Keyboard Interface

Keyboard Interface

AU : May-07, 08, 11, 18, Dec.-10, 11, 12, 19

 

1. Simple Keyboard Interface

Fig. 17.1.1  shows simple keyboard interface.

Here eight keys are individually connected to specific pins of port Pl. Each port pin gives the status of key connected to that pin. When port pin is logic 1, key is open, otherwise key is closed.

Software routine to get keycode with key debounce.

START: MOV A, P1 ; Read key status

; check if keys are open

CJNE A,#FFH,START ; if no, goto start

; otherwise continue

PRO :          LCALL DEBOUNCE_DELAY        ; call debounce delay

AGAIN : MOV A, P1; Read key status

CJNE A, #FFH PRO1 ; check if any key is

; pressed

LJMP AGAIN ; if no, goto AGAIN

; otherwise

; continue

PRO1 : LCALL DEBOUNCE_DELAY ; call debounce delay

MOV A, P1; Get key code

RET ; Return from

; subroutine

This program reads status of all keys by getting data through P1 and compares it with FFH to check whether all keys are open. If all keys are open, instruction compare sets the zero flag, and the program waits for key debounce. After waiting about 10 ms, program checks the Pl for key press. If key press is found, program waits for another 10 ms as a key debounce period. After key debounce period, program reads the keycode from P1.

 

2. Matrix Keyboard Interface

In simple keyboard interface one input line is required to interface one key and this number will increase with number of keys. Therefore, such technique is not suitable when it is necessary to interface large number of keys. To reduce number of connections keys are arranged in the matrix form as shown in the Fig. 17.1.2.

Fig. 17.1.2  shows sixteen keys arranged in four rows and four columns. When keys are open, row and column do not have any connection. When a key is pressed, it shorts corresponding one row and one column. This matrix keyboard requires eight lines to make all the connections instead of the sixteen lines required if the keys are connected individually, as shown in Fig. 18.1.1. Fig. 18.1.3 shows the interfacing of matrix keyboard. It requires two ports : an input port and an output port. Rows are connected to the input port referred to as returned lines, and columns are connected to the output port referred to as scan lines. We know that, when all keys bus are open, row and column do not have any connection. When any key is pressed it shorts corresponding row and column. If the output line of this column is low, it makes corresponding row line low; otherwise the status of row line is high. The key is identified by data sent on the output port and input code received from the input port. The following section explains the steps required to identify pressed key.

Check 1 : Whether any key is pressed or not

1. Make all column lines zero by sending low on all output lines. This activates all keys is in the keyboard matrix. (Note : When scan lines are logic high, the status on the return lines do not change, it will remain logic high.)

2. Read the status of return lines. If the status of all lines is logic high, key is not pressed; otherwise key is pressed.

Check 2 :

1. Activate keys from any one column by making any one column line zero.

2. Read the status of return lines. The zero on any return line indicates key is pressed from the corresponding row and selected column. If the status of all lines is logic high, key is not pressed from that column.

3. Activate the keys from the next column and repeat 2 and 3 for all columns.

We will see how matrix keyboard can be connected to the 8051, a single chip microprocessor/microcontroller. Fig. 17.1.4 shows the 4×4 matrix keyboard connected to the port 1 of 8051. 4 lines of port 1 (P14-P17) are used as a scan lines and remaining 4 lines (P10-P13) are used as return lines.

The steps in algorithm are as follows :

1. Initialise P1.0, P1.1, P1.2, P1.3 as inputs i.e. write '1' to these pins.

2.       Check if all the keys are released by writing 'O' to P1.4-P1.7 and check if all return lines are in state '1'. If No then wait.

If Yes then go to step 3.

3. Call debounce.

4. Wait for key closure. Ground all scan lines by writing 'O' and then check if at least one of return lines shows '0' level.

Key pressed ? No step 4

Yes step 5

5. Call debounce, (allow sufficient time for debounce)

6. Is key really pressed ? (Ground all scan lines by writing '0' and then check if at least one of the return lines shows '0' level.)

No step 4

Yes step 7

7. Find key code and display the key pressed on 7-segment display.

(By grounding one scan line at a time and checking return lines for any one line to go to '0' level. )

8. Go to step 1.

Program : 

org lookup_table address

db 30h, 31h, 32h, 33h, 34h, 35h, 36h, 37h, 38h, 39h, 41h, 42h, 43h, 44h, 45h, 46h

org program start address

beg:   mov P1 #0f h ; configure lower 4 lines of port 1 as i/p

mov dptr,#lookup_table_address ; initialise dptr with

; lookup_table_addr.

aga: mov a, P1 ;

anl a, #0fh ;

cjne a, #0fh,aga ; check for key released

1call delay  ; call delay routine for key debounce

agal:  mov a, P1

anl a, #0fh

cjne a,#0fh, go ; check for key pressed

Ijmp agal

go: Icall delay ; call delay routine for key debounce

mov a, P1

an1 a, #0fh

cjne a,#0fh, go1 ; is key really pressed ?

Ijmp agal

gol: mov r1, # 01h ; initialise counter 1

mov r0, #0efh ; store word for column selection

mov r3, #04h ; initialise column counter

aga3: mov P1, r0 ; select only 1 column

mov a, P1 ; get the status of return lines

jnb acc.0,display ; check bit 0 and if it is 0 jump to display

inc dptr ; increment lookup_table pointer

jnb acc.1,display ; check bit 1 and if it is 1 jump to display

inc dptr ; increment lookup_table pointer

inb acc.2,display ; check bit 2 and if it is 2 jump to display

inc dptr ; increment lookup_table pointer

jnb acc.3,display ; check bit 3 and if it is 3 jump to display

inc dptr ; increment lookup_table pointer

mov a, r0 ; get the word for column selection

rl a ; select next column

mov r0, a ; store word for column selection

djnz r3, aga3 ; check for last column

1jmp beg ; if any key is not pressed scan again

end

Review Questions

1. With a neat circuit diagram explain how a 4×4 keypad is interfaced with 8051 microcontroller

2. Interface an 8×8 keyboard using 8255 ports and write a program to read the code of a pressed key. AU : Dec.-10, Marks 8

3. How do you interface a 4 × 4 matrix keyboard using 8051 microcontroller ?  AU : Dec.-11, Marks 8

4. How to interface a 4×4 matrix keyboard using 8051 microcontroller and explain how to identify AU : Dec.-12, Marks 8

5. Show how to interface a 8 × 8 matrix keyboard to the 8051 microcontroller and discuss in detail the various stages for detection and identification cf key activation by a microcontroller. Also, write an assembly language program to detect and identify the pressed key. AU : Dec.-19, Marks 15