mimxrt/machine_rtc: Maintain microsecond offset.

The supplied value for microseconds in datetime() will be treated as a
starting value for the reported microseconds.  Due to internal processing
in setting the time, there is an offset about 1 ms.

Author: robert-hh

ports/mimxrt/machine_rtc.c

@@ -37,6 +37,7 @@ typedef struct _machine_rtc_obj_t {
 
 // Singleton RTC object.
 STATIC const machine_rtc_obj_t machine_rtc_obj = {{&machine_rtc_type}};
+uint32_t us_offset = 0;
 
 // Calculate the weekday from the date.
 // The result is zero based with 0 = Monday.
@@ -70,7 +71,7 @@ STATIC mp_obj_t machine_rtc_datetime_helper(size_t n_args, const mp_obj_t *args)
             mp_obj_new_int(srtc_date.hour),
             mp_obj_new_int(srtc_date.minute),
             mp_obj_new_int(srtc_date.second),
-            mp_obj_new_int(ticks_us64() % 1000000),
+            mp_obj_new_int((ticks_us64() + us_offset) % 1000000),
         };
         return mp_obj_new_tuple(8, tuple);
     } else {
@@ -91,6 +92,7 @@ STATIC mp_obj_t machine_rtc_datetime_helper(size_t n_args, const mp_obj_t *args)
         if (SNVS_LP_SRTC_SetDatetime(SNVS, &srtc_date) != kStatus_Success) {
             mp_raise_ValueError(NULL);
         }
+        us_offset = (1000000 + mp_obj_get_int(items[7]) - ticks_us64() % 1000000) % 1000000;
 
         return mp_const_none;
     }