>I just wanted to reinforce something that Drew mentioned but did not
>emphasize enough: even WRITING to port B can cause you to miss
>interrupts!  According to Andy Warren, writes are preceded with a dummy
>read.  I have verified this behavior on a 12c671.  You can't touch port B
>at all if you want the interrupt on change to be reliable.

Well, as I mentioned, you *can* use this feature for this purpose in
conjunction with normal portb i/o, but only if you are careful to access the
port in specific time intervals in which interrupts are not expected.

However, writes are not preceded by a "dummy read."  It is a normal,
legitimate read, and actually reads the values on the pins.  The bits are
modified, then the port latches are written with the new values.  Perhaps
this is what you really meant, but I think the term "dummy" is misleading.

--Andrew
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