SoftwareSerial.cpp

/*
SoftwareSerial.cpp (formerly NewSoftSerial.cpp) - 
Multi-instance software serial library for Arduino/Wiring
-- Interrupt-driven receive and other improvements by ladyada
   (http://ladyada.net)
-- Tuning, circular buffer, derivation from class Print/Stream,
   multi-instance support, porting to 8MHz processors,
   various optimizations, PROGMEM delay tables, inverse logic and 
   direct port writing by Mikal Hart (http://www.arduiniana.org)
-- Pin change interrupt macros by Paul Stoffregen (http://www.pjrc.com)
-- 20MHz processor support by Garrett Mace (http://www.macetech.com)
-- ATmega1280/2560 support by Brett Hagman (http://www.roguerobotics.com/)

This library is free software; you can redistribute it and/or
modify it under the terms of the GNU Lesser General Public
License as published by the Free Software Foundation; either
version 2.1 of the License, or (at your option) any later version.

This library is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
Lesser General Public License for more details.

You should have received a copy of the GNU Lesser General Public
License along with this library; if not, write to the Free Software
Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA  02110-1301  USA

The latest version of this library can always be found at
http://arduiniana.org.
*/

// When set, _DEBUG co-opts pins 11 and 13 for debugging with an
// oscilloscope or logic analyzer.  Beware: it also slightly modifies
// the bit times, so don't rely on it too much at high baud rates
#define _DEBUG 0
#define _DEBUG_PIN1 11
#define _DEBUG_PIN2 13
// 
// Includes
// 
#include <avr/interrupt.h>
#include <avr/pgmspace.h>
#include <Arduino.h>
#include <SoftwareSerial.h>


#if defined(__MK20DX128__) || defined(__MK20DX256__) || defined(__MKL26Z64__) || defined(__MK64FX512__) || defined(__MK66FX1M0__) || defined(__IMXRT1052__) || defined(__IMXRT1062__)


// Teensy LC, 3.0, 3.1, 3.2
#if defined(__MK20DX128__) || defined(__MK20DX256__) || defined(__MKL26Z64__)
  #define SS1_RX  0
  #define SS1_TX  1
  #define SS2_RX  9
  #define SS2_TX 10
  #define SS3_RX  7
  #define SS3_TX  8

// Teensy 3.5, 3.6
#elif defined(__MK64FX512__) || defined(__MK66FX1M0__)
  #define SS1_RX  0
  #define SS1_TX  1
  #define SS2_RX  9
  #define SS2_TX 10
  #define SS3_RX  7
  #define SS3_TX  8
  #define SS4_RX 31
  #define SS4_TX 32
  #define SS5_RX 34
  #define SS5_TX 33
  #define SS6_RX 47
  #define SS6_TX 48

// Teensy 4.0
#elif defined(__IMXRT1062__) && defined(ARDUINO_TEENSY40)
  #define SS1_RX  0
  #define SS1_TX  1
  #define SS2_RX  7
  #define SS2_TX  8
  #define SS3_RX 15
  #define SS3_TX 14
  #define SS4_RX 16
  #define SS4_TX 17
  #define SS5_RX 21
  #define SS5_TX 20
  #define SS6_RX 25
  #define SS6_TX 24
  #define SS7_RX 28
  #define SS7_TX 29

// Teensy 4.1
#elif defined(__IMXRT1062__) && defined(ARDUINO_TEENSY41)
  #define SS1_RX  0
  #define SS1_TX  1
  #define SS2_RX  7
  #define SS2_TX  8
  #define SS3_RX 15
  #define SS3_TX 14
  #define SS4_RX 16
  #define SS4_TX 17
  #define SS5_RX 21
  #define SS5_TX 20
  #define SS6_RX 25
  #define SS6_TX 24
  #define SS7_RX 28
  #define SS7_TX 29
  #define SS8_RX 34
  #define SS8_TX 35

// Teensy MICROMOD
#elif defined(__IMXRT1062__) && defined(ARDUINO_TEENSY_MICROMOD)
  #define SS1_RX  0
  #define SS1_TX  1
  #define SS2_RX 16
  #define SS2_TX 17
  #define SS3_RX 15
  #define SS3_TX 14
  #define SS4_RX  7
  #define SS4_TX  8
  #define SS5_RX 21
  #define SS5_TX 20
  #define SS6_RX 25
  #define SS6_TX 24
  #define SS7_RX 28
  #define SS7_TX 29

#endif

SoftwareSerial::SoftwareSerial(uint8_t rxPin, uint8_t txPin, bool inverse_logic /* = false */)
{
	buffer_overflow = false;

#if defined(SS1_RX) && defined(SS1_TX)
	if (rxPin == SS1_RX && txPin == SS1_TX) {
		port = &Serial1;
		return;
	}
#endif
#if defined(SS2_RX) && defined(SS2_TX)
	if (rxPin == SS2_RX && txPin == SS2_TX) {
		port = &Serial2;
		return;
	}
#endif
#if defined(SS3_RX) && defined(SS3_TX)
	if (rxPin == SS3_RX && txPin == SS3_TX) {
		port = &Serial3;
		return;
	}
#endif
#if defined(SS4_RX) && defined(SS4_TX)
	if (rxPin == SS4_RX && txPin == SS4_TX) {
		port = &Serial4;
		return;
	}
#endif
#if defined(SS5_RX) && defined(SS5_TX)
	if (rxPin == SS5_RX && txPin == SS5_TX) {
		port = &Serial5;
		return;
	}
#endif
#if defined(SS6_RX) && defined(SS6_TX)
	if (rxPin == SS6_RX && txPin == SS6_TX) {
		port = &Serial6;
		return;
	}
#endif
#if defined(SS7_RX) && defined(SS7_TX)
	if (rxPin == SS7_RX && txPin == SS7_TX) {
		port = &Serial7;
		return;
	}
#endif
#if defined(SS8_RX) && defined(SS8_TX)
	if (rxPin == SS8_RX && txPin == SS8_TX) {
		port = &Serial8;
		return;
	}
#endif
	port = NULL;
	txpin = txPin;
	rxpin = rxPin;
#if defined(__IMXRT1052__) || defined(__IMXRT1062__)
	tx_clear_reg = portClearRegister(digitalPinToPort(txPin));
	tx_set_reg = portSetRegister(digitalPinToPort(txPin));
	tx_bitmask = digitalPinToBitMask(digitalPinToPort(txPin));
	rxreg = portInputRegister(digitalPinToPort(rxPin));
#else
	txreg = portOutputRegister(digitalPinToPort(txPin));
	rxreg = portInputRegister(digitalPinToPort(rxPin));
#endif
	cycles_per_bit = 0;
}

void SoftwareSerial::begin(unsigned long speed)
{
	if (port) {
		port->begin(speed);
	} else {
		rx_head = 0;
		rx_tail = 0;
		cycles_per_bit = (uint32_t)(F_CPU + speed / 2) / speed;
		microseconds_per_bit = (float)cycles_per_bit / (float)(F_CPU / 1000000);
		// TODO: latency estimate could be better tuned to each board
		const float latency = 900.0f / (float)(F_CPU / 1000000);
		microseconds_start = microseconds_per_bit * 1.5f - latency;
		ARM_DEMCR |= ARM_DEMCR_TRCENA;
		ARM_DWT_CTRL |= ARM_DWT_CTRL_CYCCNTENA;
		pinMode(txpin, OUTPUT);
		digitalWrite(txpin, HIGH);
		pinMode(rxpin, INPUT_PULLUP);
		delayMicroseconds(5); // allow time for pullup -> logic high
		attachInterrupt(digitalPinToInterrupt(rxpin), start_bit_falling_edge, FALLING);
		active_object = this;
		//Serial.printf("begin bitbang, rxpin=%u, txpin=%u\n", rxpin, txpin);
	}
}

void SoftwareSerial::end()
{
	if (port) {
		port->end();
		port = NULL;
	} else {
		active_object = NULL;
		detachInterrupt(rxpin);
		data_bit_timer.end();
		pinMode(txpin, INPUT);
		pinMode(rxpin, INPUT);
	}
	cycles_per_bit = 0;
}

// The worst case expected length of any interrupt routines.  If an
// interrupt runs longer than this number of cycles, it can disrupt
// the transmit waveform.  Increasing this number causes SoftwareSerial
// to hog the CPU longer, delaying all interrupt response for other
// libraries, so this should be made as small as possible but still
// ensure accurate transmit waveforms.
#define WORST_INTERRUPT_CYCLES 360

static void wait_for_target(uint32_t begin, uint32_t target)
{
	if (target - (ARM_DWT_CYCCNT - begin) > WORST_INTERRUPT_CYCLES+20) {
		uint32_t pretarget = target - WORST_INTERRUPT_CYCLES;
		//digitalWriteFast(12, HIGH);
		interrupts();
		while (ARM_DWT_CYCCNT - begin < pretarget) ; // wait
		noInterrupts();
		//digitalWriteFast(12, LOW);
	}
	while (ARM_DWT_CYCCNT - begin < target) ; // wait
}

size_t SoftwareSerial::write(uint8_t b)
{
	elapsedMicros elapsed;
	uint32_t target;
	uint8_t mask;
	uint32_t begin_cycle;

	// use hardware serial, if possible
	if (port) return port->write(b);
	if (cycles_per_bit == 0) return 0;
	ARM_DEMCR |= ARM_DEMCR_TRCENA;
	ARM_DWT_CTRL |= ARM_DWT_CTRL_CYCCNTENA;
	// start bit
	target = cycles_per_bit;
	noInterrupts();
	begin_cycle = ARM_DWT_CYCCNT;
	tx0();
	wait_for_target(begin_cycle, target);
	// 8 data bits
	for (mask = 1; mask; mask <<= 1) {
		if (b & mask) {
			tx1();
		} else {
			tx0();
		}
		target += cycles_per_bit;
		wait_for_target(begin_cycle, target);
	}
	// stop bit
	tx1();
	interrupts();
	target += cycles_per_bit;
	while (ARM_DWT_CYCCNT - begin_cycle < target) ; // wait
	return 1;
}

void SoftwareSerial::flush()
{
	if (port) port->flush();
}



SoftwareSerial * SoftwareSerial::active_object = NULL;

void SoftwareSerial::start_bit_falling_edge()
{
	if (active_object) active_object->start_bit_begin();
}

void SoftwareSerial::data_bit_sampling_timer()
{
	if (active_object) active_object->data_bit_sample();
}


void SoftwareSerial::start_bit_begin()
{
	//digitalWriteFast(12, HIGH);
	if (data_bit_timer.begin(data_bit_sampling_timer, microseconds_start)) {
		data_bit_timer.update(microseconds_per_bit);
		detachInterrupt(rxpin);
		rxcount = 0;
		rxbyte = 0;
	} else {
		// TODO: should we somehow report error allocating IntervalTimer?
	}
	//digitalWriteFast(12, LOW);
}

void SoftwareSerial::data_bit_sample()
{
	//digitalWriteFast(12, HIGH);
	if (digitalRead(rxpin) == HIGH) rxbyte |= (1 << rxcount);
	rxcount = rxcount + 1;
	if (rxcount == 8) { // last data bit
		uint16_t head = rx_head + 1;
		if (head >= _SS_MAX_RX_BUFF) head = 0;
		if (head != rx_tail) {
			rx_buffer[head] = rxbyte;
			rx_head = head;
		}
	}
	if (rxcount >= 9) { // stop bit
		data_bit_timer.end();
		attachInterrupt(digitalPinToInterrupt(rxpin), start_bit_falling_edge, FALLING);
	}
	//digitalWriteFast(12, LOW);
}



int SoftwareSerial::available()
{
	if (port) return port->available();
	uint16_t head, tail;
	head = rx_head;
	tail = rx_tail;
	if (head >= tail) return head - tail;
	return _SS_MAX_RX_BUFF + head - tail;
}

int SoftwareSerial::peek()
{
	if (port) return port->peek();
	uint16_t head, tail;
	head = rx_head;
	tail = rx_tail;
	if (head == tail) return -1;
	if (++tail >= _SS_MAX_RX_BUFF) tail = 0;
	return rx_buffer[tail];
}

int SoftwareSerial::read()
{
	if (port) return port->read();
	uint16_t head, tail;
	head = rx_head;
	tail = rx_tail;
	if (head == tail) return -1;
	if (++tail >= _SS_MAX_RX_BUFF) tail = 0;
	uint8_t n = rx_buffer[tail];
	rx_tail = tail;
	return n;
}


#else

//
// Lookup table
//
typedef struct _DELAY_TABLE
{
  long baud;
  unsigned short rx_delay_centering;
  unsigned short rx_delay_intrabit;
  unsigned short rx_delay_stopbit;
  unsigned short tx_delay;
} DELAY_TABLE;

#if F_CPU == 16000000

static const DELAY_TABLE PROGMEM table[] = 
{
  //  baud    rxcenter   rxintra    rxstop    tx
  { 115200,   1,         17,        17,       12,    },
  { 57600,    10,        37,        37,       33,    },
  { 38400,    25,        57,        57,       54,    },
  { 31250,    31,        70,        70,       68,    },
  { 28800,    34,        77,        77,       74,    },
  { 19200,    54,        117,       117,      114,   },
  { 14400,    74,        156,       156,      153,   },
  { 9600,     114,       236,       236,      233,   },
  { 4800,     233,       474,       474,      471,   },
  { 2400,     471,       950,       950,      947,   },
  { 1200,     947,       1902,      1902,     1899,  },
  { 600,      1902,      3804,      3804,     3800,  },
  { 300,      3804,      7617,      7617,     7614,  },
};

const int XMIT_START_ADJUSTMENT = 5;

#elif F_CPU == 8000000

static const DELAY_TABLE table[] PROGMEM = 
{
  //  baud    rxcenter    rxintra    rxstop  tx
  { 115200,   1,          5,         5,      3,      },
  { 57600,    1,          15,        15,     13,     },
  { 38400,    2,          25,        26,     23,     },
  { 31250,    7,          32,        33,     29,     },
  { 28800,    11,         35,        35,     32,     },
  { 19200,    20,         55,        55,     52,     },
  { 14400,    30,         75,        75,     72,     },
  { 9600,     50,         114,       114,    112,    },
  { 4800,     110,        233,       233,    230,    },
  { 2400,     229,        472,       472,    469,    },
  { 1200,     467,        948,       948,    945,    },
  { 600,      948,        1895,      1895,   1890,   },
  { 300,      1895,       3805,      3805,   3802,   },
};

const int XMIT_START_ADJUSTMENT = 4;

#elif F_CPU == 20000000

// 20MHz support courtesy of the good people at macegr.com.
// Thanks, Garrett!

static const DELAY_TABLE PROGMEM table[] =
{
  //  baud    rxcenter    rxintra    rxstop  tx
  { 115200,   3,          21,        21,     18,     },
  { 57600,    20,         43,        43,     41,     },
  { 38400,    37,         73,        73,     70,     },
  { 31250,    45,         89,        89,     88,     },
  { 28800,    46,         98,        98,     95,     },
  { 19200,    71,         148,       148,    145,    },
  { 14400,    96,         197,       197,    194,    },
  { 9600,     146,        297,       297,    294,    },
  { 4800,     296,        595,       595,    592,    },
  { 2400,     592,        1189,      1189,   1186,   },
  { 1200,     1187,       2379,      2379,   2376,   },
  { 600,      2379,       4759,      4759,   4755,   },
  { 300,      4759,       9523,      9523,   9520,   },
};

const int XMIT_START_ADJUSTMENT = 6;

#else

#error This version of SoftwareSerial supports only 20, 16 and 8MHz processors

#endif

//
// Statics
//
SoftwareSerial *SoftwareSerial::active_object = 0;
char SoftwareSerial::_receive_buffer[_SS_MAX_RX_BUFF]; 
volatile uint8_t SoftwareSerial::_receive_buffer_tail = 0;
volatile uint8_t SoftwareSerial::_receive_buffer_head = 0;

//
// Debugging
//
// This function generates a brief pulse
// for debugging or measuring on an oscilloscope.
inline void DebugPulse(uint8_t pin, uint8_t count)
{
#if _DEBUG
  volatile uint8_t *pport = portOutputRegister(digitalPinToPort(pin));

  uint8_t val = *pport;
  while (count--)
  {
    *pport = val | digitalPinToBitMask(pin);
    *pport = val;
  }
#endif
}

//
// Private methods
//

/* static */ 
inline void SoftwareSerial::tunedDelay(uint16_t delay) { 
  uint8_t tmp=0;

  asm volatile("sbiw    %0, 0x01 \n\t"
    "ldi %1, 0xFF \n\t"
    "cpi %A0, 0xFF \n\t"
    "cpc %B0, %1 \n\t"
    "brne .-10 \n\t"
    : "+r" (delay), "+a" (tmp)
    : "0" (delay)
    );
}

// This function sets the current object as the "listening"
// one and returns true if it replaces another 
bool SoftwareSerial::listen()
{
  if (active_object != this)
  {
    _buffer_overflow = false;
    uint8_t oldSREG = SREG;
    cli();
    _receive_buffer_head = _receive_buffer_tail = 0;
    active_object = this;
    SREG = oldSREG;
    return true;
  }

  return false;
}

//
// The receive routine called by the interrupt handler
//
void SoftwareSerial::recv()
{

#if GCC_VERSION < 40302
// Work-around for avr-gcc 4.3.0 OSX version bug
// Preserve the registers that the compiler misses
// (courtesy of Arduino forum user *etracer*)
  asm volatile(
    "push r18 \n\t"
    "push r19 \n\t"
    "push r20 \n\t"
    "push r21 \n\t"
    "push r22 \n\t"
    "push r23 \n\t"
    "push r26 \n\t"
    "push r27 \n\t"
    ::);
#endif  

  uint8_t d = 0;

  // If RX line is high, then we don't see any start bit
  // so interrupt is probably not for us
  if (_inverse_logic ? rx_pin_read() : !rx_pin_read())
  {
    // Wait approximately 1/2 of a bit width to "center" the sample
    tunedDelay(_rx_delay_centering);
    DebugPulse(_DEBUG_PIN2, 1);

    // Read each of the 8 bits
    for (uint8_t i=0x1; i; i <<= 1)
    {
      tunedDelay(_rx_delay_intrabit);
      DebugPulse(_DEBUG_PIN2, 1);
      uint8_t noti = ~i;
      if (rx_pin_read())
        d |= i;
      else // else clause added to ensure function timing is ~balanced
        d &= noti;
    }

    // skip the stop bit
    tunedDelay(_rx_delay_stopbit);
    DebugPulse(_DEBUG_PIN2, 1);

    if (_inverse_logic)
      d = ~d;

    // if buffer full, set the overflow flag and return
    if ((_receive_buffer_tail + 1) % _SS_MAX_RX_BUFF != _receive_buffer_head) 
    {
      // save new data in buffer: tail points to where byte goes
      _receive_buffer[_receive_buffer_tail] = d; // save new byte
      _receive_buffer_tail = (_receive_buffer_tail + 1) % _SS_MAX_RX_BUFF;
    } 
    else 
    {
#if _DEBUG // for scope: pulse pin as overflow indictator
      DebugPulse(_DEBUG_PIN1, 1);
#endif
      _buffer_overflow = true;
    }
  }

#if GCC_VERSION < 40302
// Work-around for avr-gcc 4.3.0 OSX version bug
// Restore the registers that the compiler misses
  asm volatile(
    "pop r27 \n\t"
    "pop r26 \n\t"
    "pop r23 \n\t"
    "pop r22 \n\t"
    "pop r21 \n\t"
    "pop r20 \n\t"
    "pop r19 \n\t"
    "pop r18 \n\t"
    ::);
#endif
}

void SoftwareSerial::tx_pin_write(uint8_t pin_state)
{
  if (pin_state == LOW)
    *_transmitPortRegister &= ~_transmitBitMask;
  else
    *_transmitPortRegister |= _transmitBitMask;
}

uint8_t SoftwareSerial::rx_pin_read()
{
  return *_receivePortRegister & _receiveBitMask;
}

//
// Interrupt handling
//

/* static */
inline void SoftwareSerial::handle_interrupt()
{
  if (active_object)
  {
    active_object->recv();
  }
}

#if defined(PCINT0_vect)
ISR(PCINT0_vect)
{
  SoftwareSerial::handle_interrupt();
}
#endif

#if defined(PCINT1_vect)
ISR(PCINT1_vect)
{
  SoftwareSerial::handle_interrupt();
}
#endif

#if defined(PCINT2_vect)
ISR(PCINT2_vect)
{
  SoftwareSerial::handle_interrupt();
}
#endif

#if defined(PCINT3_vect)
ISR(PCINT3_vect)
{
  SoftwareSerial::handle_interrupt();
}
#endif

//
// Constructor
//
SoftwareSerial::SoftwareSerial(uint8_t receivePin, uint8_t transmitPin, bool inverse_logic /* = false */) : 
  _rx_delay_centering(0),
  _rx_delay_intrabit(0),
  _rx_delay_stopbit(0),
  _tx_delay(0),
  _buffer_overflow(false),
  _inverse_logic(inverse_logic)
{
  setTX(transmitPin);
  setRX(receivePin);
}

//
// Destructor
//
SoftwareSerial::~SoftwareSerial()
{
  end();
}

void SoftwareSerial::setTX(uint8_t tx)
{
  pinMode(tx, OUTPUT);
  digitalWrite(tx, HIGH);
  _transmitBitMask = digitalPinToBitMask(tx);
  uint8_t port = digitalPinToPort(tx);
  _transmitPortRegister = portOutputRegister(port);
}

void SoftwareSerial::setRX(uint8_t rx)
{
  pinMode(rx, INPUT);
  if (!_inverse_logic)
    digitalWrite(rx, HIGH);  // pullup for normal logic!
  _receivePin = rx;
  _receiveBitMask = digitalPinToBitMask(rx);
  uint8_t port = digitalPinToPort(rx);
  _receivePortRegister = portInputRegister(port);
}

//
// Public methods
//

void SoftwareSerial::begin(long speed)
{
  _rx_delay_centering = _rx_delay_intrabit = _rx_delay_stopbit = _tx_delay = 0;

  for (unsigned i=0; i<sizeof(table)/sizeof(table[0]); ++i)
  {
    long baud = pgm_read_dword(&table[i].baud);
    if (baud == speed)
    {
      _rx_delay_centering = pgm_read_word(&table[i].rx_delay_centering);
      _rx_delay_intrabit = pgm_read_word(&table[i].rx_delay_intrabit);
      _rx_delay_stopbit = pgm_read_word(&table[i].rx_delay_stopbit);
      _tx_delay = pgm_read_word(&table[i].tx_delay);
      break;
    }
  }

  // Set up RX interrupts, but only if we have a valid RX baud rate
  if (_rx_delay_stopbit)
  {
    if (digitalPinToPCICR(_receivePin))
    {
      *digitalPinToPCICR(_receivePin) |= _BV(digitalPinToPCICRbit(_receivePin));
      *digitalPinToPCMSK(_receivePin) |= _BV(digitalPinToPCMSKbit(_receivePin));
    }
    tunedDelay(_tx_delay); // if we were low this establishes the end
  }

#if _DEBUG
  pinMode(_DEBUG_PIN1, OUTPUT);
  pinMode(_DEBUG_PIN2, OUTPUT);
#endif

  listen();
}

void SoftwareSerial::end()
{
  if (digitalPinToPCMSK(_receivePin))
    *digitalPinToPCMSK(_receivePin) &= ~_BV(digitalPinToPCMSKbit(_receivePin));
}


// Read data from buffer
int SoftwareSerial::read()
{
  if (!isListening())
    return -1;

  // Empty buffer?
  if (_receive_buffer_head == _receive_buffer_tail)
    return -1;

  // Read from "head"
  uint8_t d = _receive_buffer[_receive_buffer_head]; // grab next byte
  _receive_buffer_head = (_receive_buffer_head + 1) % _SS_MAX_RX_BUFF;
  return d;
}

int SoftwareSerial::available()
{
  if (!isListening())
    return 0;

  return (_receive_buffer_tail + _SS_MAX_RX_BUFF - _receive_buffer_head) % _SS_MAX_RX_BUFF;
}

size_t SoftwareSerial::write(uint8_t b)
{
  if (_tx_delay == 0) {
    setWriteError();
    return 0;
  }

  uint8_t oldSREG = SREG;
  cli();  // turn off interrupts for a clean txmit

  // Write the start bit
  tx_pin_write(_inverse_logic ? HIGH : LOW);
  tunedDelay(_tx_delay + XMIT_START_ADJUSTMENT);

  // Write each of the 8 bits
  if (_inverse_logic)
  {
    for (byte mask = 0x01; mask; mask <<= 1)
    {
      if (b & mask) // choose bit
        tx_pin_write(LOW); // send 1
      else
        tx_pin_write(HIGH); // send 0
    
      tunedDelay(_tx_delay);
    }

    tx_pin_write(LOW); // restore pin to natural state
  }
  else
  {
    for (byte mask = 0x01; mask; mask <<= 1)
    {
      if (b & mask) // choose bit
        tx_pin_write(HIGH); // send 1
      else
        tx_pin_write(LOW); // send 0
    
      tunedDelay(_tx_delay);
    }

    tx_pin_write(HIGH); // restore pin to natural state
  }

  SREG = oldSREG; // turn interrupts back on
  tunedDelay(_tx_delay);
  
  return 1;
}

void SoftwareSerial::flush()
{
  if (!isListening())
    return;

  uint8_t oldSREG = SREG;
  cli();
  _receive_buffer_head = _receive_buffer_tail = 0;
  SREG = oldSREG;
}

int SoftwareSerial::peek()
{
  if (!isListening())
    return -1;

  // Empty buffer?
  if (_receive_buffer_head == _receive_buffer_tail)
    return -1;

  // Read from "head"
  return _receive_buffer[_receive_buffer_head];
}

#endif