diff --git a/hardware/arduino/avr/cores/arduino/HardwareSerial.cpp b/hardware/arduino/avr/cores/arduino/HardwareSerial.cpp
index 5cd89e5e664..ecd7918431c 100644
--- a/hardware/arduino/avr/cores/arduino/HardwareSerial.cpp
+++ b/hardware/arduino/avr/cores/arduino/HardwareSerial.cpp
@@ -26,6 +26,7 @@
 #include <stdio.h>
 #include <string.h>
 #include <inttypes.h>
+#include <util/atomic.h>
 #include "Arduino.h"
 
 #include "HardwareSerial.h"
@@ -76,6 +77,13 @@ void serialEventRun(void)
 #endif
 }
 
+// macro to guard critical sections when needed for large TX buffer sizes
+#if (SERIAL_TX_BUFFER_SIZE>256)
+#define TX_BUFFER_ATOMIC ATOMIC_BLOCK(ATOMIC_RESTORESTATE)
+#else
+#define TX_BUFFER_ATOMIC
+#endif
+
 // Actual interrupt handlers //////////////////////////////////////////////////////////////
 
 void HardwareSerial::_tx_udr_empty_irq(void)
@@ -89,8 +97,10 @@ void HardwareSerial::_tx_udr_empty_irq(void)
 
   // 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);
+  // actually got written. Other r/w bits are preserved, and zeroes
+  // written to the rest.
+
+  *_ucsra = ((*_ucsra) & ((1 << U2X0) | (1 << MPCM0))) | (1 << TXC0);
 
   if (_tx_buffer_head == _tx_buffer_tail) {
     // Buffer empty, so disable interrupts
@@ -177,15 +187,13 @@ int HardwareSerial::read(void)
 
 int HardwareSerial::availableForWrite(void)
 {
-#if (SERIAL_TX_BUFFER_SIZE>256)
-  uint8_t oldSREG = SREG;
-  cli();
-#endif
-  tx_buffer_index_t head = _tx_buffer_head;
-  tx_buffer_index_t tail = _tx_buffer_tail;
-#if (SERIAL_TX_BUFFER_SIZE>256)
-  SREG = oldSREG;
-#endif
+  tx_buffer_index_t head;
+  tx_buffer_index_t tail;
+
+  TX_BUFFER_ATOMIC {
+    head = _tx_buffer_head;
+    tail = _tx_buffer_tail;
+  }
   if (head >= tail) return SERIAL_TX_BUFFER_SIZE - 1 - head + tail;
   return tail - head - 1;
 }
@@ -218,8 +226,18 @@ size_t HardwareSerial::write(uint8_t c)
   // significantly improve the effective datarate at high (>
   // 500kbit/s) bitrates, where interrupt overhead becomes a slowdown.
   if (_tx_buffer_head == _tx_buffer_tail && bit_is_set(*_ucsra, UDRE0)) {
-    *_udr = c;
-    sbi(*_ucsra, TXC0);
+    // If TXC is cleared before writing UDR and the previous byte
+    // completes before writing to UDR, TXC will be set but a byte
+    // is still being transmitted causing flush() to return too soon.
+    // So writing UDR must happen first.
+    // Writing UDR and clearing TC must be done atomically, otherwise
+    // interrupts might delay the TXC clear so the byte written to UDR
+    // is transmitted (setting TXC) before clearing TXC. Then TXC will
+    // be cleared when no bytes are left, causing flush() to hang
+    ATOMIC_BLOCK(ATOMIC_RESTORESTATE) {
+      *_udr = c;
+      *_ucsra = ((*_ucsra) & ((1 << U2X0) | (1 << MPCM0))) | (1 << TXC0);
+    }
     return 1;
   }
   tx_buffer_index_t i = (_tx_buffer_head + 1) % SERIAL_TX_BUFFER_SIZE;
@@ -240,9 +258,14 @@ size_t HardwareSerial::write(uint8_t c)
   }
 
   _tx_buffer[_tx_buffer_head] = c;
-  _tx_buffer_head = i;
-	
-  sbi(*_ucsrb, UDRIE0);
+
+  // make atomic to prevent execution of ISR between setting the
+  // head pointer and setting the interrupt flag resulting in buffer
+  // retransmission
+  ATOMIC_BLOCK(ATOMIC_RESTORESTATE) {
+    _tx_buffer_head = i;
+    sbi(*_ucsrb, UDRIE0);
+  }
   
   return 1;
 }