Welcome to the world of CPLD!

As BDD just said, your CPLD is fast (a 7.5nS device!) so you need to pay attention to where the reference ground is and keep it as short as you can. I had removed my factory scope probe tip and associted ground clip so long ago I don't even know where to find them. They are replaced with this probe in the picture where the black ground lead can go to a standard 25mil square wirewrap post and a sharp pointed pin header to probe around. This setup minimizes the noise introduced by long leads

ATF1508 is very capable device. I'm currently re-working an EPM7128 (same as ATF1508) design for Z80 that contains a serial port, DRAM controller, 64 bytes of ROM code, bit-bang I2C, and compact flash interface. I'm adding a 5mS clock interrupt and a vectored interrupt controller for serial port & 5mS_tick. Most circuitry are buried logic and state machines. Yes, I'm running out of resources, but there are many parameters I can tweek to squeeze in a little bit more features. This is where your tool will limit what you can design. I'm designing in Altera Quartus schematic entry tool.
Bill

Yeah, I'm more or less the same way. That's why I create notes about these sorts of things. For example, my 0.1" Pin Headers note contains a digram similar to Graham's, but I also show the position of the guide protrusion and give pinouts looking in to both male and female connectors because I find it error-prone to mirror them in my head. (And I'm always very careful to consistently use "looking in" and label things that way because mirroring is my biggest source of confusion.)



That is totally brilliant! It's gone in my notes now, of course. (You'll even see that photograph there until the DMCA takedown notice arrives. :-))


I hadn't realized that diodes could pollute Vcc like that, but that's certainly good to know! I've been moving away from "dupont" pin headers and connectors and towards JST XH (for wire-to-board) and JST SM (for wire-to-wire) for power for the very reason that I have stopped trusting myself not to make mistakes with the dupont stuff.


Can you show where and how exactly that's connected to the probe? It looks rather similar to just the standard probe tip when you remove the "pin clipper" cap from it. Except for the IC clip for ground, which is brilliant. I wonder where one can get the the kind of clip used on the ground wire to attach it to the probe so I can make one like that (without destroying one of my existing alligator clip ones).

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Curt J. Sampson - github.com/0cjs

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x86?  We ain't got no x86.  We don't NEED no stinking x86!

Certainly the diode's forward drop needs consideration. But I'm drawing a blank on "switching noise polluting Vcc." WTH??? A diode wired for reverse polarity protection will conduct 100% of the time. In other words, it never switches (except when you swap the input polarity, which presumably happens when the power is off).

-- Jeff

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In 1988 my 65C02 got six new registers and 44 new full-speed instructions!
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The other option for reverse polarity protection is to put the diode across the supply line, reverse biased, then put a fuse in series with the power input. The diode doesn't conduct normally, but if you RP the board it conducts briefly and blows the fuse. For even more protection, you could put a thyristor crowbar in there as well in case you accidentally connect it to 12V instead of 5V...

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x86?  We ain't got no x86.  We don't NEED no stinking x86!

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x86?  We ain't got no x86.  We don't NEED no stinking x86!

I separated the 2-wire probe tip from the scope probe and took a picture of it to show how the 2-wire probe tip is installed on a scope probe.
Bill

Edit, I had the 2-wire probe tips for so long, I don't even remember where I got them.

Thanks for the suggestions!

You're right, that was a rats nest of antenna wires.
So the next thing I did was make a small plug-in board for the oscillator. I think the wires are now pretty short with one exception. I forgot to include VCC pins on those headers! So as you can see, I had to run VCC from another pin nearby. But the GND and OUT pin are very close to the board.

FYI, I'm messing around generating VGA so I put a 25.175MHz crystal into my board and I *think* I am diving it by four. Which should be 6.29375MHz but I'm not getting exactly that. Also, still some bounce in there but it does seem a little cleaner to me.

@BigDumbDinosaur

I tried to compile your suggestion but I'm getting an error in WINCUPL:



ERROR:

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Cat; the other white meat.

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x86?  We ain't got no x86.  We don't NEED no stinking x86!

Yes, that was directly touching the header pin out of the CPLD. No antenna on that one (other than the header pin and PCB trace).

GND was to a nearby GND pin that is ONLY using the supplied alligator clip.






Hmm. Same error.






Good question. I put the actual oscillator directly on the board (nothing else) using the closest connections I could. Even used the "springy thingy" on the probe. The scope shows it toggling between 25.000 MHz and 25.250 MHz (see attached). So I'm not so sure how accurate those oscillators are. In fact, I can't remember where I got them. I'm thinking eBay when most of the time, I get them from Mouser.

I'm going to play around with a few more oscillators on both the scope and the CPLD to see how accurate they are. I think my "mess of wires" will be somewhat OK if I try to keep them short. I've done VGA on breadboards before and it actually looked pretty good. Just not with a CPLD.

Thanks!


**EDIT**

OK, I put in a 14.000 MHz crystal into my CPLD with divide-by-four and get 3.497 MHz which is pretty close to the 3.5 MHz it should be. So 99.91% accurate. So I think it "works" but, obviously, frequency goes up then margin of error goes up.

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Cat; the other white meat.

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x86?  We ain't got no x86.  We don't NEED no stinking x86!

It looks like your scope is a 1Gs/S digital scope, so you'll have sampling error of 1nS. 25.175MHz clock is sampled either 40 times or 39 times so that calculated out to 25.0MHz or 25.6MHz. Maybe your scope has an "average" mode where it can takes multiple samples of same waveform and calculate the average frequency?
Bill