Add notes about Serial.available on ESP32

Author: PaulZC

src/SparkFun_u-blox_SARA-R5_Arduino_Library.cpp

@@ -225,7 +225,14 @@ bool SARA_R5::bufferedPoll(void)
 
   if ((hwAvailable() > 0) || (backlogLen > 0)) // If either new data is available, or backlog had data.
   {
-    // Wait for up to _rxWindowMillis for new serial data to arrive. 
+    //Check for incoming serial data. Copy it into the backlog
+
+    // Important note:
+    // On ESP32, Serial.available only provides an update every ~120 bytes during the reception of long messages:
+    // https://gitter.im/espressif/arduino-esp32?at=5e25d6370a1cf54144909c85
+    // Be aware that if a long message is being received, the code below will timeout after _rxWindowMillis = 2 millis.
+    // At 115200 baud, hwAvailable takes ~120 * 10 / 115200 = 10.4 millis before it indicates that data is being received.
+
     while (((millis() - timeIn) < _rxWindowMillis) && (avail < _RXBuffSize))
     {
       if (hwAvailable() > 0) //hwAvailable can return -1 if the serial port is NULL
@@ -5648,7 +5655,14 @@ SARA_R5_error_t SARA_R5::sendCustomCommandWithResponse(const char *command, cons
 
 void SARA_R5::sendCommand(const char *command, bool at)
 {
-  //Spend up to _rxWindowMillis milliseconds copying any incoming serial data into the backlog
+  //Check for incoming serial data. Copy it into the backlog
+  
+  // Important note:
+  // On ESP32, Serial.available only provides an update every ~120 bytes during the reception of long messages:
+  // https://gitter.im/espressif/arduino-esp32?at=5e25d6370a1cf54144909c85
+  // Be aware that if a long message is being received, the code below will timeout after _rxWindowMillis = 2 millis.
+  // At 115200 baud, hwAvailable takes ~120 * 10 / 115200 = 10.4 millis before it indicates that data is being received.
+
   unsigned long timeIn = millis();
   if (hwAvailable() > 0) //hwAvailable can return -1 if the serial port is NULL
   {

src/SparkFun_u-blox_SARA-R5_Arduino_Library.h

@@ -600,9 +600,9 @@ class SARA_R5 : public Print
 
   // This function was originally written by Matthew Menze for the LTE Shield (SARA-R4) library
   // See: https://github.com/sparkfun/SparkFun_LTE_Shield_Arduino_Library/pull/8
-  // It does the same job as ::poll but also processed any 'old' data stored in the backlog first
+  // It does the same job as ::poll but also processes any 'old' data stored in the backlog first
   // It also has a built-in timeout - which ::poll does not
-  // Use this - it is way better than ::poll. Thank you Natthew!
+  // Use this - it is way better than ::poll. Thank you Matthew!
   bool bufferedPoll(void);
 
   // This is the original poll function.
@@ -963,7 +963,7 @@ class SARA_R5 : public Print
   bool _pollReentrant = false; // Prevent reentry of poll - just in case it gets called from a callback
 
   #define _RXBuffSize 2056
-  const unsigned long _rxWindowMillis = 2; // 1ms is not quite long enough for a single char at 9600 baud. millis roll over much less often than micros.
+  const unsigned long _rxWindowMillis = 2; // 1ms is not quite long enough for a single char at 9600 baud. millis roll over much less often than micros. See notes in .cpp re. ESP32!
   char *_saraRXBuffer; // Allocated in SARA_R5::begin
   char *_pruneBuffer;
   char *_saraResponseBacklog;