I'll take a bit of a stab in the dark here ...
By a/d converter, you mean analog to digital converter?
If so, is this the same unit found in the 68HC11?
(I would have thought the 68HC12 would have been some kind of co-processor to the 68HC11 ... guess not.)
I will get back to you as soon as I have done a bit of research into your problem, but in the meantime, do you have the manual for the chip?
I have the book for the 68HC11 ... only it's packed right now, but I'll see what I can do.