Keyboard Interfacing

Keyboard Interfacing

For interfacing keyboard to the microprocessor based systems, usually push button keys are used. These push button keys when pressed, bounces a few times, closing and opening the contacts before providing a steady reading, as shown in the Fig. 12.1.1. Reading taken during bouncing period may be faulty. Therefore, microprocessor must wait until the key reach to a steady state; this is known as key debounce.

The problem of key bounce can be eliminated using key debounce technique, either hardware or software.

 

1. Key Debounce using Hardware

Fig. 12.1.2 shows the circuit diagram of key debounce. It consists of flip-flop. The output of flip-flop shown in Fig. 12.1.2 is logic 1 when key is at position A (impressed) and it is logic 0 when key is at position B, as shown in Table 12.1.1. It is important to note that, when key is in between A and B, output does not change, preventing bouncing of key output.

In other words we can say that output does not change during transition period, eliminating key debouncing.

 

2. Key Debouncing using Software

In the software technique, when a key press is found, the microprocessor waits for at least 10 ms before it accepts the key as an input. This 10 ms period is sufficient to settle key at steady state. Fig. 12.1.3 shows the flowchart with key debounce technique.

 

3. Simple Keyboard Interface

Fig. 12.1.4  shows simple keyboard interface.

Here eight keys are individually connected to specific pins of input port. 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 : IN IN_PORT ; Read key status

               CPI FFH ; check if keys are open

               JNZ START ; if no, goto start otherwise continue

              CALL DEBOUNCE_DELAY ; call debounce delay

AGAIN : IN IN PORT ; Read key status

             CPI FFH ; check if any key is pressed

             JZ AGAIN ; if no, goto AGAIN ; otherwise continue

            CALL DEBOUNCE_DELAY ; call debounce delay

            IN IN PORT ; Get key code

             RET ; Return from subroutine

This program reads status of all keys by getting data through port IN_PORT and compares it with FFH to check whether all keys are open. If all keys are open, instruction CPI sets the zero flag, and the program waits for key debounce. After waiting about 10 ms, program checks the IN_PORT 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 port IN_PORT.

 

4. 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. 12.1.5.

Fig. 12.1.5  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. 12.1.5.

Fig. 12.1.6  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 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 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.

 

Lab Experiment 12.1.1 : Hardware and software for 64-key matrix keyboard interface

Statement : Interface 64-key matrix keyboard to the 8085 microprocessor using 8255. Write an 8085 assembly language program to initialize 8255 and to read the key code.

Hardware : Fig. 12.1.7 shows a matrix keyboard with 64 keys connected to the 8085 microprocessor using 8255. A matrix keyboard reduces the number of connections, thus the number of interfacing lines. In this example, the keyboard with 64 keys, is arranged in 8 × 8 (8 rows and 8 columns) matrix. This requires sixteen lines from the microprocessor to make all the connections instead of 64 lines if the keys are connected individually. The interfacing of matrix keyboard requires two ports : one input port and other output port. Rows are connected to the input port, port A and columns are connected to the output port, port B.

Source program :

MVI A, 90H         ; Initialize Port A as input and Port B as

OUT CR ; Output

START : MVI A, 00

OUT PB ; Make all scan lines zero

BACK : IN PA

CPI FF ; Check for key release

JNZ BACK ; If not, wait for key release

CALL DELAY ; Wait for key debounce

BACK_1 : IN PA

CPI FF ; Check for key press

JZ BACK_1         ; If not, wait for key press

CALL DELAY ; Wait for key debounce

MVI L, 00H ; Initialize key

; counter

MVI C, 08H

MVI B, FEH ; Make one

; column low

NEXTCOL : MOV A, B

OUT PB

MVI D, 08H         ; Initialize row

; counter

IN PA ; Read return

; line status

NEXTROW : RRC ; Check for one row

JNC DISPLAY ; If zero, goto

; display

; otherwise

; continue

INR L ; Increment key

; counter

DCR D ; Decrement

; row counter

JNZ NEXTROW ; Check for next

; row

MOV A,B

RLC ; Select the next

; column

MOV B,A

DOR C ; Decrement

; column count

JNZ NEXTCOL ; Check for last

; column if not repeat

JMP START ; Goto start

Review Questions

1. What is keyboard interfacing? AU : May-12, Marks 2

2. Design an 8085 based system with external Keyboard. AU : May-18, Marks 5