How about this....run the thing as fast as you can.
Count the number of tics between cycles, then divide in half.  Or does
the PIC detect at the 90 phase?  If I remember right, it looks at the
full cycle.  In fact, you should be able to calculate the number of tics
(knowing the clock and the frequency of the AC), and find a edge and
count the number of clocks till you get there.

                                ----------
                                From:  John Bellini
[SMTP:JBellini@STATPOWER.COM]
                                Sent:  Wednesday, May 13, 1998 2:40 PM
                                To:  PICLIST@MITVMA.MIT.EDU
                                Subject:  Any good ideas for detecting
zero crossing?

                        I'm not sure, but this may already have come up
on the list.  If so
                        please direct me to the archive and I will look
there.  However, any
                        ideas would be great full.  

                        What I will be having to do is synchronize to
the incoming AC.  To do
                        this I will have to detect RMS voltage, Detect
the zero crossing,
                        generate the sine wave reference and then
synchronize to the line.  

                        Any ideas on detecting the zero crossing?À  I
want to do it software not
                        hardware.  I've already done it in hardware
using 2 pins on the micro.
                        I want to do it with 1 pin now.

                        John Bellini
                        Product Development, Firmware Technologist