StackRduino
diff --git a/‎ports/nrf/common-hal/rtc/RTC.c‎
Lines changed: 14 additions & 3 deletions b/‎ports/nrf/common-hal/rtc/RTC.c‎
Lines changed: 14 additions & 3 deletions
@@ -35,9 +35,14 @@
 #include "nrfx_rtc.h"
 #include "nrf_clock.h"
 
+// We clock the RTC very slowly (8Hz) so that it won't overflow often.
+// But the counter is only 24 bits, so overflow is about every 24 days ...
+// For testing, set this to 32768 and it'll overflow every few minutes
+
 #define RTC_CLOCK_HZ (8)
 
-static uint32_t rtc_offset = 0;
+volatile static uint32_t rtc_offset = 0;
+int8_t rtc_calibration = 0;
 
 const nrfx_rtc_t rtc_instance = NRFX_RTC_INSTANCE(0);
 
@@ -49,7 +54,9 @@ const nrfx_rtc_config_t rtc_config = {
 };
 
 void rtc_handler(nrfx_rtc_int_type_t int_type) {
-    // do nothing
+    if (int_type == NRFX_RTC_INT_OVERFLOW) {
+        rtc_offset += (1L<<24) / RTC_CLOCK_HZ;
+    }
 }
 
 void rtc_init(void) {
@@ -59,6 +66,7 @@ void rtc_init(void) {
     nrfx_rtc_counter_clear(&rtc_instance);
     nrfx_rtc_init(&rtc_instance, &rtc_config, rtc_handler);
     nrfx_rtc_enable(&rtc_instance);
+    nrfx_rtc_overflow_enable(&rtc_instance, 1);
 }
 
 void common_hal_rtc_get_time(timeutils_struct_time_t *tm) {
@@ -75,8 +83,11 @@ void common_hal_rtc_set_time(timeutils_struct_time_t *tm) {
 
 // A positive value speeds up the clock by removing clock cycles.
 int common_hal_rtc_get_calibration(void) {
-    return 0;
+    return rtc_calibration;
 }
 
 void common_hal_rtc_set_calibration(int calibration) {
+    if (calibration > 127 || calibration < -127)
+        mp_raise_ValueError(translate("calibration value out of range +/-127"));
+    rtc_calibration = calibration;
 }