Class 5 - 4x4 Matrix Keypad & LCD

Hi buddies!

In our previous session, we saw how to create a counter using an LCD and a single key. In this class, we’ll learn about the working principle of a 4×4 Matrix Keypad.

🎛 What is a 4×4 Matrix Keypad?

A 4×4 matrix keypad contains 16 keys arranged in 4 rows and 4 columns. These keypads are commonly used in password entry systems, calculators, and DIY electronics.

🔍 How It Works

Each key press connects a specific row and column, forming a temporary short circuit. To detect which key is pressed, we perform row-wise scanning.

1. Set columns as outputs and rows as inputs.
2. Drive one column LOW and others HIGH.
3. Read all row lines – if any row reads LOW, a key is pressed in that column.

To detect a press:

• Send 0xF0 to keypad port
• If the read value is still 0xF0, then no key is pressed.
• If a row reads LOW, proceed to scan the key position.

To find the exact key:

1. Send these values to the port one by one:
   • B11111110
   • B11111101
   • B11111011
   • B11110111
2. After each, read the port. If any of these values result in:
   • 00010000
   • 00100000
   • 01000000
   • 10000000

   …perform bitwise & to determine the exact row & column.

Now using the final row and column values, we can look up the correct character from a lookup table.

⚙️ Detecting 16 Keys

By cycling through 4 column states and checking each row, we can identify which of the 16 keys is pressed.

📚 No Need to Do This Manually!

You can simply use the KEYPAD4X4 library included in our BuddyKit tools. Here’s how:

1. In your build.bat, include:
   
   C

   rem ------------------------------------------- rem Configuration set USE=Delay LCD4 KEYPAD4X4 rem Available libraries: Delay LCD4 I2C EEPROM DS1307 SPI LED_7SEGMENT KEYPAD4X4 UART PCF8591 MPU6050 rem -------------------------------------------

   1 2 3 4 5

   rem ------------------------------------------- rem Configuration set USE=Delay LCD4 KEYPAD4X4 rem Available libraries: Delay LCD4 I2C EEPROM DS1307 SPI LED_7SEGMENT KEYPAD4X4 UART PCF8591 MPU6050 rem -------------------------------------------

2. The keypad is assigned to Port 0 by default in the header file. Change it if needed.
3. The key layout is already defined for you, for example:
   
   C

   #define KEY11 '7' #define KEY12 '8' #define KEY13 '9' #define KEY14 '%'

   1 2 3 4

   #define KEY11 '7' #define KEY12 '8' #define KEY13 '9' #define KEY14 '%'

All 16 keys are mapped to characters: numbers and symbols like +, =, X etc.

💻 Sample Code Walkthrough

1. Include the keypad.h header
2. Declare a variable to store the pressed key
3. In the loop, call KEYPAD_GetKey() repeatedly
4. If the return is non-zero, print the key on LCD at 2nd row, 0th column

Simple and efficient keypad integration!