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TTL Counter with tri-state
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SiliconWizard:

--- Quote from: obiwanjacobi on February 12, 2020, 06:25:41 pm ---One option I have considered is using two GAL 22V10's to build the counter. That would actually be a pretty nice fit (also have them in stock - and already use them in the design).

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Looks like you are often putting off using programmable logic for your projects, but I think you should definitely take the plunge (unless your projects are 100% "vintage" designs and you don't want any modern chip in them.)

GALs these days are pretty limited compared to current CPLDs/small FPGAs. Considering them would make your life greatly easier IMO.

Maybe one of your constraints is that your designs are usually on 5V, and that you'd need to use a lot of level shifters? Is this constraint completely necessary? If you're accepting to use more modern parts, I think most of what you'd need (even for discrete logic) would be available in 3.3V (except maybe the very specific logic ICs that are "obsolete") But I don't know exactly what your goal is, so maybe you don't really have a choice.

obiwanjacobi:
The idea was to build it using discrete logic components in order to fully understand the intricacies of building a VGA adapter. Reading others talk about is different from getting your hands dirty. Also this knowledge could come in handy when I do finally take the plunge and go full FPGA.

I now take a hybrid approach in using TTL logic as well as a couple of GALs - for instance for the decoding logic to save a lot of gates. Not having to deal with level shifting is a bonus. So you could say its a goal to do it in TTL as a deliberate learning curve - that is my choice. No other requirements or constraints exist - except interfacing with a Z80 (5V) eventually...


SiliconWizard:

--- Quote from: obiwanjacobi on February 13, 2020, 03:07:42 pm ---The idea was to build it using discrete logic components in order to fully understand the intricacies of building a VGA adapter. Reading others talk about is different from getting your hands dirty. Also this knowledge could come in handy when I do finally take the plunge and go full FPGA.

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Understand.


--- Quote from: obiwanjacobi on February 13, 2020, 03:07:42 pm ---I now take a hybrid approach in using TTL logic as well as a couple of GALs - for instance for the decoding logic to save a lot of gates. Not having to deal with level shifting is a bonus. So you could say its a goal to do it in TTL as a deliberate learning curve - that is my choice. No other requirements or constraints exist - except interfacing with a Z80 (5V) eventually...

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As I said, a lot of discrete logic ICs are available in 3.3V these days, so going "TTL" is not really necessary IMO, you could get the same result at 3.3V (and have more choice in ICs except maybe for the odd vintage specialized TTL ICs with specific functions....)

Now if you're going to interface with a Z80, that's another story - of course there are Z80's that can run @3.3V (CMOS series), so that wouldn't be a show-stopper either unless you absolutely need to interface with an old and existing Z80 system.

In this case, using external level shifters is still an option - you can design your whole additional circuitry for 3.3V, and just use a few level shifters. As mentioned in another thread, there are level shifters at 1 bit to 16 bits, so that would be just a few additional ICs and would still give you the opportunity to use otherwise much more recent ICs.
TomS_:

--- Quote from: SiliconWizard on February 13, 2020, 03:49:51 pm --- - of course there are Z80's that can run @3.3V (CMOS series)

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Are you sure about that? I've looked into this in the past, and the inputs very definitely wanted much more than 3.3V for a guaranteed logic high.

Also the power supply requirements were above something like 4.5V.

Edit: apologies, inputs only need 2.2V+ (I guess for TTL backward compatibility), but the power supply must be 4.5V+ for CMOS.

Unless there's a variant that I am not aware of... But that is for the Z84C00xx which are the only CMOS parts I know of.
SiliconWizard:

--- Quote from: TomS_ on February 13, 2020, 06:08:24 pm ---
--- Quote from: SiliconWizard on February 13, 2020, 03:49:51 pm --- - of course there are Z80's that can run @3.3V (CMOS series)

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Unless there's a variant that I am not aware of... But that is for the Z84C00xx which are the only CMOS parts I know of.

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Technically, the Z84C00 series was not spec'ed at 3.3V, but I've heard of a few people successfully making them run @3.3V @4MHz or so. Probably a bit on the edge here.

To avoid running things out of spec though, your best bet would be to use the Z180 series. Still easily available. Significantly lower power. Many additional goodies inside, but still 100% compatible. Can run at 3.3V up to 20MHz (eg.: Z8L18020) Kind of the Z80 many people wish they had back in the day.

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