This a description of how the FPCLOCK on Firefox works.  It has recently
changed so that it will work with the AP card.

When writing to the AP card, the following steps are taken:
.AL
.LI
Make sure that FPCLOCK has been zero long enough for the AP card to reset.
(At power up it will be zero.)
.LI
Write the first data bit.
.LI
Enable the FPCLOCK by writing a one to the FPCLOCK bit.
.LI
Write the rest of the data bits without changing FPCLOCK.
.LI
When all the bits have been sent write a zero to FPCLOCK.

Data will be valid to the AP card for at least 600 ns before FPCLOCK
goes high.  The low pulse width will be between 100 and 500 ns.  The
high pulse width will be very long (determined by software running from
PDH ROM).

The old method of writing FPCLOCK with a one and then a zero will still
work.  (This is needed for the Firefox class B test system.)

The AP card requiers that each 16 bit transmission be separated by at least
20 micro-seconds.

