Garth, BDD, floobydust, 1024MAK, drogon, BigEd ........ many many thanks for all the input and forcing me to give a good deal more thought to what I'd like to do. First things first, I've heard what you say, and the board will be spared further hacking just in case there is some value there (BDD you said "butchering" - have you seen my soldering?
). That said, I'd still be tempted to "improve" the address decoding by not having Ø1 qualify the chip enables for the 65xx devices (BDD, as you said at
http://forum.6502.org/viewtopic.php?f=4&t=511&hilit=139&start=15 "Note that RWB, chip selects and register selects for the 65C21, 65C22 and 65C51 should not be qualified by Ø2—all of these inputs on these devices must be stable before the rise of Ø2."
1024MAK wrote:
So that gets back to the old, but often repeated question: what is your objective?
Is the fun trying to improve an old, existing board?
Write code for or play with a 65C816?
Just get back into 6502 coding?
Get a 6502 board with various I/O to play with interfacing to various external devices?
Something else / none of the above…
Good question
My objective is to develop a 65xx assembler and 65xx compiler. This needs a suitable target platform, my criteria include:
1) low cost - I'm Scottish and we don't part with our money easily
2) easily available - hence my desire to reuse the existing SBC
3) 65c816 based - to provide a richer instruction set than the 65(c)02, particularly for compiler code generation
Why develop another assembler and compiler, when there are a number already available?
1) why not - many have developed new hardware, many can develop new software
2) to relive some of the excitement from my student days - learned assembler on a PDP-9, and it was tough going getting it right. Then went on to write a compiler which generated reams of correct assembler, it was like magic (true, the original high level language source also had to be correct)
3) for the fun of it
My original thinking was that "upgrading / hacking / butchering(?)" the board I already had would be low cost, and since many of the support devices were already there, also easily available. But now I'm not so sure. A 65c186 on mouser(UK) is £11, a 65c51 is £7, a 65c22 is £9, which totals ~£30 before adding memory and glue, etc. Have just checked and a W65C265SXB SBC is £43. Lots of discussion topics here related to the W65C265SXB, e.g.
http://forum.6502.org/viewtopic.php?f=4&t=3545&hilit=W65C265SXB+vs+W65c816SXB, and a read through of
https://github.com/scotws/265SXB-Guide and its difficult to see what's not to like. Mouser also do the W65c816SXB but I've yet to study the details. So maybe an off the shelf WDC board is a better way forward for me, low-ish cost, easily available, 65c816 (family) based, and a means towards achieving my objective. Will think upon this some more
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. I was aware that Ø1 was connected to the two active low inputs for the '138, and that that was essentially the same as having the '138 enabled when Ø2 is high. I was simply trying to follow the guidance that chip enables for 65xx devices should not be qualified by "the clock" as the devices (6522 and 6551) have their own Ø2 and that device address and chip select should be stable before Ø2 goes high, rather than as Ø2 goes high / Ø1 goes low.
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