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4.2 Using a 4x4 Keypad

In this lesson, we’ll learn how to interface a 4x4 matrix keypad with the Raspberry Pi Pico 2 W to detect which keys are pressed. Matrix keypads are commonly used in devices like calculators, telephones, vending machines, and security systems for numerical input.

4x4 Keypad

Required Components

In this project, we need the following components.

It’s definitely convenient to buy a whole kit, here’s the link:

Name	ITEMS IN THIS KIT	PURCHASE LINK
Pico 2 W Starter Kit	450+	Pico 2 W Kit

You can also buy them separately from the links below.

SN	COMPONENT INTRODUCTION	QUANTITY	PURCHASE LINK
1	Getting to Know Pico 2 W	1
2	Micro USB Cable	1
3	Breadboard	1	BUY
4	Jumper Wires	Several	BUY
5	Resistor	4(10KΩ)	BUY
6	4x4 Keypad	1	BUY

Understanding the 4x4 Keypad

A 4x4 keypad consists of:

16 keys arranged in 4 rows and 4 columns.
8 pins: 4 connected to rows and 4 connected to columns.

When you press a key, it connects a specific row and column, allowing us to identify the key based on the row and column numbers.

Here’s how the keys are arranged:

img_keypad

Schematic

sch_keypad_ar

The rows of the keyboard (G2 ~ G5) are programmed to go high; if one of G6 ~ G9 is read high, then we know which key is pressed.

For example, if G6 is read high, then numeric key 1 is pressed; this is because the control pins of numeric key 1 are G2 and G6, when numeric key 1 is pressed, G2 and G6 will be connected together and G6 is also high.

Wiring

wiring_keypad_ar

Writing the Code

Note

You can open the file 4.2_4x4_keypad.ino under the path of pico-2w-kit-main/arduino/4.2_4x4_keypad.
Or copy this code into Arduino IDE.
Then select the Raspberry Pi Pico board and the correct port before clicking the Upload button.
The Adafruit Keypad library is used here, you can install it from the Library Manager.

#include "Adafruit_Keypad.h"

// Define the number of rows and columns
const byte ROWS = 4;
const byte COLS = 4;

// Define the keymap for the keypad
char keys[ROWS][COLS] = {
  { '1', '2', '3', 'A' },
  { '4', '5', '6', 'B' },
  { '7', '8', '9', 'C' },
  { '*', '0', '#', 'D' }
};

// Connect to the row pinouts of the keypad
byte rowPins[ROWS] = { 2, 3, 4, 5 };

// Connect to the column pinouts of the keypad
byte colPins[COLS] = { 6, 7, 8, 9 };

// Create the Keypad object
Adafruit_Keypad myKeypad = Adafruit_Keypad(makeKeymap(keys), rowPins, colPins, ROWS, COLS);

void setup() {
  // Initialize Serial communication
  Serial.begin(115200);

  // Initialize the keypad
  myKeypad.begin();
}

void loop() {
  // Update the state of keys
  myKeypad.tick();

  // Check if there are any new keypad events
  while (myKeypad.available()) {
    // Read the keypad event
    keypadEvent e = myKeypad.read();

    // Check if the event is a key press
    if (e.bit.EVENT == KEY_JUST_PRESSED) {
      // Print the key value to the Serial Monitor
      Serial.println((char)e.bit.KEY);
    }
  }

  delay(10); // Short delay to improve stability
}

After uploading the code, press any key on the keypad. The corresponding key label (e.g., ‘1’, ‘A’) should appear in the Serial Monitor.

Ensure that each key press is accurately detected and displayed. Test all keys to confirm proper functionality.

Understanding the Code

Including the Library:

This line includes the Adafruit Keypad library, which provides functions to interact with the keypad.
```
#include "Adafruit_Keypad.h"
```

Defining the Keypad Layout:

ROWS and COLS define the dimensions of the keypad. keys is a 2D array representing the label of each key on the keypad.

const byte ROWS = 4;
const byte COLS = 4;

char keys[ROWS][COLS] = {
  { '1', '2', '3', 'A' },
  { '4', '5', '6', 'B' },
  { '7', '8', '9', 'C' },
  { '*', '0', '#', 'D' }
};

Connecting the Keypad to GPIO Pins:

rowPins and colPins are arrays that store the GPIO pins connected to the keypad’s rows and columns, respectively.
```
byte rowPins[ROWS] = { 2, 3, 4, 5 };
byte colPins[COLS] = { 6, 7, 8, 9 };
```
Initializing the Keypad Object:

This line creates an instance of the Adafruit_Keypad class, initializing it with the keymap and pin configurations.
```
Adafruit_Keypad myKeypad = Adafruit_Keypad(makeKeymap(keys), rowPins, colPins, ROWS, COLS);
```

Setup Function:

Initializes serial communication for debugging and starts the keypad.

void setup() {
  Serial.begin(115200);    // Initialize serial communication at 115200 baud
  myKeypad.begin();        // Initialize the keypad
}

Loop Function:

Continuously checks for key events.
When a key is pressed, it prints the key value to the Serial Monitor.

void loop() {
  myKeypad.tick(); // Update the state of keys

  while (myKeypad.available()) {
    keypadEvent e = myKeypad.read(); // Read the keypad event

    if (e.bit.EVENT == KEY_JUST_PRESSED) {
      Serial.println((char)e.bit.KEY); // Print the pressed key
    }
  }
  delay(10); // Short delay to improve stability
}

Further Exploration

Implementing Key Debouncing:

Improve the reliability of key detection by implementing debouncing techniques to filter out false triggers caused by mechanical noise.
Creating a Password Entry System:

Use the keypad to enter a password and control access to certain functionalities in your project.
Integrating with Other Components:

Combine the keypad with LCD displays, LEDs, or buzzers to create more complex user interfaces.
Building a Simple Calculator:

Use the keypad to input numbers and perform basic arithmetic operations displayed on an LCD.

Conclusion

In this lesson, you’ve learned how to interface a 4x4 matrix keypad with the Raspberry Pi Pico using the Adafruit Keypad library. By detecting key presses, you can create interactive projects such as key-based input systems, password entry mechanisms, and more. Understanding how to read and process keypad inputs is essential for building user-friendly interfaces in your electronics projects.