From 9aa902ef9d272e12306eb9ff7ffae4cdf47c6227 Mon Sep 17 00:00:00 2001
From: Nico <NicoHood@users.noreply.github.com>
Date: Fri, 25 Sep 2015 16:17:24 +0200
Subject: [PATCH 1/3] Fix Hardwareserial sending a byte twice

https://github.com/arduino/Arduino/issues/3745
---
 hardware/arduino/avr/cores/arduino/HardwareSerial.cpp | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/hardware/arduino/avr/cores/arduino/HardwareSerial.cpp b/hardware/arduino/avr/cores/arduino/HardwareSerial.cpp
index 5cd89e5e664..0ea71e63434 100644
--- a/hardware/arduino/avr/cores/arduino/HardwareSerial.cpp
+++ b/hardware/arduino/avr/cores/arduino/HardwareSerial.cpp
@@ -218,8 +218,11 @@ 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)) {
+    uint8_t oldSREG = SREG;
+    cli();
     *_udr = c;
     sbi(*_ucsra, TXC0);
+    SREG = oldSREG;
     return 1;
   }
   tx_buffer_index_t i = (_tx_buffer_head + 1) % SERIAL_TX_BUFFER_SIZE;
@@ -240,9 +243,12 @@ size_t HardwareSerial::write(uint8_t c)
   }
 
   _tx_buffer[_tx_buffer_head] = c;
+  uint8_t oldSREG = SREG;
+  cli();
   _tx_buffer_head = i;
 	
   sbi(*_ucsrb, UDRIE0);
+  SREG = oldSREG;
   
   return 1;
 }

From 30762518f23c5310dac505f9d79ad62556adba9b Mon Sep 17 00:00:00 2001
From: Nico <NicoHood@users.noreply.github.com>
Date: Sat, 26 Sep 2015 12:24:59 +0200
Subject: [PATCH 2/3] Changed Hardwareserial cli() to Atomic Blocks

---
 .../avr/cores/arduino/HardwareSerial.cpp      | 24 +++++++++----------
 1 file changed, 11 insertions(+), 13 deletions(-)

diff --git a/hardware/arduino/avr/cores/arduino/HardwareSerial.cpp b/hardware/arduino/avr/cores/arduino/HardwareSerial.cpp
index 0ea71e63434..483059d7581 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"
@@ -178,13 +179,12 @@ int HardwareSerial::read(void)
 int HardwareSerial::availableForWrite(void)
 {
 #if (SERIAL_TX_BUFFER_SIZE>256)
-  uint8_t oldSREG = SREG;
-  cli();
+  ATOMIC_BLOCK(ATOMIC_RESTORESTATE) {
 #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
   if (head >= tail) return SERIAL_TX_BUFFER_SIZE - 1 - head + tail;
   return tail - head - 1;
@@ -218,11 +218,10 @@ 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)) {
-    uint8_t oldSREG = SREG;
-    cli();
-    *_udr = c;
-    sbi(*_ucsra, TXC0);
-    SREG = oldSREG;
+    ATOMIC_BLOCK(ATOMIC_RESTORESTATE) {
+        *_udr = c;
+        sbi(*_ucsra, TXC0);
+    }
     return 1;
   }
   tx_buffer_index_t i = (_tx_buffer_head + 1) % SERIAL_TX_BUFFER_SIZE;
@@ -243,12 +242,11 @@ size_t HardwareSerial::write(uint8_t c)
   }
 
   _tx_buffer[_tx_buffer_head] = c;
-  uint8_t oldSREG = SREG;
-  cli();
-  _tx_buffer_head = i;
+  ATOMIC_BLOCK(ATOMIC_RESTORESTATE) {
+      _tx_buffer_head = i;
 	
-  sbi(*_ucsrb, UDRIE0);
-  SREG = oldSREG;
+      sbi(*_ucsrb, UDRIE0);
+  }
   
   return 1;
 }

From 7b87a52971815bd479de4613139ed4bdaf01080d Mon Sep 17 00:00:00 2001
From: Nico <NicoHood@users.noreply.github.com>
Date: Sat, 26 Sep 2015 16:47:32 +0200
Subject: [PATCH 3/3] Made Serial.write() buffer comparison atomic

---
 hardware/arduino/avr/cores/arduino/HardwareSerial.cpp | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/hardware/arduino/avr/cores/arduino/HardwareSerial.cpp b/hardware/arduino/avr/cores/arduino/HardwareSerial.cpp
index 483059d7581..6f0c0beb535 100644
--- a/hardware/arduino/avr/cores/arduino/HardwareSerial.cpp
+++ b/hardware/arduino/avr/cores/arduino/HardwareSerial.cpp
@@ -217,7 +217,14 @@ size_t HardwareSerial::write(uint8_t c)
   // to the data register and be done. This shortcut helps
   // 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)) {
+#if (SERIAL_TX_BUFFER_SIZE>256)
+  ATOMIC_BLOCK(ATOMIC_RESTORESTATE) {
+#endif
+    bool emptyBuffer = _tx_buffer_head == _tx_buffer_tail;
+#if (SERIAL_TX_BUFFER_SIZE>256)
+  }
+#endif
+  if (emptyBuffer && bit_is_set(*_ucsra, UDRE0)) {
     ATOMIC_BLOCK(ATOMIC_RESTORESTATE) {
         *_udr = c;
         sbi(*_ucsra, TXC0);