The simplest way:

clr et0 ;pulse width generation
clr tr0
setb ea
mov a,#00h
mov r5,#00h
mov tmod,#01h
Mov tl0,11h
mov th0,r7
setb et0
setb tr0
label:
; YOUR CODE DOING SOMETHING

sjmp label

org 000bh
cpl a
mov p1,a
cjne a,#00h,on

;if a=0 then jump to on value
;if a=1, then jump to off value

mov tl0,11h
mov th0,r7
reti

on: mov tl0,13h ;on time low byte
mov th0,r6
reti


Remove all empty loops and do something inside of them.