rp2: Implement time.time_ns with time_us_64 so it has us resolution.

Currently on rp2 the time.time_ns() function has only seconds resolution.
This commit makes it have microsecond resolution, by using the output of
time_us_64() instead of the RTC.

Tested that it does not drift from the RTC over long periods of time.

Signed-off-by: Damien George <damien.p.george@gmail.com>

ports/rp2/machine_rtc.c

@@ -58,6 +58,7 @@ STATIC mp_obj_t machine_rtc_make_new(const mp_obj_type_t *type, size_t n_args, s
         rtc_init();
         datetime_t t = { .month = 1, .day = 1 };
         rtc_set_datetime(&t);
+        mp_hal_time_ns_set_from_rtc();
     }
     // return constant object
     return (mp_obj_t)&machine_rtc_obj;
@@ -104,6 +105,7 @@ STATIC mp_obj_t machine_rtc_datetime(mp_uint_t n_args, const mp_obj_t *args) {
         if (!rtc_set_datetime(&t)) {
             mp_raise_OSError(MP_EINVAL);
         }
+        mp_hal_time_ns_set_from_rtc();
 
     }
     return mp_const_none;

ports/rp2/main.c

@@ -106,6 +106,7 @@ int main(int argc, char **argv) {
     };
     rtc_init();
     rtc_set_datetime(&t);
+    mp_hal_time_ns_set_from_rtc();
 
     // Initialise stack extents and GC heap.
     mp_stack_set_top(&__StackTop);

ports/rp2/mphalport.c

@@ -39,6 +39,10 @@
 #include "lib/cyw43-driver/src/cyw43.h"
 #endif
 
+// This needs to be added to the result of time_us_64() to get the number of
+// microseconds since the Epoch.
+STATIC uint64_t time_us_64_offset_from_epoch;
+
 #if MICROPY_HW_ENABLE_UART_REPL || MICROPY_HW_USB_CDC
 
 #ifndef MICROPY_HW_STDIN_BUFFER_LEN
@@ -176,11 +180,28 @@ void mp_hal_delay_ms(mp_uint_t ms) {
     }
 }
 
-uint64_t mp_hal_time_ns(void) {
+void mp_hal_time_ns_set_from_rtc(void) {
+    // Delay at least one RTC clock cycle so it's registers have updated with the most
+    // recent time settings.
+    sleep_us(23);
+
+    // Sample RTC and time_us_64() as close together as possible, so the offset
+    // calculated for the latter can be as accurate as possible.
     datetime_t t;
     rtc_get_datetime(&t);
+    uint64_t us = time_us_64();
+
+    // Calculate the difference between the RTC Epoch seconds and time_us_64().
     uint64_t s = timeutils_seconds_since_epoch(t.year, t.month, t.day, t.hour, t.min, t.sec);
-    return s * 1000000000ULL;
+    time_us_64_offset_from_epoch = (uint64_t)s * 1000000ULL - us;
+}
+
+uint64_t mp_hal_time_ns(void) {
+    // The RTC only has seconds resolution, so instead use time_us_64() to get a more
+    // precise measure of Epoch time.  Both these "clocks" are clocked from the same
+    // source so they remain synchronised, and only differ by a fixed offset (calculated
+    // in mp_hal_time_ns_set_from_rtc).
+    return (time_us_64_offset_from_epoch + time_us_64()) * 1000ULL;
 }
 
 // Generate a random locally administered MAC address (LAA)

ports/rp2/mphalport.h

@@ -39,6 +39,7 @@ extern int mp_interrupt_char;
 extern ringbuf_t stdin_ringbuf;
 
 void mp_hal_set_interrupt_char(int c);
+void mp_hal_time_ns_set_from_rtc(void);
 
 static inline void mp_hal_delay_us(mp_uint_t us) {
     sleep_us(us);