Serial.flush() waits for last character to be transmitted (michele.mazzucchi)

http://code.google.com/p/arduino/issues/detail?id=871

Author: damellis

hardware/arduino/cores/arduino/HardwareSerial.cpp

@@ -327,6 +327,8 @@ void HardwareSerial::begin(unsigned long baud)
   *_ubrrh = baud_setting >> 8;
   *_ubrrl = baud_setting;
 
+  transmitting = false;
+
   sbi(*_ucsrb, _rxen);
   sbi(*_ucsrb, _txen);
   sbi(*_ucsrb, _rxcie);
@@ -376,8 +378,9 @@ int HardwareSerial::read(void)
 
 void HardwareSerial::flush()
 {
-  while (_tx_buffer->head != _tx_buffer->tail)
-    ;
+  // UDR is kept full while the buffer is not empty, so TXC triggers when EMPTY && SENT
+  while (transmitting && ! (*_ucsra & _BV(TXC0)));
+  transmitting = false;
 }
 
 size_t HardwareSerial::write(uint8_t c)
@@ -394,6 +397,9 @@ size_t HardwareSerial::write(uint8_t c)
   _tx_buffer->head = i;
 
   sbi(*_ucsrb, _udrie);
+  // clear the TXC bit -- "can be cleared by writing a one to its bit location"
+  transmitting = true;
+  sbi(*_ucsra, TXC0);
 
   return 1;
 }

hardware/arduino/cores/arduino/HardwareSerial.h

@@ -43,6 +43,7 @@ class HardwareSerial : public Stream
     uint8_t _rxcie;
     uint8_t _udrie;
     uint8_t _u2x;
+    bool transmitting;
   public:
     HardwareSerial(ring_buffer *rx_buffer, ring_buffer *tx_buffer,
       volatile uint8_t *ubrrh, volatile uint8_t *ubrrl,