Fixing millis() and micros() (#87)

* change a bit shifting operation to reflect the fact the RTC is 24 bits, not 32.
* enable the overflow interrupt correctly

Author: austin1611

cores/nRF5/delay.c

@@ -29,14 +29,14 @@ static volatile uint32_t overflows = 0;
 
 uint32_t millis( void )
 {
-  uint64_t ticks = (uint64_t)((uint64_t)overflows << (uint64_t)32) | (uint64_t)(NRF_RTC1->COUNTER);
+  uint64_t ticks = (uint64_t)((uint64_t)overflows << (uint64_t)24) | (uint64_t)(NRF_RTC1->COUNTER);
 
   return (ticks * 1000) / 32768;
 }
 
 uint32_t micros( void )
 {
-  uint64_t ticks = (uint64_t)((uint64_t)overflows << (uint64_t)32) | (uint64_t)(NRF_RTC1->COUNTER);
+  uint64_t ticks = (uint64_t)((uint64_t)overflows << (uint64_t)24) | (uint64_t)(NRF_RTC1->COUNTER);
 
   return (ticks * 1000000) / 32768;
 }

cores/nRF5/wiring.c

@@ -35,11 +35,11 @@ void init( void )
   NRF_CLOCK->TASKS_LFCLKSTART = 1UL;
 
   NRF_RTC1->PRESCALER = 0;
-  NRF_RTC1->EVTENSET = offsetof(NRF_RTC_Type, EVENTS_OVRFLW);
-  NRF_RTC1->INTENSET = offsetof(NRF_RTC_Type, EVENTS_OVRFLW);
+  NRF_RTC1->EVTENSET = RTC_INTENSET_OVRFLW_Msk;
+  NRF_RTC1->INTENSET = RTC_EVTEN_OVRFLW_Msk;
   NRF_RTC1->TASKS_START = 1;
 }
 
 #ifdef __cplusplus
 }
-#endif
+#endif