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8. K E Y P A D 4 x 4
Keypads are often used as a primary input device for
embedded microcontrollers. The keypads actually consist of a number
of switches, connected in a row/column arrangement as shown in Fig 2.8.1.
In order for the microcontroller to scan the keypad,
it outputs a nibble to force one (only one) of the columns low and then
reads the rows to see if any buttons in that column have been pressed.
The rows are pulled up by the internal weak pull-ups in the 8051 ports.
Consequently, as long as no buttons are pressed, the microcontroller
sees a logic high on each of the pins attached to the keypad rows. The
nibble driven onto the columns always contains only a single 0. The
only way the microcontroller can find a 0 on any row pin is for the
keypad button to be pressed that connects the column set to 0 to a row.
The controller knows which column is at a 0-level and which row reads
0, allowing it to determine which key is pressed. For the keypad, the
pins from left to right are: R1, R2, R3, R4, C1, C2, C3, C4.
Figure 2.8.1. Keypad 4 X 4 Connection
Matrix-type keypads consist of a rectangular array
of momentary push button. Each row and each column of push buttons is
connected to a common rail. Suppose a four by four array of push button
are used. A four by four array is often used to input hexadecimal numbers.
There are four comun rails and four row rails. Each pushbutton has two
terminals, one connected to its column rail and the other to its row
rail. The row and column rails are connected to the microcontroller
ports. The columns are driven low by output port. The rows are then
read into the input ports. If no key is pressed, the rows read 1. When
a row is detected to be 0, it indicates that a key in that row is pressed.
the task now is to detect which key of the row is actually pressed.
The microcontroller loops through each column, driving only one column
low at a time as it inspects the row.The microcontroller needs to poll
the rows to see if a key is pressed. Only when the column in which the
pressed key resides is driven low is the row rail grounded, and thus
the voltage is low. The rows and columns are interchangeable, that is,
the rows may be driven low as the columns are read by the input ports.
8.1. Display Keypad Data to LED
In this lesson we are like to design, how to scan keypad
4 x 4, and then display it to LED.
Figure 2.8.2. Keypad connection and display to LED
Build the circuit as shown in figure 2.8.2. As you seen on figure 2.2.8.
P3.0 trough P3.7 is connected to keypad 4 x 4 and to drive LEDs, it's
connected to P2.0 trough P2.7.
In this step, you must tipe the assembly program to scan your keypad
data, we assume that you have already known the editor, we used MIDE-51
to edit the program. ( Download File : exp281.zip
, Download Complete Circuit File : Keypad4x4.pdf
;In this lesson we'll scan keypad and get data out to LED
;and convert it into binary data
row1 bit P2.4
row2 bit P2.5
row3 bit P2.6
row4 bit P2.7
col1 bit P2.0
col2 bit P2.1
col3 bit P2.2
col4 bit P2.3
keydata equ 70h
keybounc equ 71h
keyport equ P2
start: call keypad4x4 ;calling subroutine keypad4x4
Mov A,keydata ;A = keydata
Cjne A,#0FFh,send ;
sjmp start ;LOOPING FOREVER
send: CPL A ;A = NOT A
Mov P0,A ;P0 = A
Sjmp start ;LOOPING FOREVER PART 2
delay: mov R0,#0
; subroutine scan keypad 4x4
mov keybounc,#50 ;keybounc = 50
mov keyport,#0FFh ;keyport=P2= FF
clr col1 ;col1= P3.0 = 0
Detect:jb row1,key1 ;jump to key1 if row1=1
mov keydata,#00h ;Keydata =00h
key1: jb row2,key2 ;jump to Key2 if row2=1
mov keydata,#04h ;Keydata = 04h
key2: jb row3,key3 ; idem
key3: jb row4,key4 ; idem
key4: setb col1
key5: jb row2,key6
key6: jb row3,key7
key7: jb row4,key8
key8: setb col2
key9: jb row2,keyA
keyA: jb row3,keyB
keyB: jb row4,keyC
keyC: setb col3
keyD: jb row2,keyE
keyE: jb row3,keyF
keyF: jb row4,Nokey
;The end of Keypad 4x4 subroutine
Safe your assembly program above, and name it with key1.asm (for example)
Compile the program that you have been save by using MIDE-51, see the
Download your hex file ( key1.hex ) into the microcontroller
by using Microcontroller ATMEL ISP software, see the instruction.After
download this hex file you'll see the action of Keypad 4 x 4( of course
if your cable connection and your program are corrected )