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Mark Neil - Microprocessor Course

Decoding and Using a 4x4 Keyboard

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The Keyboard

Mark Neil - Microprocessor Course

8 lines corresponding to 4 rows and 4 columns

By Pressing a button youconnect one of the row

lines to one of the column lines.

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The PCB of the Key-Board

Mark Neil - Microprocessor Course

The buttons simply connect one row with

one column!

From this picture youCan see that the keyboard is

all PASSIVE: NoICs no Transistors

4Decoding 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

Mark Neil - Microprocessor Course

0 0 0 0

1011

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Step 2:Reading the ColumnIf 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

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000

0

01 1 1

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Pull-up Resistors on ATmeag128 Ports

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PORTx : OUTPUT Register

DDRx : Direction Reg.

PINx : INPUT (no Register)

each pin in PORTx is independent!

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.

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Reading the Keypad with the ATMega128

Mark Neil - Microprocessor Course

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 4-7Outputs

PO

RTE b

its 0

-3

Inp

uts

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Task Plan - Steps

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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 pressedEcho the PORTE input to PORTB to use the LEDs to do a

quick check of the keyboard. [also output to LCD.]

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How to configure PORTE for Step 1

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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

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Task

Mark Neil - Microprocessor Course

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