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Note 1: Interfacing a Matrix Keypad

Introduction.

This application note covers the use of matrix-encoded

keypads with PIC microcontrollers. It presents an example program in

Parallax assembly language for reading a 4 x 4 keypad.

Background.

In order to use as few input/output (I/O) pins as

possible, most keypads of eight or more switches are wired in a matrix

arrangement. Instead of interfacing each set of contacts to an I/O pin,

switches are wired to common row and column connections, as shown

in the figure. For a given number of switches, this method can save quite

a few I/O pins:

No. of

Switches

Matrix

(rows x columns)

2x4

3x4

4x4

5x4

I/O Pins

Required

The disadvantage of matrix encoding is that it requires some additional

programming. The listing shows a program that reads a 4 x 4 keypad

using an 8-bit I/O port and presents the number of the key pressed on

a 4-bit port.

How it works.

Code at the beginning of the program sets the lower four

bits of port

(connected to the columns of the keypad) to output, and

the upper four bits (rows) to input. It also sets port

, which will drive

LEDs indicating the binary number of the key pressed, to output.

The routine

scankeys

does the actual work of reading the keypad. It

performs the following basic steps:

• Assert a “1” on the current column.

• Does the “1” appear on the current row?

> No: increment key and try the next row.

> Yes: exit the subroutine.

• Try the next column.

If the first key, key 0, is pressed, the routine exits with a 0 in the variable

key,

because it ends before the first increment instruction. If no key is

pressed, the variable

key

is incremented 16 (010h) times. Therefore, this

PIC16Cxx Applications Handbook 1.1 • Parallax, Inc. • (916) 624-8333 • Page 85

Note 1: Interfacing a Matrix Keypad

number serves as a flag to the calling program that no key has been

pressed.

On the issue of switch debouncing: After a switch is pressed, it may take

20 milliseconds or more for it to settle into its new state. That means that

one key press can register as many repeated presses. The easiest way to

defeat this problem is to read the switches, and then wait a while before

reading them again. That’s the purpose of the

:delay

loop in the main

program.

470

LED

470

RA2

RA1

You may want to add 1k series resistors in

each column (RB0 - RB3). This prevents

direct shorting if two or more buttons are

pressed at the same time.

Vcc

RA3

RTCC

RA0

220 pF

OSC1

10k

MCLR

PIC

OSC2

16C54

Vss

RC/P

Vdd

RB0

RB1

RB2

RB3

RB7

RB6

Vcc

10k

RB5

RB4

Modifications.

In circumstances where electrical noise might be a

problem, Microchip’s data sheet on the PIC indicates that it might be

wise to move the port I/O assignments to the beginning of

scankeys.

The

reason is that electrostatic discharge (ESD) from the user’s fingertips, or

presumably any other strong electrical noise, could corrupt an I/O

control register and switch an input pin to output. This would prevent

the routine from reading one or more rows of the keypad.

Page 86 • PIC16Cxx Applications Handbook 1.1 • Parallax, Inc. • (916) 624-8333

Note 1: Interfacing a Matrix Keypad

A milder zap could conceivably cause a false input to the keypad. Some

routines check for this condition by comparing two or more consecutive

readings of the keys. Unless several readings match, no data is returned

to the main program.

Program listing.

This program may be downloaded from the Parallax

BBS as

KEYPAD

SRC

. You can reach the BBS at (916) 624-7101.

; PROGRAM: KEYPAD

; This program demonstrates a simple method for scanning a matrix-encoded

; keypad. The 4 columns of the pad are connected to

.0 through

.3; 4 rows to

;

.4 through

.7. The scanning subroutine returns the code of key pressed

; (0h—0Fh) in the variable key. If no switch is pressed, the out-of-range value 10h is

; returned in key (this avoids the use of a separate flag variable).

keypad

row1

row2

row3

row4

rb.4

rb.5

rb.6

rb.7

; Variable storage above special-purpose registers.

org

cols

key

index

; Remember to change device info if programming a different PIC.

device

pic16c54,rc_osc,wdt_off,protect_off

reset

start

; Set starting point in program ROM to zero.

org

start

mov

!rb, #11110000b

mov

!ra, #0

:keys

call

scankeys

cje

key, #16, :delay

mov

ra, key

:delay

nop

djnz

index,:delay

goto

:keys

; cols out, rows in

; ra all outputs

; Subroutine to scan the keypad. Assumes that the calling routine will delay long

; enough for debounce.

scankeys

clr

key

PIC16Cxx Applications Handbook 1.1 • Parallax, Inc. • (916) 624-8333 • Page 87

Note 1: Interfacing a Matrix Keypad

clr

mov

setb

keypad

cols,#4

; 4 x 4 keypad

; put a 1 into carry

; On the first time through the following loop, the carry bit (1) is pulled into keypad.

; On subsequent loops, the lone 1 moves across the column outputs.

:scan

keypad

clrb

; follow the 1 with zeros

row1, press

; If a 1 is detected, quit the routine with the current value of key. If row1, column1 is

; pressed, the value of key is 0 (on the first loop). If not, increment key and try the

; next row.

inc

key

row2, press

inc

key

row3, press

inc

key

row4, press

inc

key

djnz

cols, :scan

; Try all 4 columns.

press

ret

; Return with value in key.

; If no key is pressed, the value will be 10h due to execution of the final increment

; instruction. The program should interpret this out-of-range value as ‘no press.’

Page 88 • PIC16Cxx Applications Handbook 1.1 • Parallax, Inc. • (916) 624-8333

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PIC16Cxx Applications Handbook 1.1 • Parallax, Inc. • (916) 624-8333 • Page 89

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