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WDC 65C22 vs. MOS 6522
blusnowkitty:
I recently obtained a giant pile of scrap alarm system boards which are all basically textbook 1980s-era 6502 computers - just with the addition of an ethernet interface. Of course they're all made with modern WDC parts instead of vintage MOS parts, but they're still the same core on the inside.
Is there any reason why a modern WDC 65C22 couldn't be used as a drop-in replacement for a vintage MOS 6522 on the I/O or parallel boards?
Lisa2:
According to WDC, the W65C22N is a "drop-in" replacement for the older VIA chips and should be compatible. The W65C22S version is not.
blusnowkitty:
--- Quote from: Lisa2 on April 21, 2022, 12:46:18 pm ---According to WDC, the W65C22N is a "drop-in" replacement for the older VIA chips and should be compatible. The W65C22S version is not.
--- End quote ---
Thanks for the tip, with that in mind I started digging through the WDC datasheet. It looks to me like the biggest differences are that,
- S variant is fabbed with CMOS and thus can be clocked faster than the N variant
- S variant has a "bus holding device" on the data pins while the N variant does not
- S variant has a different on-die IRQB circuit than the vintage and N variants, but WDC states you can wire an external diode to the IRQB pin on the S variant to make it behave like the N
- S variant does not have current limiting resistors on the output pins while the N variant does, but a footnote in the datasheet suggests you can use external limiting resistors to make it behave like the N
So yeah, it looks like the S variant may not be a 100% compatible drop-in, but it may be possible to use the S variant anyway with enough external support components. At least I have a scope and a logic analyzer?
Now to find out if the 6522s in my scrap alarm boards are the S or the N variants...
For future study: https://www.eevblog.com/forum/beginners/combining-open-drain-and-totem-pole-outputs/
Lisa2:
Speaking of replacement 65xx chips, I have had success using the 6507 in place of the of 6504 in the FDC section of the I/O board. 6507's are easier to find due to them being used in millions of Atari game systems.
After the supplies of 6504's and 6507's dry up, someone will have to work up an adapter to use a 6502 ( maybe a modern WDC version) on the Lisa IO card.
blusnowkitty:
--- Quote from: Lisa2 on April 22, 2022, 11:16:07 am ---Speaking of replacement 65xx chips, I have had success using the 6507 in place of the of 6504 in the FDC section of the I/O board. 6507's are easier to find due to them being used in millions of Atari game systems.
After the supplies of 6504's and 6507's dry up, someone will have to work up an adapter to use a 6502 ( maybe a modern WDC version) on the Lisa IO card.
--- End quote ---
I did a quick bit of research and, at least according to the Atari guys there appears to be no reason why you couldn't use a full-blown vintage 6502 in place of a 6507. Things get more interesting by the time we get to the 65C02 (and by extension, the modern WDC parts) - I don't speak 6502 assembly, but according to the Atari guys code that uses illegal opcodes on a vintage 650x will no longer function on the 65C02. Another page I found said that illegal opcodes simply NOP in the 65C02, and some instructions may take an extra cycle or two to complete now.
Not speaking 6502 I couldn't disassemble the floppy ROM and tell you if this would be a problem... but it might be worth looking at to see if the floppy ROM would play nice with a WDC 65C02. On the bright side, if the floppy ROMs do have to be patched, at least there's only 3 floppy ROMs to patch (Twiggy, 400k, SunRem 800k) instead of thousands of Atari 2600 games?
https://atariage.com/forums/topic/194587-use-a-6502-instead-of-6507/
http://www.oxyron.de/html/opcodesc02.html
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