Keyboard interface(64-key-matrix-keyboard)

Statement: Interface a 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 FOR MATRIX KEYBOARD INTERFACE
Fig. 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 x 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
SOFTWARE FOR MATRIX KEYBOARD INTERFACE

Source program
MVI A, 90H : Initialize Port A as input and
OUT CR : Port B as Output
START: MVI A, 00 : Make all scan lines zero
OUT PB
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 else 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
DCR C : Decrement column count
JNZ NEXTCOL : Check for last column if not repeat
JMP START : Go to start
INTERFACING SCHEME


Delay subroutine:
Delay: LXI D, Count
Back: DCX D
MOV A, D
ORA E
JNZ Back
RET
FLOWCHART


Source Program
Delay routine