Use TX buffer for slave writes, empty on master read mode + DRDY IRQ

cores/arduino/SERCOM.cpp

@@ -557,11 +557,8 @@ bool SERCOM::sendDataSlaveWIRE(uint8_t data)
   //Send data
   sercom->I2CS.DATA.bit.DATA = data;
 
-  //Wait data transmission successful
-  while(!sercom->I2CS.INTFLAG.bit.DRDY);
-
   //Problems on line? nack received?
-  if(sercom->I2CS.STATUS.bit.RXNACK)
+  if(!sercom->I2CS.INTFLAG.bit.DRDY || sercom->I2CS.STATUS.bit.RXNACK)
     return false;
   else
     return true;

libraries/Wire/Wire.cpp

@@ -152,8 +152,6 @@ uint8_t TwoWire::endTransmission()
 }
 
 size_t TwoWire::write(uint8_t ucData)
-{
-  if(sercom->isMasterWIRE())
 {
   // No writing, without begun transmission or a full buffer
   if ( !transmissionBegun || txBuffer.isFull() )
@@ -165,16 +163,6 @@ size_t TwoWire::write(uint8_t ucData)
 
   return 1 ;
 }
-  else
-  {
-    if(sercom->sendDataSlaveWIRE( ucData ))
-    {
-      return 1;
-    }
-  }
-
-  return 0;
-}
 
 size_t TwoWire::write(const uint8_t *data, size_t quantity)
 {
@@ -246,9 +234,9 @@ void TwoWire::onService(void)
 
       if(sercom->isMasterReadOperationWIRE()) //Is a request ?
       {
-        // wait for data ready flag,
-        // before calling request callback
-        while(!sercom->isDataReadyWIRE());
+        txBuffer.clear();
+
+        transmissionBegun = true;
 
         //Calling onRequestCallback, if exists
         if(onRequestCallback)
@@ -257,8 +245,18 @@ void TwoWire::onService(void)
         }
       }
     }
-    else if(sercom->isDataReadyWIRE()) //Received data
+    else if(sercom->isDataReadyWIRE())
     {
+      if (sercom->isMasterReadOperationWIRE())
+      {
+        uint8_t c = 0xff;
+
+        if( txBuffer.available() ) {
+          c = txBuffer.read_char();
+        }
+
+        transmissionBegun = sercom->sendDataSlaveWIRE(c);
+      } else { //Received data
         if (rxBuffer.isFull()) {
           sercom->prepareNackBitWIRE(); 
         } else {
@@ -272,6 +270,7 @@ void TwoWire::onService(void)
       }
     }
   }
+}
 
 #if WIRE_INTERFACES_COUNT > 0
   /* In case new variant doesn't define these macros,