rw0 equ P0.4;
rw1 equ P0.5;
rw2 equ P0.6;
rw3 equ P0.7;
cl0 equ P0.0;
cl1 equ P0.1;
cl2 equ P0.2;
cl3 equ P0.3;
scan_key:mov P0,#0ffh
clr cl0
mov a, P0
anl a,#00001111b
cjne a,#00001111b,Row0
setb cl0
clr cl1
mov a, P0
anl a,#00001111b
cjne a,#00001111b,Row1
setb cl1
clr cl2
mov a, P0
anl a,#00001111b
cjne a,#00001111b,Row2
setb cl2
clr cl3
mov a, P0
anl a,#00001111b
cjne a,#00001111b,Row3
setb cl3
ret