Unity: Use UInt16.MaxValue rather than hardcoding 65536

Unity provides constants that reflect the maximum value of integer types,
use it for checking the checksum overflow rather than using 65536 directly.
This makes the intent more clear.

Arduino/ReceiveCart/ReceiveCart.ino

@@ -45,6 +45,7 @@ void setup() {
   Serial.begin(115200);
   pinMode(LED_BUILTIN, OUTPUT);
   Wire.begin();
+  Wire.setClock(400000);
 }
 
 void loop() {

Arduino/SendCart/SendCart.ino

@@ -33,6 +33,7 @@ void setup() {
   Serial.begin(115200);
   pinMode(LED_BUILTIN, OUTPUT);
   Wire.begin();
+  Wire.setClock(400000);
 }
 
 /*

Assets/Scripts/Arduino_Com.cs

@@ -14,6 +14,7 @@ public class Arduino_Com : MonoBehaviour
     {
         Déconnecté,
         Connecté,
+        Connexion,
         Lecture,
         Écriture,
         Invalide
@@ -68,16 +69,26 @@ public class Arduino_Com : MonoBehaviour
         return sum;
     }
     
+    // Core communication function.
+    // Receives all the data from the cartridge and writes it out to the png file.
+    // As the computer will probably read the serial data faster than the Arduino
+    // /and/ the transfer is quite slow (lots of bits, low baudrate), make this a
+    // coroutine.
+    // This prevents the whole application freezing up why waiting for data and
+    // allow presenting visual feedback.
     private IEnumerator<WaitUntil> ReceiveSerialData()
     {
         do
         {
+            // We could use an event instead but as Unity is lagging behind .NET versions,
+            // it wouldn't be cross-platform.
             yield return new WaitUntil(() => _serial.BytesToRead >= 1);
+            
             // Read all available data, up to the PageSize.
             _readBytes += _serial.Read(_pageBuffer, _readBytes, PageSize - _readBytes);
 
             // Full page received, check checksum and ACK/NAK, write received page.
-            if (_readBytes >= 128)
+            if (_readBytes >= PageSize)
             {
                 Byte[] checksum = new byte[2];
                 // There might be less than the two characters of the checksum available, loop until complete.
@@ -88,7 +99,8 @@ public class Arduino_Com : MonoBehaviour
                 } while (checksumRead < 2);
                 
                 // Implicit type of the sum is 4 bytes, so the overflow doesn't occur. Compute manually.
-                if (65536 - (BitConverter.ToUInt16(checksum) + ComputeChecksum(_pageBuffer)) == 0)
+                if ((UInt16.MaxValue + 1) -
+                    (BitConverter.ToUInt16(checksum) + ComputeChecksum(_pageBuffer)) == 0)
                 {
                     // Checksum valid, write it out to the cart.
                     _cart.Write(_pageBuffer);
@@ -104,6 +116,7 @@ public class Arduino_Com : MonoBehaviour
                 _readBytes = 0;
             }
         } while (_currentPage < PageCount);
+        
         // We are done receiving the cart, we don't expect more data to be received so the coroutine can return.
         _currentPage = 0;
         _state = ArduinoState.Connecté;
@@ -222,14 +235,7 @@ public class Arduino_Com : MonoBehaviour
                 return;
             }
             
-            // TODO: Wait ~2s for Arduino startup
+            StartCoroutine(DoSerialConnection());
-            _arduinoControlButton.text = "Déconnecter";
-            
-            _cartLoadButton.SetEnabled(true);
-            // _cartWriteButton.SetEnabled(true);  TODO: Not implemented
-            _arduinoSerialPort.SetEnabled(false);
-
-            _state = ArduinoState.Connecté;
         }
     }
     
@@ -244,4 +250,24 @@ public class Arduino_Com : MonoBehaviour
         _state = SerialPort.GetPortNames().Contains(newPort.newValue) ? ArduinoState.Déconnecté : ArduinoState.Invalide;
         _arduinoControlButton.SetEnabled(_state != ArduinoState.Invalide);
     }
+
+    // Update UI during connection and when ready, prevent interacting too quickly.
+    IEnumerator<WaitForSeconds> DoSerialConnection()
+    {
+        _arduinoControlButton.text = "Connexion...";
+        _arduinoControlButton.SetEnabled(false);
+        _arduinoSerialPort.SetEnabled(false);
+        _state = ArduinoState.Connexion;
+        
+        // Delay enabling transfers because the Arduino is slow to set-up the serial.
+        yield return new WaitForSeconds(1.5f);
+        
+        _arduinoControlButton.text = "Déconnecter";
+        _arduinoControlButton.SetEnabled(true);
+            
+        _cartLoadButton.SetEnabled(true);
+        // _cartWriteButton.SetEnabled(true);  TODO: Not implemented
+
+        _state = ArduinoState.Connecté;
+    }
 }