keyport equ P2 ;Keypad port connected here
col1 equ P2.0 ;Column 1
col2 equ P2.1 ;Column 2
col3 equ P2.2 ;Column 3
col4 equ P2.3 ;Column 4

keyval equ 30H ;To store key number
pressed bit 0H ;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

jb pressed, done ;check if flag is set

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

jb pressed, done ;check if flag is set

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
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


please read the code and understand and make changes in the code as per your hardware.
Please let me know if u have any issues.