2_CLOCK_Type clockTime;
-  clockTime.reg = RTC->MODE2.CLOCK.reg;
+  clockTime.reg = RTC->MODE2.CLOCK.reg; // same for SAMD21 and SAMD51
 
   struct tm tm;
 
@@ -424,7 +417,7 @@ void RTCZero::setEpoch(uint32_t ts)
 
     gmtime_r(&t, &tmp);
 
-    RTC_MODE2_CLOCK_Type clockTime;
+    RTC_MODE2_CLOCK_Type clockTime; // same for SAMD21 and SAMD51
 
     clockTime.bit.YEAR = tmp.tm_year - EPOCH_TIME_YEAR_OFF;
     clockTime.bit.MONTH = tmp.tm_mon + 1;
@@ -433,7 +426,7 @@ void RTCZero::setEpoch(uint32_t ts)
     clockTime.bit.MINUTE = tmp.tm_min;
     clockTime.bit.SECOND = tmp.tm_sec;
 
-    RTC->MODE2.CLOCK.reg = clockTime.reg;
+    RTC->MODE2.CLOCK.reg = clockTime.reg; // same for SAMD21 and SAMD51
 
     while (RTCisSyncing())
       ;
@@ -449,6 +442,17 @@ void RTCZero::setY2kEpoch(uint32_t ts)
 
 /* Attach peripheral clock to 32k oscillator */
 void RTCZero::configureClock() {
+#ifdef __SAMD51__
+
+#ifdef CRYSTALLESS
+  OSC32KCTRL->RTCCTRL.reg = OSC32KCTRL_RTCCTRL_RTCSEL_ULP1K;
+#else
+  OSC32KCTRL->RTCCTRL.reg = OSC32KCTRL_RTCCTRL_RTCSEL_XOSC1K;
+#endif
+
+#else // SAMD21
+
+  // Setup clock GCLK2 with OSC32K divided by 32
   GCLK->GENDIV.reg = GCLK_GENDIV_ID(2)|GCLK_GENDIV_DIV(4);
   while (GCLK->STATUS.reg & GCLK_STATUS_SYNCBUSY)
     ;
@@ -462,15 +466,47 @@ void RTCZero::configureClock() {
   GCLK->CLKCTRL.reg = (uint32_t)((GCLK_CLKCTRL_CLKEN | GCLK_CLKCTRL_GEN_GCLK2 | (RTC_GCLK_ID << GCLK_CLKCTRL_ID_Pos)));
   while (GCLK->STATUS.bit.SYNCBUSY)
     ;
+
+#endif
 }
 
 /*
  * Private Utility Functions
  */
 
+/* whether cpu rebooted from a reasonably good running state */
+inline bool RTCZero::isHotReset(void) {
+#ifdef __SAMD51__
+  return (RSTC->RCAUSE.reg & (RSTC_RCAUSE_SYST | RSTC_RCAUSE_WDT | RSTC_RCAUSE_EXT));
+#else // SAMD21
+  return (PM->RCAUSE.reg & (PM_RCAUSE_SYST | PM_RCAUSE_WDT | PM_RCAUSE_EXT));
+#endif
+}
+
 /* Configure the 32768Hz Oscillator */
 void RTCZero::config32kOSC() 
 {
+#ifdef __SAMD51__
+
+#ifdef CRYSTALLESS
+  // enable 1k output
+  OSC32KCTRL->OSCULP32K.reg |= OSC32KCTRL_OSCULP32K_EN1K;
+#else
+  // XOSC32K activated in startup.c but activating again in case USER-code stopped it
+  // enable 1k output
+  OSC32KCTRL->XOSC32K.reg = OSC32KCTRL_XOSC32K_ENABLE |
+    OSC32KCTRL_XOSC32K_EN32K |
+    OSC32KCTRL_XOSC32K_EN1K |
+    OSC32KCTRL_XOSC32K_CGM_XT |
+    OSC32KCTRL_XOSC32K_RUNSTDBY |
+    OSC32KCTRL_XOSC32K_ONDEMAND |
+    OSC32KCTRL_XOSC32K_XTALEN;
+  while( (OSC32KCTRL->STATUS.reg & OSC32KCTRL_STATUS_XOSC32KRDY) == 0 )
+    ;
+#endif
+
+#else // SAMD21
+
 #ifndef CRYSTALLESS
   SYSCTRL->XOSC32K.reg = SYSCTRL_XOSC32K_ONDEMAND |
                          SYSCTRL_XOSC32K_RUNSTDBY |
@@ -479,12 +515,55 @@ void RTCZero::config32kOSC()
                          SYSCTRL_XOSC32K_STARTUP(6) |
                          SYSCTRL_XOSC32K_ENABLE;
 #endif
+
+#endif
+}
+
+/* whether RTC is configured as a clock (MODE2) */
+inline bool RTCZero::RTCisClock(void) {
+#ifdef __SAMD51__
+  return (RTC->MODE2.CTRLA.reg & RTC_MODE2_CTRLA_MODE_CLOCK);
+#else // SAMD21
+  return (RTC->MODE2.CTRL.reg & RTC_MODE2_CTRL_MODE_CLOCK);
+#endif
+}
+
+/* configure the device registers */
+inline void RTCZero::RTCconfigureRegs(void) {
+#ifdef __SAMD51__
+  uint16_t tmp_reg = 0;
+  tmp_reg |= RTC_MODE2_CTRLA_MODE_CLOCK; // set clock operating mode
+  tmp_reg |= RTC_MODE2_CTRLA_PRESCALER_DIV1024; // set prescaler to 1024 for MODE2
+  tmp_reg &= ~RTC_MODE2_CTRLA_MATCHCLR; // disable clear on match
+  //According to the datasheet RTC_MODE2_CTRLA_CLKREP = 0 for 24h
+  tmp_reg &= ~RTC_MODE2_CTRLA_CLKREP; // 24h time representation
+  tmp_reg |= RTC_MODE2_CTRLA_CLOCKSYNC; // synchronize reads
+
+  RTC->MODE2.CTRLA.reg = tmp_reg;
+#else // SAMD21
+  uint16_t tmp_reg = 0;
+  tmp_reg |= RTC_MODE2_CTRL_MODE_CLOCK; // set clock operating mode
+  tmp_reg |= RTC_MODE2_CTRL_PRESCALER_DIV1024; // set prescaler to 1024 for MODE2
+  tmp_reg &= ~RTC_MODE2_CTRL_MATCHCLR; // disable clear on match
+  //According to the datasheet RTC_MODE2_CTRL_CLKREP = 0 for 24h
+  tmp_reg &= ~RTC_MODE2_CTRL_CLKREP; // 24h time representation
+
+  RTC->MODE2.READREQ.reg &= ~RTC_READREQ_RCONT; // disable continuously mode
+
+  RTC->MODE2.CTRL.reg = tmp_reg;
+#endif
+  while (RTCisSyncing())
+    ;
 }
 
 /* Synchronise the CLOCK register for reading*/
 inline void RTCZero::RTCreadRequest() {
   if (_configured) {
+#ifdef __SAMD51__
+    // CTRLA.CLOCKSYNC set so no action needed here
+#else // SAMD21
     RTC->MODE2.READREQ.reg = RTC_READREQ_RREQ;
+#endif
     while (RTCisSyncing())
       ;
   }
@@ -493,33 +572,53 @@ inline void RTCZero::RTCreadRequest() {
 /* Wait for sync in write operations */
 inline bool RTCZero::RTCisSyncing()
 {
+#ifdef __SAMD51__
+  return (RTC->MODE2.SYNCBUSY.reg);
+#else // SAMD21
   return (RTC->MODE2.STATUS.bit.SYNCBUSY);
+#endif
 }
 
 void RTCZero::RTCdisable()
 {
+#ifdef __SAMD51__
+  RTC->MODE2.CTRLA.reg &= ~RTC_MODE2_CTRLA_ENABLE; // disable RTC
+#else // SAMD21
   RTC->MODE2.CTRL.reg &= ~RTC_MODE2_CTRL_ENABLE; // disable RTC
+#endif
   while (RTCisSyncing())
     ;
 }
 
 void RTCZero::RTCenable()
 {
+#ifdef __SAMD51__
+  RTC->MODE2.CTRLA.reg |= RTC_MODE2_CTRLA_ENABLE; // enable RTC
+#else // SAMD21
   RTC->MODE2.CTRL.reg |= RTC_MODE2_CTRL_ENABLE; // enable RTC
+#endif
   while (RTCisSyncing())
     ;
 }
 
 void RTCZero::RTCreset()
 {
+#ifdef __SAMD51__
+  RTC->MODE2.CTRLA.reg |= RTC_MODE2_CTRLA_SWRST; // software reset
+#else // SAMD21
   RTC->MODE2.CTRL.reg |= RTC_MODE2_CTRL_SWRST; // software reset
+#endif
   while (RTCisSyncing())
     ;
 }
 
 void RTCZero::RTCresetRemove()
 {
+#ifdef __SAMD51__
+  RTC->MODE2.CTRLA.reg &= ~RTC_MODE2_CTRLA_SWRST; // software reset remove
+#else // SAMD21
   RTC->MODE2.CTRL.reg &= ~RTC_MODE2_CTRL_SWRST; // software reset remove
+#endif
   while (RTCisSyncing())
     ;
 }
diff --git a/src/RTCZero.h b/src/RTCZero.h
index e0bfb64..08745d5 100644
--- a/src/RTCZero.h
+++ b/src/RTCZero.h
@@ -104,8 +104,11 @@ class RTCZero {
 private:
   bool _configured;
 
+  bool isHotReset(void);
   void config32kOSC(void);
   void configureClock(void);
+  bool RTCisClock(void);
+  void RTCconfigureRegs(void);
   void RTCreadRequest();
   bool RTCisSyncing(void);
   void RTCdisable();