From f00c1c14f307e9152c1bc1e5c0b07413338f844b Mon Sep 17 00:00:00 2001
From: peteruithoven <peter@peteruithoven.nl>
Date: Mon, 13 Nov 2017 17:18:28 +0100
Subject: [PATCH] [SoftSerial] Half duplex support

Squash and rebase of https://github.com/arduino/Arduino/pull/4377
---
 .../SoftwareSerial/src/SoftwareSerial.cpp     | 19 ++++++++++++++++---
 libraries/SoftwareSerial/src/SoftwareSerial.h |  4 +++-
 2 files changed, 19 insertions(+), 4 deletions(-)

diff --git a/libraries/SoftwareSerial/src/SoftwareSerial.cpp b/libraries/SoftwareSerial/src/SoftwareSerial.cpp
index 474fe4a..b83fcf3 100644
--- a/libraries/SoftwareSerial/src/SoftwareSerial.cpp
+++ b/libraries/SoftwareSerial/src/SoftwareSerial.cpp
@@ -246,13 +246,14 @@ ISR(PCINT3_vect, ISR_ALIASOF(PCINT0_vect));
 //
 // Constructor
 //
-SoftwareSerial::SoftwareSerial(uint8_t receivePin, uint8_t transmitPin, bool inverse_logic /* = false */) : 
+SoftwareSerial::SoftwareSerial(uint8_t receivePin, uint8_t transmitPin, bool inverse_logic /* = false */, bool full_duplex /* = true */) : 
   _rx_delay_centering(0),
   _rx_delay_intrabit(0),
   _rx_delay_stopbit(0),
   _tx_delay(0),
   _buffer_overflow(false),
-  _inverse_logic(inverse_logic)
+  _inverse_logic(inverse_logic),
+  _full_duplex(full_duplex)
 {
   setTX(transmitPin);
   setRX(receivePin);
@@ -273,7 +274,11 @@ void SoftwareSerial::setTX(uint8_t tx)
   // output high. Now, it is input with pullup for a short while, which
   // is fine. With inverse logic, either order is fine.
   digitalWrite(tx, _inverse_logic ? LOW : HIGH);
-  pinMode(tx, OUTPUT);
+  if(_full_duplex) 
+    pinMode(tx, OUTPUT);
+  else
+    pinMode(tx, INPUT);
+  _transmitPin = tx;
   _transmitBitMask = digitalPinToBitMask(tx);
   uint8_t port = digitalPinToPort(tx);
   _transmitPortRegister = portOutputRegister(port);
@@ -418,6 +423,9 @@ size_t SoftwareSerial::write(uint8_t b)
     setWriteError();
     return 0;
   }
+  
+  if(!_full_duplex)
+    pinMode(_transmitPin, OUTPUT);
 
   // By declaring these as local variables, the compiler will put them
   // in registers _before_ disabling interrupts and entering the
@@ -461,6 +469,11 @@ size_t SoftwareSerial::write(uint8_t b)
   else
     *reg |= reg_mask;
 
+  if(!_full_duplex){ 
+    pinMode(_transmitPin, INPUT);
+    *reg |= reg_mask; // send 1 
+  }
+  
   SREG = oldSREG; // turn interrupts back on
   tunedDelay(_tx_delay);
   
diff --git a/libraries/SoftwareSerial/src/SoftwareSerial.h b/libraries/SoftwareSerial/src/SoftwareSerial.h
index b1a37c4..63132cc 100644
--- a/libraries/SoftwareSerial/src/SoftwareSerial.h
+++ b/libraries/SoftwareSerial/src/SoftwareSerial.h
@@ -54,6 +54,7 @@ private:
   uint8_t _receivePin;
   uint8_t _receiveBitMask;
   volatile uint8_t *_receivePortRegister;
+  uint8_t _transmitPin;
   uint8_t _transmitBitMask;
   volatile uint8_t *_transmitPortRegister;
   volatile uint8_t *_pcint_maskreg;
@@ -67,6 +68,7 @@ private:
 
   uint16_t _buffer_overflow:1;
   uint16_t _inverse_logic:1;
+  uint16_t _full_duplex:1;
 
   // static data
   static uint8_t _receive_buffer[_SS_MAX_RX_BUFF]; 
@@ -89,7 +91,7 @@ private:
 
 public:
   // public methods
-  SoftwareSerial(uint8_t receivePin, uint8_t transmitPin, bool inverse_logic = false);
+  SoftwareSerial(uint8_t receivePin, uint8_t transmitPin, bool inverse_logic = false, bool full_duplex = true);
   ~SoftwareSerial();
   void begin(long speed);
   bool listen();