VIA 6522 Bugs
-------------

This is an attempt to document the bugs present in MOS technologies VIA6522. For
some reason this has never been done in great detail, leading to confusing and
inaccurate emulation.


Shift Register Bug:
-------------------

Due to a design defect, if the edge on CB1 falls within a few nanoseconds of the
falling edge of the phi2 (phase–2) clock, the CB1 edge will be ignored, causing
the loss of a bit and framing errors on subsequent data.

As a workaround, put the external clock signal into the D input of a 74AC74
flip-flop, run the flop's Q output to the 6522's CB1 pin, and clock the
flip-flop with phi0 or phi2.2

This infamous serial shift register bug was fixed in the California Micro
Devices CMD G65SC22. California Micro Devices was acquired by ON Semiconductor.


Register corruption Problem:
----------------------------

Aside from the aforementioned shift register bug, there was a potential register
corruption problem that usually only occurred in systems using the 6522 with a
microprocessor having a non-6502 bus architecture, such as a Motorola 68000.

If the address lines changed while chip select was low (inactive) but the phi2
clock input was high (active), register contents could be changed despite the
fact that the chip was not selected.

This was fixed in some but not all of the CMOS versions.

Timer Race Conditions:
----------------------

??? mentioned a few times but never detailed


References:
-----------

https://en.wikipedia.org/wiki/MOS_Technology_6522#Bugs
https://plus.google.com/108984290462000253857/posts/1Ei3qVWZHwn
http://visual6502.org/images/pages/MOS_6522.html
http://forum.6502.org/viewtopic.php?t=40&highlight=wd65c22s

--------------------------------------------------------------------------------
perhaps related:

http://forum.6502.org/viewtopic.php?f=7&t=342&start=7
"To get the 6522's T1 running in free-run mode, you must set bit 6 of the ACR
before writing to the counters. Then, write to T1's counters to get it started.
After that you can write to the latches. (This requirement may not hold for all
manufacturers.) It took me a long time to get this going the first time since
this information is not in the data books."