7.19. OneInOneOutASCII Arduino Sketch

This sketch is an Arduino program which acts as an simplified hardware I/O server using a simple readable message protocol. The intent is to provide an easily modified and extended real-time embedded hardware controller which can interface easily with a non-real-time client running on a laptop or Raspberry Pi.

The following documentation was extracted from the OneInOneOutASCII sample sketch and highlights particular functions, variables, and classes within the code.

Note that if your only objective is basic hardware access the Firmata firmware is more efficient and complete. It however is significantly harder to extend, and the binary protocol is harder to debug.

Contents

• OneInOneOutASCII Arduino Sketch
  • Top-Level Functions
  • ASCII Messaging Protocol
  • Full Source Code

7.19.1. Top-Level Functions

void setup()

void loop()

7.19.2. ASCII Messaging Protocol

void parse_input_message(int argc, char * argv)

Process an input message. Unrecognized commands are silently ignored.

Parameters

• argc -

  number of argument tokens

• argv -

  array of pointers to strings, one per token

void hardware_input_poll(void)

Polling function to read and send specific input values at periodic intervals.

void serial_input_poll(void)

Polling function to process messages arriving over the serial port. Each iteration through this polling function processes at most one character. It records the input message line into a buffer while simultaneously dividing it into ‘tokens’ delimited by whitespace. Each token is a string of non-whitespace characters, and might represent either a symbol or an integer. Once a message is complete, parse_input_message() is called.

void send_debug_message(const char * str)

Send a single debugging string to the console.

void send_debug_message(int i)

Send a single debugging integer to the console.

void send_message(const char * command, long value)

Send a single-argument message back to the host.

void send_message(const char * command, long value1, long value2)

Send a two-argument message back to the host.

void user_message_0(char * command)

Convenience function provided to help with extending the messaging protocol; this function receives zero-argument messages which just contain a token as a string, e.g. “stop”. The protocol can also be extended by modifying parse_input_message().

void user_message_1(char * command, int value)

Similar to user_message_0; process one-argument messages with a single value. E.g. “speed 33”.

void user_message_2(char * command, int value1, int value2)

Similar to user_message_0; process two-argument messages. E.g. “pantilt 0 33”.

7.19.3. Full Source Code

The full code is all in one file OneInOneOutASCII.ino.

/// \file OneInOneOutASCII.ino
/// \brief Arduino program to act as an simplified hardware I/O server using a simple message protocol.

/// \copyright Copyright (c) 2014, Garth Zeglin.  All rights reserved. Licensed
///            under the terms of the BSD 3-clause license as included in
///            LICENSE.

/// \details This example is intended as a starting point for adding low-latency
///          hardware-level computing on an Arduino coupled to dynamic code
///          (e.g. Pure Data or Python) on a laptop or Raspberry Pi.  The
///          communications between the Arduino and the host uses a simple
///          message protocol based on lines of ASCII text.

/****************************************************************/
/**** ASCII messaging scheme ************************************/
/****************************************************************/

// The message protocol is based on commands encoded as a sequence of string
// tokens and integers in a line of text.  One line is one message.  All the
// input message formats begin with a string naming a specific command or
// destination followed by one or two argument integers.  The output formats are
// similar but include more general debugging output with a variable number of
// tokens.

// The following message formats are recognized by this program.  Note that not
// all pins or channels are supported, e.g., servo output is only supported on a
// particular pin.

// Command	Arguments		Meaning
// led		<value>			controls the built-in LED, value is 0 or non-zero
// poll         <value>                 set the input polling rate, value is milliseconds 
// pwm		<pin> <value>		PWM control on given pin
// dig		<pin> <value>		digital output on given pin, value is 0 or non-zero
// svo		<pin> <value>	 	hobby servo PWM control signal on given pin, value is angle in degrees

// Additional messages can be added by inserting code in the user_message_#() functions below.

// This program generates the following messages:

// Command	Arguments		Meaning
// dbg		<value-or-token>+	debugging message to print for user
// clk		<microseconds>		Arduino clock time in microseconds
// led		<value>			reply with current LED state
// ana		<channel> <value>	analog input value on given channel, value is 0 to 1023
// dig		<pin> <value>		digital input value on PIN8, value is 0 or 1

/****************************************************************/
/**** Library imports *******************************************/
/****************************************************************/

// Use the Servo library for generating control signals for hobby servomotors.
// Hobby servos require a specific form of pulse-width modulated control signal,
// usually with positive-going pulses between 1 and 2 milliseconds repeated at
// 50 Hz.  Note that this is a significantly different signal than the PWM
// usually required for powering a motor at variable torque.
#include <Servo.h>

/****************************************************************/
/**** Global variables and constants ****************************/
/****************************************************************/

// The baud rate is the number of bits per second transmitted over the serial port.
#define BAUD_RATE 115200

// Interval in milliseconds between input samples.
static unsigned int hardware_polling_interval = 50; // 20 Hz samples to start

// Create a hobby servo control signal generator.
static Servo servo_output;
static const int servo_output_pin = 4;

// The maximum message line length.
#define MAX_LINE_LENGTH 80

// The maximum number of tokens in a single message.
#define MAX_TOKENS 10

// Some version of the Arduino IDE don't correctly define this symbol for an
// Arduino Uno.
#ifndef LED_BUILTIN
#define LED_BUILTIN 13
#endif

/****************************************************************/
/**** Utility functions *****************************************/
/****************************************************************/

/// Send a single debugging string to the console.
static void send_debug_message( const char *str )
{
  Serial.print("dbg ");
  Serial.println( str );
}

/****************************************************************/
/// Send a single debugging integer to the console.
static void send_debug_message( int i )
{
  Serial.print("dbg ");
  Serial.println( i );
}

/****************************************************************/
/// Send a single-argument message back to the host.
static void send_message( const char *command, long value )
{
  Serial.print( command );
  Serial.print( " " );
  Serial.println( value );
}

/****************************************************************/
/// Send a two-argument message back to the host.
static void send_message( const char *command, long value1, long value2 )
{
  Serial.print( command );
  Serial.print( " " );
  Serial.print( value1 );
  Serial.print( " " );
  Serial.println( value2 );
}

/****************************************************************/
// Wrapper on strcmp for clarity of code.  Returns true if strings are
// identical.
static int string_equal( char *str1, char *str2) 
{
  return !strcmp(str1, str2);
}

/****************************************************************/
/****************************************************************/
// Application-specific message processing.  You can customize these functions
// to add additional message types.

/// Convenience function provided to help with extending the messaging protocol;
/// this function receives zero-argument messages which just contain a token as
/// a string, e.g. "stop".  The protocol can also be extended by modifying
/// parse_input_message().
static void user_message_0( char *command )
{
  if (string_equal(command, "stop")) {
    // do something to set the stop state here

    send_debug_message("now stopped");

  } else  if (string_equal(command, "start")) {
    // do something to set the start state here

    send_debug_message("starting");
  }
  // ...
}

/// Similar to user_message_0; process one-argument messages with a single
/// value. E.g. "speed 33".
static void user_message_1( char *command, int value )
{
  if (string_equal(command, "speed")) {
    // do something to set the stop state using 'value'

  }
  // ...
}

/// Similar to user_message_0; process two-argument messages. E.g. "pantilt 0
/// 33".
static void user_message_2( char *command, int value1, int value2 )
{
  if (string_equal(command, "pantilt")) {
    // do something using value1 and value2
  }
  // ...
}

/****************************************************************/
/// Process an input message.  Unrecognized commands are silently ignored.
///   \param argc   number of argument tokens
///   \param argv   array of pointers to strings, one per token
static void parse_input_message(int argc, char *argv[])
{
  // Interpret the first token as a command symbol.
  char *command = argv[0];

  /* -- process zero-argument commands --------------------------- */
  if (argc == 1) {

    // just pass it along
    user_message_0( command );
  }

  /* -- process one-argument commands --------------------------- */
  else if (argc == 2) {
    int value = atoi(argv[1] );

    // Process the 'led' command.
    if ( string_equal( command, "led" )) {
#ifdef LED_BUILTIN
      pinMode( LED_BUILTIN, OUTPUT );
      // turn on the LED if that value is true, then echo it back as a handshake
      digitalWrite(LED_BUILTIN, (value != 0) ? HIGH : LOW);
#endif
      send_message( "led", value );
    }
    else if ( string_equal( command, "poll" )) {
      if (value > 0)  hardware_polling_interval = value;
      else send_debug_message("invalid poll value");
    }

    // else just pass it along
    else user_message_1( command, value );
  } 

  /* -- process two-argument commands --------------------------- */
  else if (argc == 3) {
    int pin   = atoi(argv[1] );
    int value = atoi(argv[2] );


    // Process the 'pwm' command to generate a variable duty-cycle PWM signal on
    // any digital pin.  The value must be between 0 and 255.
    if ( string_equal( command, "pwm" )) {
      analogWrite( pin, value );
      return;
    }

    // Process the 'dig' command to set a pin to output mode and control its level.
    else if ( string_equal( command, "dig" )) {
      pinMode( pin, OUTPUT );
      digitalWrite( pin, value );
      return;

    }

    // Process the 'svo' command to generate a hobby-servo PWM signal on a particular pin.
    // The value must be an angle between 0 and 180.
    else if ( string_equal( command, "svo" )) {
      if (pin == servo_output_pin) {
	servo_output.write( value );
      } else {
	send_debug_message("unsupported servo pin");
      }
      return;
    }

    // else just pass it along
    else user_message_2( command, pin, value );
  }
}

/****************************************************************/
/// Polling function to process messages arriving over the serial port.  Each
/// iteration through this polling function processes at most one character.  It
/// records the input message line into a buffer while simultaneously dividing it
/// into 'tokens' delimited by whitespace.  Each token is a string of
/// non-whitespace characters, and might represent either a symbol or an integer.
/// Once a message is complete, parse_input_message() is called.

static void serial_input_poll(void)
{
  static char input_buffer[ MAX_LINE_LENGTH ];   // buffer for input characters
  static char *argv[MAX_TOKENS];                 // buffer for pointers to tokens
  static int chars_in_buffer = 0;  // counter for characters in buffer
  static int chars_in_token = 0;   // counter for characters in current partially-received token (the 'open' token)
  static int argc = 0;             // counter for tokens in argv
  static int error = 0;            // flag for any error condition in the current message

  // Check if at least one byte is available on the serial input.
  if (Serial.available()) {
    int input = Serial.read();

    // If the input is a whitespace character, end any currently open token.
    if ( isspace(input) ) {
      if ( !error && chars_in_token > 0) {
	if (chars_in_buffer == MAX_LINE_LENGTH) error = 1;
	else {
	  input_buffer[chars_in_buffer++] = 0;  // end the current token
	  argc++;                               // increase the argument count
	  chars_in_token = 0;                   // reset the token state
	}
      }

      // If the whitespace input is an end-of-line character, then pass the message buffer along for interpretation.
      if (input == '\r' || input == '\n') {

	// if the message included too many tokens or too many characters, report an error
	if (error) send_debug_message("excessive input error");

	// else process any complete message
	else if (argc > 0) parse_input_message( argc, argv ); 

	// reset the full input state
	error = chars_in_token = chars_in_buffer = argc = 0;                     
      }
    }

    // Else the input is a character to store in the buffer at the end of the current token.
    else {
      // if beginning a new token
      if (chars_in_token == 0) {

	// if the token array is full, set an error state
	if (argc == MAX_TOKENS) error = 1;

	// otherwise save a pointer to the start of the token
	else argv[ argc ] = &input_buffer[chars_in_buffer];
      }

      // the save the input and update the counters
      if (!error) {
	if (chars_in_buffer == MAX_LINE_LENGTH) error = 1;
	else {
	  input_buffer[chars_in_buffer++] = input;
	  chars_in_token++;
	}
      }
    }
  }
}

/****************************************************************/
/// Polling function to read and send specific input values at periodic
/// intervals.

// N.B. The timing calculation could be improved to reduce jitter.

static void hardware_input_poll(void)
{
  static unsigned long last_time = 0;
  unsigned long now = millis();

  if ((now - last_time) > hardware_polling_interval) {
    last_time = now;
    
    // send A0 analog state
    send_message( "ana", 0, analogRead(0) );

    // send PIN8 digital state
    send_message( "dig", 8, digitalRead(8) );

    // send a time reading
    long clock = micros();
    send_message( "clk", clock );
  }
}

/****************************************************************/
/**** Standard entry points for Arduino system ******************/
/****************************************************************/

/// Standard Arduino initialization function to configure the system.
void setup()
{
  // initialize the Serial port
  Serial.begin( BAUD_RATE );

  // send a message as a diagnostic
  send_debug_message("wakeup");

  // set up the hobby servo control output
  servo_output.attach( servo_output_pin );

  // additional hardware configuration can go here
}

/****************************************************************/
/// Standard Arduino polling function to handle all I/O and periodic processing.
/// This loop should never be allowed to stall or block so that all tasks can be
/// constantly serviced.
void loop()
{
  serial_input_poll();
  hardware_input_poll();

  // other polled tasks can go here
}

/****************************************************************/
/****************************************************************/