We use 16C57 with 1 second sleep periods for an application.  On wakeup, we
test if a pin has gone high.  It works fine, the only limitation is that
the line must stay high for a minimum of 1 second.  If that isn't true for
your application, you can either use another chip with interrupt on port
capability (costs a bit more than a 57) or external circuitry to hold the
line high for a second.  If you expect several pulses during a sleep
period, you will need some sort of adder.
  <edtodd@sni.net>    Ed Todd