I tried ISR addressing in this code .
Iam unable to get desired result . couldn't find where i went wrong.

i programmed as below..
1) Timer0 of AT89c51 to count for 50 sec.
2) Timer0 ISR to decrement registers ro & r1 to measure 50 sec
The problem i face is... the ports(of port1) are not getting set at appropriate timing intervals when tested using LED's.
can any one suggest me to find a solution...

org 0
ljmp main

org 0Bh
LJMP Timer0_ISR

main:
Mov ro,#13d ;when decremented to 00 "one sec" is over
Mov r1,#49d Decremented every 1 sec for 50sec
mov Tmod,#01h(16 bit timer mode)
mov TH0,#00h
mov TL0,#00h
setB TR0
cjne r1,#02d,$
Mov P1,#54h
cjne r1,#13d,$
mov p1,#FEh
cjne r1,#35d,$
mov p1,#BDh,$
clr TR0
ljmp main

Timero_ISR:
Dec ro
cjne ro,#00d,$
mov ro,#13d
dec r1
RETI

end