hi jeff:

;initialize external intr in edge
;mode
;initialize timer
;clear flag x
mov ret_add_lsb,#low(ret_add)
mov ret_add_msb,#high(ret_add)
jnb x,$
;check for valid time


intx_isr:
;start/load timer
push ret_add_lsb
push ret_add_msb
reti

ret_add:
jnb px.y,$
;stop timer
;read timer
setb x
ret

regards
pranav