Add TX buffer for Serial

cores/arduino/RingBuffer.cpp

@@ -67,6 +67,14 @@ int RingBuffer::available()
 		return delta;
 }
 
+int RingBuffer::availableForStore()
+{
+	if (_iHead >= _iTail)
+		return SERIAL_BUFFER_SIZE - 1 - _iHead + _iTail;
+	else
+		return _iTail - _iHead - 1;
+}
+
 int RingBuffer::peek()
 {
 	if(_iTail == _iHead)

cores/arduino/RingBuffer.h

@@ -40,6 +40,7 @@ class RingBuffer
 	void clear();
 	int read_char();
 	int available();
+	int availableForStore();
 	int peek();
 	bool isFull();
 

cores/arduino/SERCOM.cpp

@@ -173,6 +173,16 @@ int SERCOM::writeDataUART(uint8_t data)
   return 1;
 }
 
+void SERCOM::enableDataRegisterEmptyInterruptUART()
+{
+  sercom->USART.INTENSET.reg |= SERCOM_USART_INTENSET_DRE;
+}
+
+void SERCOM::disableDataRegisterEmptyInterruptUART()
+{
+  sercom->USART.INTENCLR.reg = SERCOM_USART_INTENCLR_DRE;
+}
+
 /*	=========================
  *	===== Sercom SPI
  *	=========================

cores/arduino/SERCOM.h

@@ -163,6 +163,8 @@ class SERCOM
 		int writeDataUART(uint8_t data) ;
 		bool isUARTError() ;
 		void acknowledgeUARTError() ;
+		void enableDataRegisterEmptyInterruptUART();
+		void disableDataRegisterEmptyInterruptUART();
 
 		/* ========== SPI ========== */
 		void initSPI(SercomSpiTXPad mosi, SercomRXPad miso, SercomSpiCharSize charSize, SercomDataOrder dataOrder) ;

cores/arduino/Uart.cpp

@@ -50,10 +50,13 @@ void Uart::end()
 {
   sercom->resetUART();
   rxBuffer.clear();
+  txBuffer.clear();
 }
 
 void Uart::flush()
 {
+  while(txBuffer.available()); // wait until TX buffer is empty
+
   sercom->flushUART();
 }
 
@@ -63,6 +66,16 @@ void Uart::IrqHandler()
     rxBuffer.store_char(sercom->readDataUART());
   }
 
+  if (sercom->isDataRegisterEmptyUART()) {
+    if (txBuffer.available()) {
+      uint8_t data = txBuffer.read_char();
+
+      sercom->writeDataUART(data);
+    } else {
+      sercom->disableDataRegisterEmptyInterruptUART();
+    }
+  }
+
   if (sercom->isUARTError()) {
     sercom->acknowledgeUARTError();
     // TODO: if (sercom->isBufferOverflowErrorUART()) ....
@@ -79,7 +92,7 @@ int Uart::available()
 
 int Uart::availableForWrite()
 {
-  return (sercom->isDataRegisterEmptyUART() ? 1 : 0);
+  return txBuffer.availableForStore();
 }
 
 int Uart::peek()
@@ -94,7 +107,16 @@ int Uart::read()
 
 size_t Uart::write(const uint8_t data)
 {
-  sercom->writeDataUART(data);
+  if (sercom->isDataRegisterEmptyUART() && txBuffer.available() == 0) {
+    sercom->writeDataUART(data);
+  } else {
+    while(txBuffer.isFull()); // spin lock until a spot opens up in the buffer
+
+    txBuffer.store_char(data);
+
+    sercom->enableDataRegisterEmptyInterruptUART();
+  }
+
   return 1;
 }
 

cores/arduino/Uart.h

@@ -46,6 +46,7 @@ class Uart : public HardwareSerial
   private:
     SERCOM *sercom;
     RingBuffer rxBuffer;
+    RingBuffer txBuffer;
 
     uint8_t uc_pinRX;
     uint8_t uc_pinTX;