Editing MOS Technology 6522 (section)

== Bugs ==

Due to a design defect, if the edge on {{mono|CB1}} falls within a few nanoseconds of the falling edge of the ϕ2 (phase&ndash;2) clock, the {{mono|CB1 edge}} will be ignored, causing the loss of a bit and framing errors on subsequent data. 

A workaround is put the external clock signal into the {{mono|D}} input of a 74ACT74 [[Flip-flop (electronics)|flip-flop]], run the flop's {{mono|Q}} output to the 6522's {{mono|CB1}} pin, and clock the flip-flop with ϕ0 or ϕ2.<ref>[http://forum.6502.org/viewtopic.php?t=342#p2310 Wilson, Garth]</ref>

The serial shift register bug was corrected in the [[California Micro Devices]] CMD G65SC22{{citation_needed | date=December 2021}} and in the [[MOS Technology CIA|MOS 6526]], the latter device which Commodore used in place of the 6522 from the [[Commodore 64]] onwards.

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 (computing)|bus]] architecture, such as a [[Motorola 68000]]. If the address lines changed while [[chip select]] was low (inactive) but the ϕ2 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.