MP: Add clock sync support for 150 BPS

Rework the clock sync math to determine the number of hops that occurred
during the transmitted frame.  The resulting math seems to gets the
clock sync within 2 mSec.

Author: LeeLeahy2

Firmware/LoRaSerial/Radio.ino

@@ -3250,7 +3250,7 @@ void stopChannelTimer()
 
 //Given the remote unit's number of ms before its next hop,
 //adjust our own channelTimer interrupt to be synchronized with the remote unit
-void syncChannelTimer(uint32_t frameAirTimeUsec)
+void syncChannelTimer(uint32_t frameAirTimeUsec, bool clockStarting)
 {
   int16_t adjustment;
   uint8_t caseNumber;
@@ -3318,6 +3318,48 @@ void syncChannelTimer(uint32_t frameAirTimeUsec)
   //timer update will add only microseconds to when the hop is done.
   delayedHopCount = timeToHop ? 1 : 0;
 
+  // 4800 BPS operation
+  //
+  //            |<---------------- Millis to HOP ---------------->|
+  //     _____  |                                                 |_____________
+  //          |_|_________________________________________________|
+  //            |                          |                      |
+  //   TX Start ^         TX Complete ^    |<-- rmtHopTimeMsec -->|
+  //                           RX Complete ^
+  //
+  //
+  // 150 BPS operation
+  //
+  //            |<--- Millis to HOP --->|
+  //     _____  |                       |_________________________
+  //          |_|_______________________|                  |      |_____________
+  //            |                                          |      |
+  //   TX Start ^                         TX Complete ^    |      |
+  //                                           RX Complete ^      |
+  //                                                       |<-  ->|
+  //                                                    rmtHopTimeMsec
+  //
+  //            Millis to HOP
+  //               |<- ->|
+  //               |     |_________________________                           __
+  //    ___________|_____|                         |_________________________|
+  //               |                                          |              |
+  //      TX Start ^                         TX Complete ^    |              |
+  //                                              RX Complete ^              |
+  //                                                          |<----    ---->|
+  //                                                           rmtHopTimeMsec
+  //
+  //For low speed operation move the TX start into the current dwell time period
+  //to make the rest of the math look like the 4800 BPS operation.
+  frameAirTimeMsec = settings.maxDwellTime - msToNextHopRemote
+                   + (frameAirTimeUsec + settings.txToRxUsec + micros() - transactionCompleteMicros) / 1000;
+  while (frameAirTimeMsec >= (settings.maxDwellTime + (settings.maxDwellTime >> 6)))
+  {
+    frameAirTimeMsec -= settings.maxDwellTime;
+    if (clockStarting)
+      delayedHopCount += 1; //Account for the missing hop when the timer is stopped
+  }
+
   //The radios are using the same frequencies since the frame was successfully
   //received.  The goal is to adjust the channel timer to fire in close proximity
   //to the firing of the remote sysstem's channel timer.  The following cases
@@ -3338,8 +3380,7 @@ void syncChannelTimer(uint32_t frameAirTimeUsec)
   //Compute the remote system's channel timer firing time offset in milliseconds
   //using the channel timer value and the adjustments for transmit and receive
   //time (time of flight)
-  frameAirTimeMsec = (frameAirTimeUsec + settings.txToRxUsec + micros() - transactionCompleteMicros) / 1000;
-  rmtHopTimeMsec = msToNextHopRemote - frameAirTimeMsec;
+  rmtHopTimeMsec = settings.maxDwellTime - frameAirTimeMsec;
 
   //Compute when the local system last hopped
   lclHopTimeMsec = currentMillis - channelTimerStart;

Firmware/LoRaSerial/States.ino

@@ -750,7 +750,7 @@ void updateRadioState()
             COMPUTE_TIMESTAMP_OFFSET(rxData, 1, txDataAckUsec);
 
             //The datagram we are expecting
-            syncChannelTimer(txDataAckUsec); //Adjust freq hop ISR based on remote's remaining clock
+            syncChannelTimer(txDataAckUsec, 0); //Adjust freq hop ISR based on remote's remaining clock
 
             triggerEvent(TRIGGER_RX_ACK);
 
@@ -1067,9 +1067,9 @@ void updateRadioState()
       //then skip active discovery and go to standby
       if (((frameAirTimeUsec + txDataAckUsec + settings.txToRxUsec) / 1000) > (settings.maxDwellTime / 2))
       {
-        stopChannelTimer();
         channelNumber = 0;
         setRadioFrequency(false);
+        stopChannelTimer();
         changeState(RADIO_DISCOVER_STANDBY);
       }
       else
@@ -1160,7 +1160,7 @@ void updateRadioState()
               //Start and adjust freq hop ISR based on remote's remaining clock
               startChannelTimer();
               channelTimerStart -= settings.maxDwellTime;
-              syncChannelTimer(txSyncClocksUsec);
+              syncChannelTimer(txSyncClocksUsec, 1);
               triggerEvent(TRIGGER_RX_SYNC_CLOCKS);
 
               if (settings.debugSync)
@@ -1304,7 +1304,7 @@ void updateRadioState()
               //Start and adjust freq hop ISR based on remote's remaining clock
               startChannelTimer();
               channelTimerStart -= settings.maxDwellTime;
-              syncChannelTimer(txSyncClocksUsec);
+              syncChannelTimer(txSyncClocksUsec, 1);
 
               if (settings.debugSync)
               {
@@ -1422,7 +1422,7 @@ void updateRadioState()
             if (settings.server == false)
             {
               //Adjust freq hop ISR based on server's remaining clock
-              syncChannelTimer(txHeartbeatUsec);
+              syncChannelTimer(txHeartbeatUsec, 0);
 
               //Received heartbeat - do not ack.
               triggerEvent(TRIGGER_RX_HEARTBEAT);
@@ -3224,7 +3224,7 @@ void vcReceiveHeartbeat(uint32_t rxMillis)
 
   //Adjust freq hop ISR based on server's remaining clock
   if ((rxSrcVc == VC_SERVER) || (memcmp(rxVcData, myUniqueId, sizeof(myUniqueId)) == 0))
-    syncChannelTimer(txHeartbeatUsec);
+    syncChannelTimer(txHeartbeatUsec, 0);
 
   //Update the timestamp offset
   if (rxSrcVc == VC_SERVER)