On Thu, 23 Nov 1995, Andrew Warren wrote:

> It was bad enough when only instructions that performed an EXPLICIT
> read or read-modify-write kept the interrupt from happening... This
> new information makes the change-on-portB interrupt almost
> COMPLETELY worthless.  Does Microchip see this as a bug and plan to
> correct it in future revs of the chip?

I have no connection with Microchip other than that I'm designing a 16C73
into a product currently being prototyped, but I believe I can predict
the answer.  They will say "no".  The data sheet has always [always = in
the 1994 and 1995/1996 books] implied that this feature was intended for
use in applications where the input change is used to wake the chip from
a sleep state, and of course when the chip is sleeping it won't be
reading or writing the port B pins and disturbing the change sensing...

There's even an application note that demonstrates exactly this use,
isn't there?

So I believe that this is _exactly_ the operation that Microchip designed
into those pins on port B, and the bug is only that you wish they worked
otherwise than they do.  The 1995/1996 databook is pretty explicit about
this:  "the interrupt on change feature is reccomended for wake-up on key
depression operation and operations where port B is used only for the
interrupt on change feature."  I think they could allow as well
applications where bit 0 is used as an external interrupt input (for
non-sleeping applications).