Decoding and Using a 4x4 Keyboard

1. Decoding and Using a 4x4 Keyboard Jordan Nash - Microprocessor Course

2. The Keyboard By Pressing a button you connect one of the row lines to one of the columnlines. 8 lines corresponding to 4 rows and 4 columns Jordan Nash - Microprocessor Course

3. The PCB of the Key-Board From this picture you Can see that the keyboard is all PASSIVE: No ICs no Transistors The buttons simply connect one row with one column! Jordan Nash - Microprocessor Course

4. 0 0 0 0 Decoding what key is pressedStep 1: Row We can provide power to the four rows, and readout the data We write 0 to all columns In the example on the right (the 5 is pressed) we will see a 0 on row 2 and 1 on the rest of the rows We then have to determine which column has been pressed 1 0 1 1 Jordan Nash - Microprocessor Course

5. 1 0 1 1 Step 2:Reading the Column If instead of providing power to the rows, we provide power to the columns we can determine which column has a key pressed We write 0 to all the rows In this case we will read 0 on column 1 and 1 on the other rows Hence we know that the ‘5’ was pressed as row 2 and col 1 have a key pressed 0 0 0 0 Jordan Nash - Microprocessor Course

6. Pull-up Resistors on ATmeag128 Ports There are pull-up resistors present on-board. Which allow us to put power on the PORT pins. These need to be enabled by writing high levels to input pins. DDRx: Direction Reg. PORTx: OUTPUT Register PINx: INPUT (no Register) each pin in PORTx is independent! Jordan Nash - Microprocessor Course

7. Reading the Keypad with the ATMega128 PORTE bits 4-7 Outputs We will use PORTE for both Reading the pins and providing the Power. This example is the initial configuration to read the row PORTE bits 0-3 Inputs Jordan Nash - Microprocessor Course

8. Task Plan - Steps • Enable the pull-up resistors on bits 0-3 of PORTE and connect the 4 pins of the keyboard. Configure these pins as inputs, the 4 lines will be held high by the pull-up resistors. • Connect the rest of keyboard to pins 4-7 of PORTE and configure them as outputs. • Write a program that drives the output bits (4-7) of PORTE low all at once • read the 4 PORTE input pins (0-3). • Determine which row has a key pressed • Now configure bits 0-3 as outputs and bits 4-7 as inputs • Determine which column has a key pressed • Determine which key has been pressed • Echo the PORTE input to PORTB to use the LEDs to do a quick check of the keyboard. [also output to LCD.] Jordan Nash - Microprocessor Course

9. How to configure PORTE for Step 1 • To configure PORTE 4-7 as outputs and PORTE 0-3 as inputs • set DDRE:$F0 • Set the PORT REGISTERS in such a way so that the inputs have pull up resistors and the outputs drive low • set PORTE:$0F Jordan Nash - Microprocessor Course

10. Task • Write a routine that decodes the key on the 4x4 Keyboard and tells you what key was pressed on the LCD display • We need a reliable setup where the ATmega128 interprets relatively quickly any key pressed on the keyboard . • Notice that the microprocessor is faster than your finger and can ‘see’ the bouncing up and down that happens when you press the a button of the keyboard. • Write a routine which reliably inputs a key press. • Write a routine which reliably inputs a string of key presses Jordan Nash - Microprocessor Course