smaller rtc fixes

README.md

@@ -114,7 +114,7 @@ void receiver_loop() {
   }
 
   if (time_to_send_timesync()) {
-    time_send_timesync(self_short_id); // 60s for first 10 min, then hourly
+    time_send_timesync(self_short_id); // 60s for first 10 min, then hourly if RTC is present
   }
 
   mqtt_loop();
@@ -242,8 +242,10 @@ inline constexpr uint16_t EXPECTED_SENDER_IDS[NUM_SENDERS] = { 0xF19C };
 - Defaults: `pool.ntp.org` and `time.nist.gov`.
 ## RTC (DS3231)
 - Optional DS3231 on the I2C bus. Connect SDA to GPIO21 and SCL to GPIO22 (same bus as the OLED).
+- Enable/disable with `ENABLE_DS3231` in `include/config.h`.
 - Receiver time sync packets set the RTC.
 - On boot, if no LoRa time sync has arrived yet, the sender uses the RTC time as the initial `ts_utc`.
+- When no RTC is present or enabled, the receiver keeps sending time sync every 60 seconds.
 
 ## Build Environments
 - `lilygo-t3-v1-6-1`: production build

src/rtc_ds3231.cpp

@@ -13,6 +13,23 @@ static uint8_t dec_to_bcd(uint8_t val) {
   return static_cast<uint8_t>(((val / 10) << 4) | (val % 10));
 }
 
+static time_t timegm_fallback(struct tm *tm_utc) {
+  if (!tm_utc) {
+    return static_cast<time_t>(-1);
+  }
+  char *old_tz = getenv("TZ");
+  setenv("TZ", "UTC0", 1);
+  tzset();
+  time_t t = mktime(tm_utc);
+  if (old_tz) {
+    setenv("TZ", old_tz, 1);
+  } else {
+    unsetenv("TZ");
+  }
+  tzset();
+  return t;
+}
+
 static bool read_registers(uint8_t start_reg, uint8_t *out, size_t len) {
   if (!out || len == 0) {
     return false;
@@ -77,7 +94,7 @@ bool rtc_ds3231_read_epoch(uint32_t &epoch_utc) {
   tm_utc.tm_year = year - 1900;
   tm_utc.tm_isdst = 0;
 
-  time_t t = timegm(&tm_utc);
+  time_t t = timegm_fallback(&tm_utc);
   if (t <= 0) {
     return false;
   }