Dr Jefyll wrote:
BigDumbDinosaur wrote:
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Removed the 1502 in favor of a pair of ATF16V8Cs (figured out how to do this without chaining them together)*
Why? A single 1504 CPLD makes more sense. As you have it, you’ve got the potential for a timing race with the two GALs. There is no guarantee the two will have the same pin-to-pin prop time.
Wired as they are, I don't see why it would matter if the two ATF16V8Cs happen to differ in pin-to-pin prop time. Yuri's circuit would be equally valid using discrete logic in place of the PLD(s)... and even in that scenario I don't see any cause for concern about a timing race.
Not that I'm arguing
against the 1502. There are pros and cons to both approaches. For example, board area and component cost/availability are legitimate factors that could sway the decision one way or the other.
-- Jeff
I can certainly go back to using the 1502, that isn't an issue. I had considered that there would be differences between the two chips, but I also figured that they would be so slight it wouldn't be a problem at the speeds I'm working with. Sounds like BDD disagrees with my assessment, and I will defer to his expertise on that.
Right now on the breadboard I have a 74HC138 hanging off of a 74HC139 to do the address decoding. At 6Mhz this appears to be working fine, but if I want to up the clock rate closer to what the 6502 can handle, I'm quite sure the propagation delay on both those chips will become an issue.
I switched to the 16V8Cs because flooby had mentioned that I was barely using the 1502. Which is true, for the simple address decoding I'm doing it's kind of overkill. I don't like the thought of very underutilizing the chip, but I'm not sure what choice I have at the moment.
It sounds like a lot of folks find my address splitting logic very complicated, though to me it makes perfect sense.
It particularly made a lot of sense given the parts I had available to me when I first built this on breadboards.
GARTHWILSON wrote:
Yuri wrote:
I have added some 245s for the bus, which I'm sure someone will probably call me out on. I know the primer suggested NOT adding these because why would you want to add more gate delays; but I'm not sure what else I plan on connecting in the future and I imagine I can only drive the pins off the 6502 so far.
WDC's outputs on the '02S and '22S are really strong—much much stronger than the data sheet lets on, according to my tests. They don't need any help like the NMOS 6502s of yore did. Depending on what kind of '245 you were thinking of adding, you might find that it's weaker than the W65C02S's outputs are. Okay, took them out.
BigDumbDinosaur wrote:
[color=#000000]
Yuri wrote:
Okay, think if I continue to stare at this any longer I might go cross eyed. XD
To quote something I wrote in my previous reply, “Diode D1 is superfluous.”
Removed
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There is no industry-standard pinout for a JTAG connector. However, most CPLD vendors have used a 10-pin header on their programming rigs with a consistent pinnout. Here’s what Atmel uses with their programming rigs:
...
Okay, updated to those pin outs. Thank you.
I'll update with the caps you mentioned.
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Using the output of a GAL to clock the MPU is not a good idea. GALs produce TTL-level outputs, with a theoretical maximum VOH of 3.4 volts in a five volt system. WDC MPUs require CMOS levels on their clock inputs, with VIH being a minimum of 3.5 volts in a five volt system. Your machine might work with a GAL driving Ø2, but will likely be unstable.
Dag nabbit! Just when I think I've figured a good way to remove a bunch of ICs and simplify.... >_<
Okay, back to the discrete ICs I go!
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Still think I need more around the DMA/RDY pin. (Or am I just over thinking it again?)
I think your design is too complicated for a first try.
...
Once again, you are making it unnecessarily complicated, in my opinion.
I know I can overthink things sometimes, though I'm still curious as to what specifically makes you feel this is overly complicated. Right now, other than the addition of the RTC and the PIC, this isn't radically different than what I have on my breadboard. (I can definitely remove them if you think that would be best.)
If it's the RDY/BE pins; that's something I'm looking to branch out and explore and learn about. (I'd like to have those so I can play with some DMA for sound/video circuits in the future)
If it's the bus transceivers then rest assured I have removed those.
If it's something else I'd like to know what; this ultimately is to be something I can hack other circuits into later.
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