_____
         ||      |    |
9v -*--*-| |-*--*-|____|--------- 5V
    |  | |/| |  |   |
   _|_ |    _|_ |   |
   ___ |    ___ |   |
    |  |     |  |   |
gnd *--|-----*--|---*----------------
       |        |            |    '51
       |        *--resistor--| __ interrupt
       *-----------resistor--| /   power 
                             |/    off 
                         comparator

I have used this circuit in cases where I had to write a few values to an EE at power off.

Erik