Merge pull request #652 from facchinm/fix_flush_locks_on_startup

fix Serial.flush() blocks forever #597

Author: facchinm

cores/arduino/SERCOM.cpp

@@ -112,11 +112,10 @@ void SERCOM::enableUART()
 void SERCOM::flushUART()
 {
   // Skip checking transmission completion if data register is empty
-//   if(isDataRegisterEmptyUART())
-//     return;
+  // Wait for transmission to complete, if ok to do so.
+  while(!sercom->USART.INTFLAG.bit.TXC && onFlushWaitUartTXC);
 
-  // Wait for transmission to complete
-  while(!sercom->USART.INTFLAG.bit.TXC);
+  onFlushWaitUartTXC = false;
 }
 
 void SERCOM::clearStatusUART()
@@ -183,6 +182,10 @@ int SERCOM::writeDataUART(uint8_t data)
 
   //Put data into DATA register
   sercom->USART.DATA.reg = (uint16_t)data;
+
+  // indicate it's ok to wait for TXC flag when flushing
+  onFlushWaitUartTXC = true;
+
   return 1;
 }
 

cores/arduino/SERCOM.h

@@ -218,6 +218,11 @@ class SERCOM
 		uint8_t calculateBaudrateSynchronous(uint32_t baudrate) ;
 		uint32_t division(uint32_t dividend, uint32_t divisor) ;
 		void initClockNVIC( void ) ;
+
+		// Flag set when data is loaded into sercom->USART.DATA.reg.
+		// Helps with preventing UART lockups when flushing on startup
+		// and the asyncronous nature of the DRE and TXC interrupt flags.
+		bool onFlushWaitUartTXC = false;
 };
 
 #endif