Studio 4: Hex to Binary LEDs

Click here to access the repository for this studio.


By the end of studio, you should know:

  • how to read from the Arduino’s Serial Port,
  • how to convert from Hexadecimal to Binary, and
  • how to differentiate between
    • Hex a and ASCII a
    • int 0 and ASCII 0.
  • how to receive user input in Java on your Computer,
  • how to write to the Serial Port in Java on your Computer,

Today’s studio

Today you will be reading a Nibble (also spelled “Nybble”), which is one hexadecimal digit, from your keyboard and displaying it on your Arduino using 4 LEDs to represent the bits of the nibble. So you should be able to type 1-9 and a-f on your computer and see the binary representation show up on the LEDs.


Recap on Hexadecimal

0 0 0000
9 9 1001
10 a 1010
11 b 1011
12 c 1100
13 d 1101
14 e 1110
15 f 1111

Why Hexadecimal? — Humanizing Binary

Most common data types require many binary digits. For example, a Java int is 32-bits wide and uses Two’s Complement. In binary, a -12 would look like 11111111111111111111111111110100, which isn’t very easy to read. (Can you quickly tell if it even has 32 bits?) Since each digit of hexadecimal corresponds to exactly 4 bits of binary, we can break this down into just 8 hex digits: 0xFFFFFFF4, which is much easier to work with. Since it’s easier to read and can still be used to quickly find the value of a particular bit, hexadecimal is often used rather than binary.

Notice that the conversion from hexadecimal to binary (or from binary to hexadecimal) is like a simple substitution cipher. Given a number like: 11111111111111111111111111110100 you:

  1. Break it into groups of 4 bits: 1111 1111 1111 1111 1111 1111 1111 0100
  2. Look up each group of 4 bits in the table and substitute it with the corresponding hexadecimal digit: F F F F F F F 4

If you wanted to convert hex to binary you just use the table in reverse. You’d look up each hexadecimal digit and replace it with the corresponding four binary digits.

The Studio

Part 1: The Arduino End

An empty Arduino sketch has been created in CSE132-studio4 > HexLEDs > HexLEDs.ino. Use it for your Arduino work.

Start by assuming the User’s Input will be sent to the Arduino via the serial port. We can use the Serial Monitor window to emulate the values that will eventually be coming from a Java program on the computer. Notice the text field that can be used for input, like the ‘a’ that is entered here:

Serial Monitor with Echoed Input

In this example the Arduino program has “echoed” the value back to the Serial Monitor.

Reading from Serial Port

We will be using Serial.available() and to read from the serial port.

To Echo Data like in the above example:

  • check to see if there are available bytes to read

      if(Serial.available() > 0) {
  • read available bytes into an appropriate variable

      	myVar =
  • print back to the Serial Monitor


Complete a sketch using the above to merely “echo” whatever characters you type in the input prompt of the Serial Monitor. Test it to be sure you are reading data correctly.

Note: For all Serial Port Matters this reference is your friend.

Displaying on LEDs

Once you can successfully echo back the input using the serial monitor:

  1. Build a circuit that includes four LEDs. Suggested components:
    • 4 LEDs (any color)
    • 4 resistors
    • 5 wires
  2. Update the sketch to test the four LEDs and run it.
  3. Update the sketch to display the appropriate binary value. Note that the input will be an ASCII character, which won’t correspond to the binary representation of a hexadecimal digit. You’ll have to use some code to convert or interpret the meaning of the ASCII value correctly.

Converting ASCII to Binary

Update your sketch to use an int variable to store the result from and print the value. Test your code. What does it print when you enter an ‘0’?

ASCII is also like a simple substitution cipher, where each symbol is replaced by a numeric value. This ASCII table shows symbols in the center column and the decimal value that is substituted for it in the left column. For example, the character ‘0’ is represented by the decimal value of 48 (or 0x30 if we use hex). When the computer sends a ‘0’ (a character) it’s really sending the equivalent of 48 (decimal).

The order and values assigned in the ASCII table weren’t chosen at random. There are several things that are particularly convenient about the orders and values of symbols in the table:

  • The “differences” between lower and upper case. In binary, how different is an ‘A’ from an ‘a’? What about a ‘B’ from a ‘b’? If you notice a pattern you may realize that there’s an easy way to convert from one case to another using bitwise operations. There’s also an easy way to “ignore case” using bitwise operations.
  • The “difference” between digits. How much difference is there between the decimal value of ‘0’ and the decimal value of ‘9’? What would happen if you take the decimal value of ‘5’ and subtract the decimal value of ‘0’? There’s a pattern here that can make conversions of characters to corresponding decimal digits easy. A similar trick can be used with the a-f characters used in hexadecimal.

Complete code that will convert the ASCII value into an appropriate decimal value (that is, an ‘a’ should become the decimal value 10, a ‘7’ should be the decimal value of 7, etc.). (For a challenge, try to accomplish this in four or fewer simple lines of code)

Writing to the Serial Port

  1. CSE132-studio4 > communication contains a partially complete class named SerialComm. It has some TODO items that need to be completed.

  2. This program will send data via the serial port (to your Arduino). In order to do that, you will need to use the SerialPort class—that is available as a library, the javadoc is available here—and connect to the correct port.

The SerialPort methods might throw an Exception. For example, the port you provide could not exist, or your drivers could be set improperly. When code you write calls other code that might do this, you have two options: pass the exception up for someone else to deal with (by specifying that your function throws an exception sometimes, or handle it yourself using a try-catch block. Eclipse will help you write one.

Note the constructor code sets the baud rate (feel free to leave it at the current 9600 baud), the rate at which your serial expects data from your Arduino.

Add a new writeByte() method to SerialComm. It can simply use the writeByte() method of SerialPort. Your writeByte() should also have a debugging capability. When debug is true, writeBtye() should both send the byte to the serial port (using SerialPort’s writeByte()) and display it (in hex) on the console. Use the format <0xff>, where ff is the hex value. In this way, displayed hex values in [] are input bytes and hex values in <> are output bytes (from the perspective of the Java program). Note, we’ll add the capability to handle input bytes next week, for now just worry about the output bytes.

Resolving problems on a Mac

If you are on a Mac and you are having problems, try the following:

Open up Terminal (oh boy)
Type `sudo mkdir /var/lock` (without the quotes) and press enter.
Type `sudo chmod go+rwx /var/lock` (without the quotes) and press enter.
Communication should work now


Make sure you’ve closed all your Java programs with the Terminate button in the console. Make sure you’ve closed Serial Monitor. Make sure you’ve run the Terminal commands in the list above.


See what SerialPort is connecting to— it should match the port name in the Arduino IDE (/dev/cu.usbmodem0000 or something similar on Macs, COM1, COM2, or something similar on PCs).

Get User Input

Open the (in the communication package).

Now that the Arduino is setup to receive user input you need to prepare a Java program to get user input and transmit it to the Arduino. For this studio we will be using the Console panel in Eclipse.

You might have noticed the console is where all of your System.out.print()s are sent:

Console: Hello World

Input can also be entered at the Console:

  • Instead of System.out we use
  • Like most Streams in Java, can be wrapped for increased usability, and performance.
  • Take a look at InputStreamReaders , BufferedReaders , and Scanners to see which wrapper(s) you want to use. You may have code that looks something like:
    Wrapper sys = new Wrapper( new innerWrapper(;
  • Try to “echo” the user input (print what you receive), like:
  • Run your Java Program to see if “echos” the input typed into the console pane in Eclipse.
  • You might be wondering why you’d use wrappers? This StackOverFlow response has a great explanation.

Never trust User Input / Sanitizing Input

Now that your program can receive Input, we must be cautious before sending to the Arduino.

The LEDs can represent 0-9 , a-f

  • What if:
    • the user types “g”
    • the user types “ab”
    • the user types “a03443sadfasd”
    • the user types “” (I.e., just hits enter without typing anything)

Often user input must be “validated” or “sanitized” to remove undesirable values. Both validation and sanitization are about avoiding undesirable values in the input. The term validation usually refers to ensuring input is in a valid format and range and sanitization usually means that no “covert” input is included. Unsanitized input is a prime culprit in many forms of attacks on internet sites (See: Prevent Web Attacks Using Input Sanitization, or Keeping Web Users Safe By Sanitizing Input Data, or Bobby Tables (Bobby Tables Explained)).

Before writing code to validate input you first have to identify what constitutes valid vs. invalid input and then you have to decide how to handle the problem.

For example, we know that with just 4 LEDs the Arduino can only display 1 Nibble (1 hex digit) at a time. If the input is longer than 1 character it’s invalid, but do you…

  • Keep just the first letter or
  • Keep just the last letter or
  • Ignore everything or
  • Raise an error and ask for user input again

Whenever receiving user input try to predict what will happen if they don’t follow your guidelines and plan appropriate responses. Planning responses to invalid input can avoid many run-time errors.

Next, send valid input from the console to the Arduino, and then prompt the user for additional input (i.e., put the prompt to the user and the sending of the input to the Arduino in a loop). Be careful how you take a char type and send only a byte. They are not the same thing in Java.

Finishing up

Check out and get out.

  1. Make sure to commit your workspace

    In Eclipse’s package explorer, any files you have modified since your last commit are prefixed with a >.

    Right-click the outer-most folder (you want to commit everything within), and choose Team>Commit.... Write a helpful message and press OK.

    You can verify that your changes went to the server by opening the repository URL in any web browser.

  2. Show both your programs and your repository to a TA.

Repository structure for this lab:

  • communication/
  • HexLEDs/
    • HexLEDs.ino

Key Concepts

  • Java program
    • Acceptable User Input
      • 1-9 , a-f
      • displays an error message for invalid input and does not pass invalid input to the Arduino.
    • Writing to the Serial port
      • writeByte()
  • Arduino Program
    • Reading from Serial port
      • available()
      • read()
    • Convert ASCII to Binary for the corresponding value
    • Display 4-bit Binary with 4 LEDs