2 files changed, 17 insertions, 6 deletions
diff --git a/cores/arduino/HardwareSerial.cpp b/cores/arduino/HardwareSerial.cpp
index 806fe5f..d78c205 100644
--- a/cores/arduino/HardwareSerial.cpp
+++ b/cores/arduino/HardwareSerial.cpp
@@ -276,6 +276,8 @@ void HardwareSerial::begin(unsigned long baud, byte config)
   *_ubrrh = baud_setting >> 8;
   *_ubrrl = baud_setting;
 
+  _written = false;
+
   //set the data bits, parity, and stop bits
 #if defined(__AVR_ATmega8__)
   config |= 0x80; // select UCSRC register (shared with UBRRH)
@@ -331,9 +333,15 @@ int HardwareSerial::read(void)
 
 void HardwareSerial::flush()
 {
-  // UDR is kept full while the buffer is not empty, so TXC triggers when EMPTY && SENT
-  while (transmitting && bit_is_clear(*_ucsra, TXC0));
-  transmitting = false;
+  // If we have never written a byte, no need to flush. This special
+  // case is needed since there is no way to force the TXC (transmit
+  // complete) bit to 1 during initialization
+  if (!_written)
+    return;
+
+  // UDR is kept full while the buffer is not empty, so TXC triggers
+  // when EMPTY && SENT
+  while (bit_is_clear(*_ucsra, TXC0));
 }
 
 size_t HardwareSerial::write(uint8_t c)
@@ -350,8 +358,10 @@ size_t HardwareSerial::write(uint8_t c)
   _tx_buffer_head = i;
 	
   sbi(*_ucsrb, UDRIE0);
-  // clear the TXC bit -- "can be cleared by writing a one to its bit location"
-  transmitting = true;
+  _written = true;
+  // clear the TXC bit -- "can be cleared by writing a one to its bit
+  // location". This makes sure flush() won't return until the bytes
+  // actually got written
   sbi(*_ucsra, TXC0);
   
   return 1;
diff --git a/cores/arduino/HardwareSerial.h b/cores/arduino/HardwareSerial.h
index cf3f6bd..0ae51f4 100644
--- a/cores/arduino/HardwareSerial.h
+++ b/cores/arduino/HardwareSerial.h
@@ -72,7 +72,8 @@ class HardwareSerial : public Stream
     volatile uint8_t *_ucsrb;
     volatile uint8_t *_ucsrc;
     volatile uint8_t *_udr;
-    bool transmitting;
+    // Has any byte been written to the UART since begin()
+    bool _written;
 
     volatile uint8_t _rx_buffer_head;
     volatile uint8_t _rx_buffer_tail;