nrf: port: save rtc value across reboots

As part of the reset process, save the current tick count to an
uninitialized memory location.  That way, the current tick value will be
preserved across reboots.

A reboot will cause us to lose a certain number of ticks, depending on
how long a reboot takes, however if reboots are infrequent then this
will not be a large amount of time lost.

Signed-off-by: Sean Cross <sean@xobs.io>

1 files changed, 24 insertions, 3 deletions

diff --git a/ports/nrf/supervisor/port.c b/ports/nrf/supervisor/port.c
index 9fe72905c..5c76f6aa1 100644
--- a/ports/nrf/supervisor/port.c
+++ b/ports/nrf/supervisor/port.c
@@ -78,13 +78,19 @@ const nrfx_rtc_config_t rtc_config = {
     .interrupt_priority = 6
 };
 
-static volatile uint64_t overflowed_ticks = 0;
+#define OVERFLOW_CHECK_PREFIX 0x2cad564f
+#define OVERFLOW_CHECK_SUFFIX 0x11343ef7
+static volatile struct {
+    uint32_t prefix;
+    uint64_t overflowed_ticks;
+    uint32_t suffix;
+} overflow_tracker __attribute__((section(".uninitialized")));
 
 void rtc_handler(nrfx_rtc_int_type_t int_type) {
     if (int_type == NRFX_RTC_INT_OVERFLOW) {
         // Our RTC is 24 bits and we're clocking it at 32.768khz which is 32 (2 ** 5) subticks per
         // tick.
-        overflowed_ticks += (1L<< (24 - 5));
+        overflow_tracker.overflowed_ticks += (1L<< (24 - 5));
     } else if (int_type == NRFX_RTC_INT_TICK && nrfx_rtc_counter_get(&rtc_instance) % 32 == 0) {
         // Do things common to all ports when the tick occurs
         supervisor_tick();
@@ -101,6 +107,17 @@ void tick_init(void) {
     nrfx_rtc_init(&rtc_instance, &rtc_config, rtc_handler);
     nrfx_rtc_enable(&rtc_instance);
     nrfx_rtc_overflow_enable(&rtc_instance, true);
+
+    // If the check prefix and suffix aren't correct, then the structure
+    // in memory isn't correct and the clock will be wildly wrong. Initialize
+    // the prefix and suffix so that we know the value is correct, and reset
+    // the time to 0.
+    if (overflow_tracker.prefix != OVERFLOW_CHECK_PREFIX ||
+        overflow_tracker.suffix != OVERFLOW_CHECK_SUFFIX) {
+        overflow_tracker.prefix = OVERFLOW_CHECK_PREFIX;
+        overflow_tracker.suffix = OVERFLOW_CHECK_SUFFIX;
+        overflow_tracker.overflowed_ticks = 0;
+    }
 }
 
 safe_mode_t port_init(void) {
@@ -205,6 +222,10 @@ void reset_to_bootloader(void) {
 }
 
 void reset_cpu(void) {
+    // We're getting ready to reset, so save the counter off.
+    // This counter will get reset to zero during the reboot.
+    uint32_t ticks = nrfx_rtc_counter_get(&rtc_instance);
+    overflow_tracker.overflowed_ticks += ticks;
     NVIC_SystemReset();
 }
 
@@ -248,7 +269,7 @@ uint64_t port_get_raw_ticks(uint8_t* subticks) {
     if (subticks != NULL) {
         *subticks = (rtc % 32);
     }
-    return overflowed_ticks + rtc / 32;
+    return overflow_tracker.overflowed_ticks + rtc / 32;
 }
 
 // Enable 1/1024 second tick.