add clearFrameError UART

from arduino#349

Author: sabas1080

cores/arduino/SERCOM.cpp

@@ -200,6 +200,12 @@ bool SERCOM::isFrameErrorUART()
   return sercom->USART.STATUS.bit.FERR;
 }
 
+void SERCOM::clearFrameErrorUART()
+{
+  // clear FERR bit writing 1 status bit
+  sercom->USART.STATUS.bit.FERR = 1;
+}
+
 bool SERCOM::isParityErrorUART()
 {
   //PERR : Parity Error

cores/arduino/SERCOM.h

@@ -163,6 +163,7 @@ class SERCOM
 		bool availableDataUART( void ) ;
 		bool isBufferOverflowErrorUART( void ) ;
 		bool isFrameErrorUART( void ) ;
+		void clearFrameErrorUART( void ) ;
 		bool isParityErrorUART( void ) ;
 		bool isDataRegisterEmptyUART( void ) ;
 		uint8_t readDataUART( void ) ;

cores/arduino/Uart.cpp

@@ -122,6 +122,13 @@ void Uart::errorHandler()
 #else
 void Uart::IrqHandler()
 {
+  if (sercom->isFrameErrorUART()) {
+    // frame error, next byte is invalid so read and discard it
+    sercom->readDataUART();
+
+    sercom->clearFrameErrorUART();
+  }
+  
   if (sercom->availableDataUART()) {
     rxBuffer.store_char(sercom->readDataUART());
 
@@ -146,7 +153,6 @@ void Uart::IrqHandler()
   if (sercom->isUARTError()) {
     sercom->acknowledgeUARTError();
     // TODO: if (sercom->isBufferOverflowErrorUART()) ....
-    // TODO: if (sercom->isFrameErrorUART()) ....
     // TODO: if (sercom->isParityErrorUART()) ....
     sercom->clearStatusUART();
   }