For Serial, if assumeSuccess is true, also try to clear the buffers

For Serial, if the Serial port is congested, .begin can fail when the NAV RATE messages are delayed. This is usually a sign that the baud rate is too low. Clearing the serial buffer first seems to help.

src/SparkFun_u-blox_GNSS_Arduino_Library.cpp

@@ -514,6 +514,30 @@ bool SFE_UBLOX_GNSS::begin(Stream &serialPort, uint16_t maxWait, bool assumeSucc
   //New in v2.0: allocate memory for the file buffer - if required. (The user should have called setFileBufferSize already)
   createFileBuffer();
 
+  //Get rid of any stale serial data already in the processor's RX buffer
+  while (_serialPort->available())
+    _serialPort->read();
+
+  //If assumeSuccess is true, the user must really want begin to succeed. So, let's empty the module's serial transmit buffer too!
+  //Keep discarding new serial data until we see a gap of 2ms - hopefully indicating that the module's TX buffer is empty.
+  if (assumeSuccess)
+  {
+    unsigned long startTime = millis();
+    unsigned long lastActivity = startTime;
+    bool keepGoing = true;
+    while (keepGoing && (millis() < (startTime + (unsigned long)maxWait)))
+    {
+      while (_serialPort->available()) // Discard any new data
+      {
+        _serialPort->read();
+        lastActivity = millis();
+      }
+
+      if (millis() > (lastActivity + (unsigned long)2)) // Check if we have seen no new data for at least 2ms
+        keepGoing = false;
+    }
+  }
+
   // Call isConnected up to three times - tests on the NEO-M8U show the CFG RATE poll occasionally being ignored
   bool connected = isConnected(maxWait);
 
@@ -3642,7 +3666,7 @@ sfe_ublox_status_e SFE_UBLOX_GNSS::waitForACKResponse(ubxPacket *outgoingUBX, ui
   packetAuto.classAndIDmatch = SFE_UBLOX_PACKET_VALIDITY_NOT_DEFINED;
 
   unsigned long startTime = millis();
-  while (millis() - startTime < maxTime)
+  while (millis() < (startTime + (unsigned long)maxTime))
   {
     if (checkUbloxInternal(outgoingUBX, requestedClass, requestedID) == true) //See if new data is available. Process bytes as they come in.
     {
@@ -3781,7 +3805,7 @@ sfe_ublox_status_e SFE_UBLOX_GNSS::waitForACKResponse(ubxPacket *outgoingUBX, ui
     } //checkUbloxInternal == true
 
     delay(1); // Allow an RTOS to get an elbow in (#11)
-  } //while (millis() - startTime < maxTime)
+  } //while (millis() < (startTime + (unsigned long)maxTime))
 
   // We have timed out...
   // If the outgoingUBX->classAndIDmatch is VALID then we can take a gamble and return DATA_RECEIVED

src/SparkFun_u-blox_GNSS_Arduino_Library.h

@@ -599,6 +599,7 @@ public:
 	void setPacketCfgPayloadSize(size_t payloadSize); // Set packetCfgPayloadSize
 
 	//Begin communication with the GNSS. Advanced users can assume success if required. Useful if the port is already outputting messages at high navigation rate.
+	//Begin will then return true if "signs of life" have been seen: reception of _any_ valid UBX packet or _any_ valid NMEA header.
 	//By default use the default I2C address, and use Wire port
 	bool begin(TwoWire &wirePort = Wire, uint8_t deviceAddress = 0x42, uint16_t maxWait = defaultMaxWait, bool assumeSuccess = false); //Returns true if module is detected
 	//serialPort needs to be perviously initialized to correct baud rate