Serial Call and Response (handshaking)

This example demonstrates multi-byte communication from the MSP430 LaunchPad to the computer using a call-and-response (handshaking) method. This sketch sends an ASCII A (byte of value 65) on startup and repeats that until it gets a serial response from the computer. Then it sends three sensor values as single bytes, and waits for another response from the computer. You can use the Energia serial monitor to view the sent data, or it can be read by Processing (see code below).

Hardware Required

  • MSP-EXP430G2 LaunchPad

  • Two potentiometers

  • Momentary switch/button

  • Three 10K ohm resistors

  • Breadboard

  • Hook-up wire

Relevant Groundwork

In this code, we use the function Serial.write(). This function writes the value of what is in the function as binary data to the Serial Monitor. For instance if we put Serial.write(100); into Energia sketch, when we open the Serial monitor, we will see a string of 'd' written on the Serial Monitor which 100 in decimal corresponds to 'd' in ASCII.

Connect analog sensors to analog input pin 0 and 1 with 10K ohm resistors used as voltage dividers. Connect a pushbutton or switch to digital I/O pin 2 with a 10Kohm resistor as a reference to ground.


SerialCallResponse bb

Image developed using Fritzing. For more circuit examples, see the Fritzing project page.


SerialCallResponse schem

Code Explanation


At the start of the program, the Serial Monitor will display a string of 'A' until another ASCII character is written in the Serial Monitor. Once another character is received, the values of the three sensor attached to the LaunchPad will be displayed on the Serial Monitor as ASCII characters.


The Processing code will display a window with a grey circle inside. Changing the sensors will change the position of the grey circle inside the Processing window.


  Serial Call and Response

  This program sends an ASCII A (byte of value 65) on startup
  and repeats that until it gets some data in.
  Then it waits for a byte in the serial port, and
  sends three sensor values whenever it gets a byte in.

  Thanks to Greg Shakar and Scott Fitzgerald for the improvements

  The circuit:
  * potentiometers attached to analog inputs 0 and 1
  * pushbutton attached to digital I/O 11

  Created 26 Sept. 2005
  by Tom Igoe
  modified 24 April 2012
  by Tom Igoe and Scott Fitzgerald
  Modified 19 April 2013
  By Sean Alvarado

  This example code is in the public domain.

int firstSensor = 0; // first analog sensor
int secondSensor = 0; // second analog sensor
int thirdSensor = 0; // digital sensor
int inByte = 0; // incoming serial byte

void setup()
  // start serial port at 9600 bps:

  pinMode(11, INPUT); // digital sensor is on digital pin 11
  establishContact(); // send a byte to establish contact until receiver responds

void loop()
  // if we get a valid byte, read analog ins:
  if (Serial.available() > 0) {
    // get incoming byte:
    inByte =;
    // read first analog input, divide by 4 to make the range 0-255:
    firstSensor = analogRead(A0)/4;
    // delay 10ms to let the ADC recover:
    // read second analog input, divide by 4 to make the range 0-255:
    secondSensor = analogRead(A1)/4;
    // read switch, map it to 0 or 255L
    thirdSensor = map(digitalRead(11), 0, 1, 0, 255);
    // send sensor values:

void establishContact() {
  while (Serial.available() <= 0) {
    Serial.println('A'); // send a capital A

Processing sketch to run with this example:

// This example code is in the public domain.

import processing.serial.*;

int bgcolor; // Background color
int fgcolor; // Fill color
Serial myPort; // The serial port
int[] serialInArray = new int[3]; // Where we'll put what we receive
int serialCount = 0; // A count of how many bytes we receive
int xpos, ypos; // Starting position of the ball
boolean firstContact = false; // Whether we've heard from the microcontroller

void setup() {
  size(256, 256); // Stage size
  noStroke(); // No border on the next thing drawn

  // Set the starting position of the ball (middle of the stage)
  xpos = width/2;
  ypos = height/2;

  // Print a list of the serial ports, for debugging purposes:

  // I know that the first port in the serial list on my mac
  // is always my FTDI adaptor, so I open Serial.list()[0].
  // On Windows machines, this generally opens COM1.
  // Open whatever port is the one you're using.
  String portName = Serial.list()[0];
  myPort = new Serial(this, portName, 9600);

void draw() {
  // Draw the shape
  ellipse(xpos, ypos, 20, 20);

void serialEvent(Serial myPort) {
  // read a byte from the serial port:
  int inByte =;
  // if this is the first byte received, and it's an A,
  // clear the serial buffer and note that you've
  // had first contact from the microcontroller.
  // Otherwise, add the incoming byte to the array:
  if (firstContact == false) {
    if (inByte == 'A') {
      myPort.clear(); // clear the serial port buffer
      firstContact = true; // you've had first contact from the microcontroller
      myPort.write('A'); // ask for more
  else {
    // Add the latest byte from the serial port to array:
    serialInArray[serialCount] = inByte;

    // If we have 3 bytes:
    if (serialCount > 2 ) {
      xpos = serialInArray[0];
      ypos = serialInArray[1];
      fgcolor = serialInArray[2];

      // print the values (for debugging purposes only):
      println(xpos + "\t" + ypos + "\t" + fgcolor);

      // Send a capital A to request new sensor readings:
     // Reset serialCount:
     serialCount = 0;

Working Video

Try it out

See Also

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