To avoid the "00" do this:

start: 
;   count 3Fh..00
    mov    30h,#3Fh
    djnz   30h,$
    jmp    start 

 

And then in the Interrupt routine, increment the value:

     :
     : 
;   report 40h..01
    mov    A,30h
    inc    A
    mov    SBUF,A
     :
     : 

 

--Rich