LA traces on interesting video signals

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troed
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Re: LA traces on interesting video signals

Postby troed » Sat Jan 13, 2018 9:17 pm

troed wrote:I'll ponder about how to build such a WS cycling circuit.


EDIT: Scrap the below. It WORKS. It does seem as if I need to time it to actual RESET very accurately though. I'm not sure why?


Congratulations Ijor - yet another feather in your hat! :D On one single power cycle and three RESETs I've gone through WS1, WS3, WS3 and WS4.



Old content, on what I built:

So I've built it now, but either I misunderstood how it would work or it doesn't work as theoreticized.

GLUE TEST nudge.png


I have a push button connected to a 74121 which outputs a 170ns pulse (apparently, I was shooting for 125 .. ) onto the GLUE TEST pin. It definitely does _something_, if I push the button while the desktop is running it "jumps" vertically about one scanline up'n'down. However, pushing it either during holding RESET or otherwise does not change the GLUE-MMU wakestate* as detected from software.

For the ones interested: I initially tried disconnecting GLUE PLCC pin 52 (TEST) from GND by cutting the traces on the motherboard. There's a wide trace on the top, and a small one on the bottom, which would leave a nice via to solder to close to the socket. However, after cutting and scraping and cutting and scraping and not succeeding, I extracted the chip from the socket and saw that there was yet another one underneath. I gave up the trace cutting method and instead desoldered the pin from the PLCC socket and soldered a new pin directly to the GLUE, which now sticks up a bit and is easy to solder to. This is on a Mega ST Rev 5.0 motherboard.

/Troed

*) While I need to do more testing, it seems my WS2 loving Mega ST suddenly likes WS1 a lot more since I rewired the TEST pin. Could be a fluke, but so far I've only seen WS1 afterwards.
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ijor
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Re: LA traces on interesting video signals

Postby ijor » Sat Jan 13, 2018 11:09 pm

troed wrote:It WORKS. It does seem as if I need to time it to actual RESET very accurately though. I'm not sure why?
Congratulations Ijor - yet another feather in your hat! :D On one single power cycle and three RESETs I've gone through WS1, WS3, WS3 and WS4.


:) :) So glad it works. Jaja

Not sure why you need to sync it with Reset. As far as I can see, as long as you perform the pulse while Reset is asserted, it should work. That is, TEST high together with RESET low, triggers the counter reset.

It definitely does _something_, if I push the button while the desktop is running it "jumps" vertically about one scanline up'n'down.


Yes, that's the expected functionality if performed without Reset. This is used for testing and simulation. Don't remember the exact details, but it advances the vertical sync counter. The idea is to accelerate the vertical sync process to be able to test it without waiting a full frame.

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troed
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Re: LA traces on interesting video signals

Postby troed » Sat Jan 13, 2018 11:42 pm

ijor wrote:Not sure why you need to sync it with Reset. As far as I can see, as long as you perform the pulse while Reset is asserted, it should work. That is, TEST high together with RESET low, triggers the counter reset.


It might be that my Mega ST due to age is having difficulties keeping RESET asserted when pushing the button. I think I'll remake this into a toggle switch which in one mode will pulse GLUE TEST at every reset, or keep the existing original behaviour. That way I use the actual /RESET to select with and know it will be timed correctly.

Anyway, this is absolutely perfect :) Will clean up a description and post for the slight possibility that someone else but me will build it up ... hehe. Thanks again.

edit: Now done. Schematics and writeup here: https://blog.troed.se/projects/atari-st ... te-nudger/


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