SoftwareSerialZero.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 1
#define _DEBUG_PIN1 8
#define _DEBUG_PIN2 9
//
// Includes
//
#include <Arduino.h>
#include "SoftwareSerialZero.h"

//
// Statics
//
SoftwareSerial *SoftwareSerial::active_object = 0;
uint8_t 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.
#if _DEBUG
inline void DebugPulse(uint8_t pin, uint8_t count)
{
  volatile uint32_t *pport = portOutputRegister(digitalPinToPort(pin));

  uint32_t val = *pport;
  while (count--)
  {
    *pport = val | digitalPinToBitMask(pin);
    *pport = val;
  }
}
#else
inline void DebugPulse(uint8_t, uint8_t) {}
#endif

//
// Private methods
//

/* static */
__attribute__ ((section(".ramfunc")))
inline void SoftwareSerial::tunedDelay(uint32_t delay) { 
  if ( delay == 0 )
  {
    return ;
  }
  __asm__ __volatile__ (
    "loop:          \n"
    "sub r0, r0, #1 \n"
    "cmp r0, #0     \n"
    "bne loop         "
  );
}

// This function sets the current object as the "listening"
// one and returns true if it replaces another
bool SoftwareSerial::listen()
{
  if (!_rx_delay_stopbit)
    return false;

  if (active_object != this)
  {
    if (active_object)
      active_object->stopListening();

    _buffer_overflow = false;
    _receive_buffer_head = _receive_buffer_tail = 0;
    active_object = this;

    setRxIntMsk(true);
    return true;
  }

  return false;
}

// Stop listening. Returns true if we were actually listening.
bool SoftwareSerial::stopListening()
{
  if (active_object == this)
  {
    setRxIntMsk(false);
    active_object = NULL;
    return true;
  }
  return false;
}

//
// The receive routine called by the interrupt handler
//
void SoftwareSerial::recv()
{
  DebugPulse(_DEBUG_PIN2, 1);
  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())
  {
    // Disable further interrupts during reception, this prevents
    // triggering another interrupt directly after we return, which can
    // cause problems at higher baudrates.
    setRxIntMsk(false);

    // 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 = 8; i > 0; --i)
    {
      tunedDelay(_rx_delay_intrabit);
      d >>= 1;
      DebugPulse(_DEBUG_PIN2, 1);
      if (rx_pin_read())
        d |= 0x80;
    }
    DebugPulse(_DEBUG_PIN2, 1);
    if (_inverse_logic)
      d = ~d;

    // if buffer full, set the overflow flag and return
    uint8_t next = (_receive_buffer_tail + 1) % _SS_MAX_RX_BUFF;
    if (next != _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 = next;
    }
    else
    {
      DebugPulse(_DEBUG_PIN1, 1);
      _buffer_overflow = true;
    }

    // skip the stop bit
    tunedDelay(_rx_delay_stopbit);
    DebugPulse(_DEBUG_PIN1, 1);

    // Re-enable interrupts when we're sure to be inside the stop bit
    setRxIntMsk(true);

  }
}

uint32_t SoftwareSerial::rx_pin_read()
{
  return *_receivePortRegister & _receiveBitMask;
}

//
// Interrupt handling
//

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

//
// 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)
{
  // First write, then set output. If we do this the other way around,
  // the pin would be output low for a short while before switching to
  // 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);
  _transmitBitMask = digitalPinToBitMask(tx);
  _transmitPortRegister = portOutputRegister(digitalPinToPort(tx));
}

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

uint16_t SoftwareSerial::subtract_cap(uint32_t num, uint32_t sub) {
  if (num > sub)
    return num - sub;
  else
    return 1;
}

//
// Public methods
//

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

  // Precalculate the various delays, in number of 4-cycle delays
  uint32_t bit_delay = VARIANT_MCK / speed / 4;
  
  // Loop/IO overhead- less than 1us
  _tx_delay = subtract_cap(bit_delay, 8);

  // Only setup rx when we have a valid interrupt pin (this should be done better)
  uint8_t in = digitalPinToInterrupt(_receivePin);
  if (in != NOT_AN_INTERRUPT && in != EXTERNAL_INT_NMI){
    // 5us interrupt overhead, 5us interrupt disable?
    _rx_delay_centering = subtract_cap(bit_delay / 2, 10 * VARIANT_MCK/1000000ul / 4 );

    // 3us to account for loop/delay overhead
    _rx_delay_intrabit = subtract_cap(bit_delay, 3 * VARIANT_MCK/1000000ul / 4 );

    // No longer checked? Set to something that seems reasonable
    // Original aim was to be ~3/4 bit time
    _rx_delay_stopbit = subtract_cap(bit_delay * 3 / 4, 3 * VARIANT_MCK/1000000ul / 4);

    attachInterrupt(_receivePin, SoftwareSerial::handle_interrupt, CHANGE);

    tunedDelay(_tx_delay); // if we were low this establishes the end
  }

  listen();
}

void SoftwareSerial::setRxIntMsk(bool enable)
{
  if (enable)
    attachInterrupt(_receivePin, SoftwareSerial::handle_interrupt, CHANGE);
  else
    detachInterrupt(_receivePin);
}

void SoftwareSerial::end()
{
  stopListening();
}


// 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;
  }

  // By declaring these as local variables, the compiler will put them
  // in registers _before_ disabling interrupts and entering the
  // critical timing sections below, which makes it a lot easier to
  // verify the cycle timings
  volatile uint32_t *reg = _transmitPortRegister;
  uint32_t reg_mask = _transmitBitMask;
  uint32_t inv_mask = ~_transmitBitMask;
  bool inv = _inverse_logic;
  uint16_t delay = _tx_delay;

  if (inv)
    b = ~b;

  noInterrupts();  // turn off interrupts for a clean txmit

  // Write the start bit
  if (inv)
    *reg |= reg_mask;
  else
    *reg &= inv_mask;

  tunedDelay(delay);

  // Write each of the 8 bits
  for (uint8_t i = 8; i > 0; --i)
  {
    if (b & 1) // choose bit
      *reg |= reg_mask; // send 1
    else
      *reg &= inv_mask; // send 0

    tunedDelay(delay);
    b >>= 1;
  }

  // restore pin to natural state
  if (inv)
    *reg &= inv_mask;
  else
    *reg |= reg_mask;

  interrupts(); // turn interrupts back on
  
  tunedDelay(_tx_delay);

  return 1;
}

void SoftwareSerial::flush()
{
  // There is no tx buffering, simply return
}

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];
}