working priciple of keypad and its interfacing

7/28/2019 working priciple of Keypad and its Interfacing

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Introduction

Keypads are a part of HMI or Human Machine Interface and play really important role in a small embedded system where human interaction orhuman input is needed. Martix keypads are well known for their simple architecture and ease of interfacing with any microcontroller. In thispart of tutorial we will learn how to interface a 4x4 matrix keypad with AVR and 8051 microcontroller. Also we will see how to program then inAssembly and C.

Constructing a Matrix KeypadConstuction of a keypad is really simple. As per the outline shown in the figure below we have four rows and four columns. In between eachoverlapping row and column line there is a key.

So keeping this outline we can constuct a keypad using simple SPST Switches as shown below:

Now our keypad is ready, all we have to do is connect the rows and columns to a port of microcontroller and program the controller to read

the input.

Scanning a Matrix Keypad

There are many methods depending on how you connect your keypad with your controller, but the basic logic is same. We make the coloums

as i/p and we drive the rows making them o/p, this whole procedure of reading the keyboard is called scanning.

In order to detect which key is pressed from the matrix, we make row lines low one by one and read the coloums. Lets say we first make Row1

low, then read the columns. If any of the key in row1 is pressed will make the corrosponding column as low i.e if second key is pressed in

Row1, then column2 will give low. So we come to know that key 2 of Row1 is pressed. This is how scanning is done.

So to scan the keypad completely, we need to make rows low one by one and read the columns. If any of the button is pressed in a row, it will


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take the corrosponding column to a low state which tells us that a key is pressed in that row. If button 1 of a row is pressed then Column 1 will

become low, if button 2 then column2 and so on...

Martix Keypad Interfacing with Microcontrollers: Connection Diagram

Keypad Connections with AVR Microcontroller

[lightbox=keypad-avr.gif|Keypad connections with AVR Microcontroller|Keypad Interfacing]

[/lightbox]Please click on the image to enlarge it

Circuit shown above is for demonstration and does not include any reset and crystal circuit. For practical use you need to have a reset circuitand crystal.

Keypad Connections with 8051 Microcontroller

[lightbox=keypad-8051.gif|Keypad connections with 8051 Microcontroller|Keypad Interfacing]

[/lightbox]Please click on the image to enlarge it

Circuit shown above is for demonstration and does not include any reset and crystal circuit. For practical use you need to have a reset circuit


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and crystal.

Introduction | 8051 Program

Programming AVR Microcontroller

AVR Assembly Programming for 4x4 Keypad Matrix

CODE:

.include"8515def.inc"

.equ col1 = PINA0

.equ col2 = PINA1

.equ col3 = PINA2

.equ col4 = PINA3

.def keyval = r16

.def temp = r17

.def flags = r18

.equ keyport = PORTA.equ pressed = 0

key_init: ldi keyval,$F0 ;Make Cols as i/p outDDRA, keyval ;and Rows as o/p ldi keyval,$0F ;Enable pullups out keyport, keyval ;on columns ret

get_key: ldi keyval,$0 ;Scanning Row1 ldi temp,$7F ;Make Row1 low out keyport,temp ;Send to keyport

rcall read_col ;Read Columns

sbrc flags,pressed ;If key pressed rjmp done ;Exit the routine

ldi keyval,$4 ;Scanning Row2 ldi temp,$BF ;Make Row2 Low out keyport,temp ;Send to keyport rcall read_col ;Read Columns

sbrc flags,pressed ;If key pressed rjmp done ;Exit from routine

ldi keyval,$8 ;Scanning Row3 ldi temp,$DF ;Make Row3 Low out keyport,temp ;Send to keyport rcall read_col ;Read columns

sbrc flags,pressed ;If key pressed rjmp done ;Exit the routine

ldi keyval,$C ;Scanning Row4 ldi temp,$EF ;Make Row4 Low
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out keyport,temp ;send to keyport rcall read_col ;Read columns

done: ret

read_col: cbr flags,(1


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Programming AVR in C for 4x4 Keypad Matrix

CODE:

#include // Include file for AVR#define keyport PORTA //Keypad Port#define keyportddr DDRA //Data Direction Register#define keyportpin PINA //Keypad Port Pins

#define col1 PA0 //Column1 PortA.0#define col2 PA1 //Column2 PortA.1#define col3 PA2 //Column3 PortA.2#define col4 PA3 //Column4 PortA.3

#define TRUE 1#define FALSE 0

unsignedchar keyval; //A variable

/*+---------------------------------------+

| Prototype: void key_init(void); || Return Type: void || Arguments: None || Description: Initialize ports and || Keypad. |+---------------------------------------+*/void key_init(){

keyportddr =0xF0;keyport =0x0F;

}

/*+-----------------------------------------------+

| Prototype: unsigned char get_key(void); || Return Type: unsigned char || Arguments: None || Description: To read key from the keypad |+-----------------------------------------------+*/unsignedchar get_key(){ unsignedchar i,key=1; for(i=0;i>i); //Make rows low one by one if(!(keyportpin &(1


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return key; //return key value } if(!(keyportpin &(1


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8051 Assembly Program for 4x4 Keypad Matrix

CODE:

keyport equP2 ;Keypad port connected herecol1 equP2.0 ;Column 1

col2 equP2.1 ;Column 2col3 equP2.2 ;Column 3col4 equP2.3 ;Column 4

keyval equ30H ;To store key numberpressed bit0H ;Flag

key_init: mov keyport,#0FH ;Make rows as o/p and col as i/p ret

get_key: mov keyval,#0 ;reset the number mov keyport,#7FH ;make Row1 low acall read_col ;read columns

jb pressed, done ;check if flag is set

mov keyval,#4 ;if not then read next row mov keyport,#0BFH ;make Row2 low acall read_col ;read columns


mov keyval,#8 ;if not then read next row mov keyport,#0DFH ;make row3 low acall read_col ;read columns


mov keyval,#12 ;if not read row4 mov keyport,#0EFH ;make row4 low acall read_col ;read columns

done: ret

read_col: ;read columns routine clr pressed ;reset the flag

jb col1, nextcol ;check if first key is pressed jnb col1,$ ;if yes then wait for key release

setb pressed ;set the flag ret

nextcol: ;read col2 jb col2, nextcol1 ;check if second key is pressed jnb col2,$ ;if yes then wait for key release inc keyval ;its key number 2 setb pressed ;set the flag


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ret

nextcol1: ;read col3 jb col3, nextcol2 ;check if third key is pressed jnb col3,$ ;if yes then wait for key release inc keyval ;its key 3 inc keyval setb pressed ;set the flag ret

nextcol2: ;read column 4 jb col4,exit ;check if fourth key pressed jnb col4,$ ;if yes then wait for key release inc keyval ;its key 4 inc keyval inc keyval setb pressed ;set the flag ret

exit: ;if no key is pressed

clr pressed ;clr the flag clr keyval ;reset the number ret

end

C Program for 4x4 Keypad Matrix

CODE:

#include //Include file for 8051#define keyport P2 //keypad connected to P2#define col1 P2_0 //column 1

#define col2 P2_1 //column 2#define col3 P2_2 //column 3#define col4 P2_3 //column 4#define TRUE 1 //some defines#define FALSE 0

/*+---------------------------------------+| Prototype: void key_init(void); || Return Type: void || Arguments: None || Description: Initialize ports and || Keypad. |

+---------------------------------------+*/void key_init(){

keyport &=0x0F;//make Rows as o/p and cols are i/p}

/*+-----------------------------------------------+| Prototype: unsigned char get_key(void); || Return Type: unsigned char |


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| Arguments: None || Description: To read key from the keypad |+-----------------------------------------------+*/unsignedchar get_key(){ unsignedchar i,k,key=0;

k=1; for(i=0;i>i); //to make rows low 1 by 1 if(!col1){ //check if key1 is pressed

key = k+0; //set key number while(!col1); //wait for release return key; //return key number } if(!col2){ //check if key2 is pressed



key = k+3; //set key number while(!col4); //wait for release return key; //return key number }

k+=4; //next row key numberkeyport |=0x80>>i; //make the row high again

} return FALSE; //return false if no key pressed}

Example of how to use the above functions is given here.
http://www.8051projects.net/keypad-interfacing/avr-programming.php#examplehttp://www.8051projects.net/keypad-interfacing/avr-programming.php#examplehttp://www.8051projects.net/keypad-interfacing/avr-programming.php#example

working priciple of keypad and its interfacing

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