org 00h
jmp start
start:	mov  tmod,#10h
	mov th1,#3ch
	mov tl1,#0bh
        setb tr0

	clr p1.0
        mov r0,#0
again:  jbc tf0,ok
	jmp again

ok:	inc r0
       	cjne r0,#20,ok2
	cpl p1.0
        mov r0,#0
ok2:
	mov th1,#3ch
	mov tl1,#0bh
	ljmp again
end