mimxrt/machine_rtc: Start RTC at boot and set datetime if not set.

Changes in this commit:
- Start the RTC Timer at system boot.  Otherwise time.time() will advance
  only if an RTC() object was created.
- Set the time to a more recent date than Jan 1, 1970, if not set.  That is
  2013/10/14, 19:53:11, MicroPython's first commit.
- Compensate an underflow in in timeutils_seconds_since_2000(), called by
  time.time(), if the time is set to a pre-2000 date.

ports/mimxrt/board_init.c

@@ -100,6 +100,8 @@ void board_init(void) {
     #if MICROPY_PY_MACHINE_I2S
     machine_i2s_init0();
     #endif
+    // RTC
+    machine_rtc_start();
 }
 
 void USB_OTG1_IRQHandler(void) {

ports/mimxrt/machine_rtc.c

@@ -40,13 +40,31 @@ typedef struct _machine_rtc_obj_t {
 STATIC const machine_rtc_obj_t machine_rtc_obj = {{&machine_rtc_type}};
 uint32_t us_offset = 0;
 
+// Start the RTC Timer.
+void machine_rtc_start(void) {
+
+    SNVS_LP_SRTC_StartTimer(SNVS);
+    // If the date is not set, set it to a more recent start date,
+    // MicroPython's first commit.
+    snvs_lp_srtc_datetime_t srtc_date;
+    SNVS_LP_SRTC_GetDatetime(SNVS, &srtc_date);
+    if (srtc_date.year <= 1970) {
+        srtc_date = (snvs_lp_srtc_datetime_t) {
+            .year = 2013,
+            .month = 10,
+            .day = 14,
+            .hour = 19,
+            .minute = 53,
+            .second = 11,
+        };
+        SNVS_LP_SRTC_SetDatetime(SNVS, &srtc_date);
+    }
+}
+
 STATIC mp_obj_t machine_rtc_make_new(const mp_obj_type_t *type, size_t n_args, size_t n_kw, const mp_obj_t *args) {
     // Check arguments.
     mp_arg_check_num(n_args, n_kw, 0, 0, false);
 
-    // Start up the RTC if needed.
-    SNVS_LP_SRTC_StartTimer(SNVS);
-
     // Return constant object.
     return (mp_obj_t)&machine_rtc_obj;
 }

ports/mimxrt/modmachine.h

@@ -48,5 +48,6 @@ void machine_sdcard_init0(void);
 void mimxrt_sdram_init(void);
 void machine_i2s_init0();
 void machine_i2s_deinit_all(void);
+void machine_rtc_start(void);
 
 #endif // MICROPY_INCLUDED_MIMXRT_MODMACHINE_H

ports/mimxrt/modutime.c

@@ -95,7 +95,16 @@ MP_DEFINE_CONST_FUN_OBJ_1(time_mktime_obj, time_mktime);
 STATIC mp_obj_t time_time(void) {
     snvs_lp_srtc_datetime_t t;
     SNVS_LP_SRTC_GetDatetime(SNVS, &t);
-    return mp_obj_new_int_from_ull(timeutils_seconds_since_epoch(t.year, t.month, t.day, t.hour, t.minute, t.second));
+    // EPOCH is 1970 for this port, which leads to the following trouble:
+    // timeutils_seconds_since_epoch() calls timeutils_seconds_since_2000(), and
+    // timeutils_seconds_since_2000() subtracts 2000 from year, but uses
+    // an unsigned number for seconds, That causes an underrun, which is not
+    // fixed by adding the TIMEUTILS_SECONDS_1970_TO_2000.
+    // Masking it to 32 bit for year < 2000 fixes it.
+    return mp_obj_new_int_from_ull(
+        timeutils_seconds_since_epoch(t.year, t.month, t.day, t.hour, t.minute, t.second)
+        & (t.year < 2000 ? 0xffffffff : 0xffffffffffff)
+        );
 }
 STATIC MP_DEFINE_CONST_FUN_OBJ_0(time_time_obj, time_time);
 

shared/timeutils/timeutils.h

@@ -69,6 +69,7 @@ static inline uint64_t timeutils_mktime(mp_uint_t year, mp_int_t month, mp_int_t
 
 static inline uint64_t timeutils_seconds_since_epoch(mp_uint_t year, mp_uint_t month,
     mp_uint_t date, mp_uint_t hour, mp_uint_t minute, mp_uint_t second) {
+    // TODO this will give incorrect results for dates before 2000/1/1
     return timeutils_seconds_since_2000(year, month, date, hour, minute, second) + TIMEUTILS_SECONDS_1970_TO_2000;
 }