tasking design for keylock

["Rolf" <rolf.ebert_nospam_@gmx.net>]

The following problem arose when I was thinking about the design for a
minimal tasking runtime system.

Many doors are locked by electronic keylocks where one has to type four
correct keys on a small keyboard (mostly 0 .. 9, *, #) before the lock
is opened. Now there are requirements

- that the time between the key presses must not exceed 0.5 seconds,
- and that after the 4th correct key you must not type any key at all
for at least 0.5 seconds.

How do you model the timing requirements using Ada tasking? (I can
attach the h/w interrupt from the key press to a protected procedure,
but I don't know how to proceed from there, i.e. how to cancel or serve
a timed entry call now)

    Rolf

P.S. I made a small state diagram expressing the requirements but don't
know where to upload it.