In my example above for the interrupt routine, you'll need to add all the other envirnoment maintenance code like pushing and popping the flags (as you must do for most all interrupts).

I only mention it because this sounded like a novice question and if you forget to PUSH PSW and POP PSW on the way out, you'll get a very hard system wide bug to find. The example code I wrote affects no other register values during its interrupt:

. NTR_T1:
. push psw ;save foreground flags
. ... ;enable next interrupt
. djnz countdown,adios
. cpl px.y ;toggle LED output pin
. mov countdown,#8 ;reload passcount
.adios:
. pop psw ;restore foreground flags
. reti

-Jay C. Box