wakeup sync (experimental)

docs/configuration/index.md

@@ -88,6 +88,10 @@ Supported external time sources are GPS, LORAWAN network time and LORAWAN applic
 
 	If your LORAWAN network does not support network time, you can run a Node-Red timeserver application using the enclosed [**Timeserver code**](https://github.com/cyberman54/ESP32-Paxcounter/blob/master/src/Node-RED/Timeserver.json). Configure the MQTT nodes in Node-Red for the LORAWAN application used by your paxocunter device. Time can also be set without precision liability, by simple remote command, see section remote control.
 
+## Syncing multiple paxcounters 
+
+A fleet of paxcounters can be synchronized to keep all devices wake up and start scanning at the same time. Synchronization is based on top-of-hour as common time point of reference. This feature requires time-of-day to be present on each device. Thus, `TIME_SYNC_INTERVAL` option, as explained above, must be enabled. Wake up syncing is enabled by setting `SYNCWAKEUP` in `paxcounter.conf` to a value X, greater than zero, and smaller than `SLEEPCYCLE`. This defines a time window, centered at top-of-hour, sized +/- X seconds. If a device, returning from sleep, would wakeup within this time windows, the wakeup will be adjusted to top-of-hour.
+
 ## Wall clock controller
 
 Paxcounter can be used to sync a wall clock which has a DCF77 or IF482 time telegram input. Set `#define HAS_IF482` or `#define HAS_DCF77` in board's hal file to setup clock controller. Use case of this function is to integrate paxcounter and clock. Accurary of the synthetic DCF77 signal depends on accuracy of on board's time base, see above.