Efficient alternative to SoftwareSerial with attachInterrupt for RX chars, simultaneous RX & TX
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README.md

The NeoSWSerial class is intended as an more-efficient drop-in replacement for the Arduino built-in class SoftwareSerial. If you could use Serial, Serial1, Serial2 or Serial3, you should use NeoHWSerial instead. If you could use an Input Capture pin (ICP1, pins 8 & 9 on an UNO), you should consider NeoICSerial instead.

NeoSWSerial is limited to four baud rates: 9600 (default), 19200, 31250 (MIDI) and 38400.

The class methods are nearly identical to the built-in SoftwareSerial, except for two new methods, attachInterrupt and detachInterrupt:

    typedef void (* isr_t)( uint8_t );
    void attachInterrupt( isr_t fn );
    void detachInterrupt() { attachInterrupt( (isr_t) NULL ); };

  private:
    isr_t  _isr;

There are five, nay, six advantages over SoftwareSerial:

1) It uses much less CPU time.

2) Simultaneous transmit and receive is fully supported.

3) Interrupts are not disabled for the entire RX character time. (They are disabled for most of each TX character time.)

4) It is much more reliable (far fewer receive data errors).

5) Characters can be handled with a user-defined procedure at interrupt time. This should prevent most input buffer overflow problems. Simply register your procedure with the 'NeoSWSerial' instance:

    #include <NeoSWSerial.h>
    NeoSWSerial ss( 4, 3 );
    
    volatile uint32_t newlines = 0UL;
    
    static void handleRxChar( uint8_t c )
    {
      if (c == '\n')
        newlines++;
    }
    
    void setup()
    {
      ss.attachInterrupt( handleRxChar );
      ss.begin( 9600 );
    }

Remember that the registered procedure is called from an interrupt context, and it should return as quickly as possible. Taking too much time in the procedure will cause many unpredictable behaviors, including loss of received data. See the similar warnings for the built-in attachInterrupt for digital pins.

The registered procedure will be called from the ISR whenever a character is received. The received character will not be stored in the rx_buffer, and it will not be returned from read(). Any characters that were received and buffered before attachInterrupt was called remain in rx_buffer, and could be retrieved by calling read().

If attachInterrupt is never called, or it is passed a NULL procedure, the normal buffering occurs, and all received characters must be obtained by calling read().

6) The NeoSWSerial ISRs can be disabled. This can help you avoid linking conflicts with other PinChangeInterrupt libraries, like EnableInterrupt:

void myDeviceISR()
{
  NeoSWSerial::rxISR( *portInputRegister( digitalPinToPort( RX_PIN ) ) );
  // if you know the exact PIN register, you could do this:
  //    NeoSWSerial::rxISR( PIND );
}

void setup()
{
  myDevice.begin( 9600 );
  enableInterrupt( RX_PIN, myDeviceISR, CHANGE );
  enableInterrupt( OTHER_PIN, otherISR, RISING );
}

This class supports the following MCUs: ATtinyx61, ATtinyx4, ATtinyx5, ATmega328P (Pro, UNO, Nano), ATmega32U4 (Micro, Leonardo), ATmega2560 (Mega), ATmega2560RFR2, ATmega1284P and ATmega1286