Serial Duplex using an Arduino

Overview

In the first serial lab, you sent data from one sensor to a personal computer. In this lab, you'll send data from multiple sensors to a program on a personal computer. You'll use the data from the sensors to create a pointing-and-selecting device (i.e. a mouse).

1.  Parts

For this lab you'll need:

Solderless breadboard

22-AWG hookup wire

Arduino Microcontroller
module

accelerometer
(or two other analog sensors)

switch

2.  Connect the sensors

Connect two analog sensors to analog pins 0 and 1 like you did in the analog lab. Connect a switch to digital pin 2 like you did in the digital lab.

The photos and schematic in this lab show an accelerometer and a pushbutton. You don't have to use these, though. Use whatever sensors are appropriate to your final application. While you're figuring what sensors to use, use the most convenient sensors you've got in hand; perhaps two potentiometers for the analog sensors and a pushbutton?

Accelerometer and switch on an Arduino

(Diagram made with Fritzing

3.  Read and send the serial data

To begin with, just send the value from one sensor, the first analog sensor (the first axis of the accelerometer in the photos) and divide the output to give a maximum value of 255:

When you open the serial monitor, you should see a number between 0 and 255 scrolling down the window. That's because Serial.println() formats the value it prints as an ASCII-encoded DECimal value, with a linefeed at a carriage return at the end. Also, the serial monitor assumes it should show you the ASCII character corresponding to each byte it receives. Try changing the Serial.println to a Serial.write().

Now you get a range of garbage characters. What's going on?

For example, imagine that analogValue = 32:

• Serial.println(analogValue) results in "32" with a linefeed and carriage return
• Serial.write(analogValue) results in " ", the space character, which has the ASCII value 32.

4.  Send the data in many formats

Try this program and view the results in the Serial Monitor:

You should get output like this:

â	11100010	226	E2	342
á	11100001	225	E1	341
á	11100001	225	E1	341
á	11100001	225	E1	341
à	11100000	224	E0	340
à	11100000	224	E0	340
ß	11011111	223	DF	337
ß	11011111	223	DF	337
ß	11011111	223	DF	337

It's printing the raw binary value, then the ASCII-encoded binary value, then the ASCII-encoded decimal, hexadecimal, and octal values. You may never need all of these differnt formats, but you'll likely need at least the decimal and the raw binary versions at some point.

5.  Send the values for all three sensors

In the first serial lab, you sent one byte representing one sensor's value, over and over. When you're sending multiple sensor values, it gets a little more complicated. You need to a way to know which value represents which sensor. For example, imagine if you used the following loop to send your sensor values:

you'd get a string like this:

452,345,416,234,534,417,325,452,231

There are two ways to get your sensor values in order. You can use punctuation or you can use a call-and-response or handshaking method. Use whichever makes the most sense to you.

6.  Punctuation Method

One way to send the data such that it can be interpreted clearly is to punctuate each set of data uniquely. Just as a sentence ends with a period, you can end your data with a carriage return and a newline. Change the for loop above so that a carriage return and newline are printed at the end of each string of three values.

452,345,416
234,534,417
325,452,231

Write a program that reads the two analog sensors on your board and the one digital switch, and prints them out in this format:

analog1, analog2, switch
analog1, analog2, switch
analog1, analog2, switch

Start by setting up a constant for the switch pin's number. Then in the setup, initialize serial communications at 9600bps, and declare the switch pin as an input.

In the main loop, use a local variable called sensorValue to read each input. Read the two analog inputs first, and print them with a comma after each one. Then read the digital input, and print it with a carriage return and linefeed at the end.

Once you've got a data format, all you have to do is read it in the receiving program. Now write a Processing sketch that reads the data as formatted by the Arduino program above.

First, set up the beginning of your program as you did in the first serial lab, to import the serial library and make a global variable to hold an instance of it. Then in the setup(), print out a list of serial ports, and put the one you want in a string. Then instantiate the serial library in the global variable you made.

Note: you might get an error message when trying to use the Processing Serial Library for the first time. Here are instructions on what to do if this happens.

Add one extra line at the end of the setup() method. This line tells the sketch not to call serialEvent() unless an ASCII newline byte (value 10) comes in the serial port. It will save any other bytes it gets in the serial buffer, so you can read them all at once when the newline arrives:

The draw() method doesn't do anything, just like the first serial lab:

Now your serialEvent() method only occurs when you get a newline, so you can assume that the entire string that came before the newline is in the serial buffer. Read the string, and check to see if it's null. If it isn't, print it out. This way, you'll be sure you've got a full string before you do anything with it.

The Serial.println() in Arduino attaches a newline and a carriage return. Newlines, carriage returns, spaces, and tabs are often called whitespace and there's a command that removed whitespace from a string, called trim(). Replace the println() with the following two lines. The first trims the whitespace characters off. The second splits the string into three separate strings at the commas, the converts those strings to integers:

Next, print out those three integers using a for() loop, like so:

Run the sketch now and you should get output like this:

Now you've got all the sensor values as integers, and you can do whatever you want with them. Use them to move a circle on the screen.

To do that, you'll need a few global variables at the beginning of the sketch:

Add a line at the beginning of the setup() to set the window size:

Put the following lines in the draw method. These will draw the circle using the variables you just declared:

In order to see anything happen, you need to assign the sensor values to these variables. Do this at the end of the serialEvent():

If you run this, you should see the ball moving onscreen whenever you press the switch and move the analog sensors.

If your analog values are greater than 800 or 600, the ball will be offscreen, so you may have to map your sensor range to the screen size. For example, the accelerometer sensor ranges are approximately 430 to 580. Since the screen is 0 to 800 (horizontal) and 0 to 600 (vertical), map the ranges of the sensors to fill the screeen.

7.  Call and Response (Handshaking) Method

Another way to keep multiple bytes of serial data in order is to send one set of values at a time, rather than sending repeatedly. If you use this method, the receiving program has to request new data every time it finishes reading what it's got.

You can convert the punctuation method shown above to a call-and-response method fairly simply. First, modify the Arduino code. Add a new method at the end of the sketch called establishContact(). This method sends out a message on startup until it gets a byte of data from Processing. Later you'll modify the Processing sketch to look for this message and send a response:

To call this, add a line at the end of the setup() method:

Now, modify the loop() by adding an if() statement to look for incoming serial data and read it.

The rest of the Arduino sketch remains the same. When you run this and open the serial monitor, you'll see:

Type any character in the output box and click Send. You'll get a string of sensor values at the end of your hellos:

Type another character and click Send. It doesn't matter what character you send, but the loop will always wait for an incoming byte before sending a new set of sensor values.

Next modify the Processing program. Add a new global variable called firstContact before the setup():

Then modify the serialEvent(). Until that firstContact is true, the serialEvent() will look for the "hello" message from the microcontroller. When it gets the initial message, it will send out a byte to request data. From then on, it will read in the string, parse it, and send a byte to request more data when it's done.

If you did everything right, the ball should move in response to the analog sensors, and appear or disappear when you press the button.

8.  Advantages of Raw Binary vs. ASCII

All the examples shown here sent the sensor values as ASCII-encoded strings. As mentioned above, that means you sent three bytes to send a three-digit value. If that same value was less than 255, you could send it in one raw binary byte. So ASCII is definitely less efficient. However, it's more readable for debugging purposes, and if the receiving program is well-suited to convert strings to numbers, then ASCII is a good way to go. If the receiver's not so good at converting strings to numbers (for example, it's more challenging to read a multiple byte string in Arduino than in Processing) then you may want to send your data as binary values.

9.  Advantages of Punctuation or Call-and-Response

The punctuation method for sending multiple serial values may seem simpler, but it has its limitations. You can't easily use it to send binary values, because you need to have a byte with a unique value for the punctuation. In the example above, you're using the value 10 (ASCII newline) as punctuation, so if you were sending your sensor values as raw bytes, you'd be in trouble when the sensor's value is 10. The receiver would interpret the 10 as punctuation, not as a sensor value. In contrast, call-and-response can be used whether you're sending data as raw binary values or as ASCII-encoded values.

Sometimes the receiver reads serial data slower than the sender sends it. For example, if you have a program that does a lot of graphic work, it may only read serial data every few milliseconds. The serial buffer will get full in that case, you'll notice a lag in response time. This is when it's good to switch to a call-and-response method.

10.  Get creative

This is just a suggestion for a short project. Apply these principles to your media controller or final project, you don't have to do this exact application.

You just duplicated the basic functionality of a mouse; that is, a device with two analog sensors that affect X and Y, and a digital sensor (mouse button). What applications can you think of that could use a better physical interface for a mouse? A video editor that scrubs forward and back when you tilt a wand? An action game that reacts to how hard you hit a punching bag? An instructional presentation that speeds up if you shift in your chair too much? A music program driven by a custom musical instrument that you design?

Create a prototype in Arduino and Processing, or whatever programming environment you choose. Come up with a physical interface that makes it clear what actions map to what movements and actions. Figure out which actions can and should be possible at the same time. Present a working software and hardware model of your idea.