Project: HDMI/DVI out for STFM

Progress on our FPGA cores.
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exxos
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Re: Project: HDMI/DVI out for STFM

Post by exxos » Wed Aug 29, 2018 9:39 pm

I can donate a osc if needs be, not sure what they are, but got stacks of MB for breakers..
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Re: Project: HDMI/DVI out for STFM

Post by troed » Wed Aug 29, 2018 10:59 pm

I will discuss my interfacing of this into my doubleST here: viewtopic.php?f=18&t=283&p=8998#p8998

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Re: Project: HDMI/DVI out for STFM

Post by Smonson » Thu Aug 30, 2018 12:23 am

troed wrote:
Wed Aug 29, 2018 4:33 pm
One solution would of course be to run the FPGA off the system clock - but I understand you need to rewrite all timing code for that to work. And also, I guess you would need different code in the FPGA depending on which motherboard clock it is ...
exxos wrote:
Wed Aug 29, 2018 9:39 pm
I can donate a osc if needs be, not sure what they are, but got stacks of MB for breakers..
The problem is the FPGA needs a full digital clock signal, and the ST's board (mine, at least) only has the crystal, with part of the clock circuit completed inside the ST shifter. Maybe it's possible to add a clock generator circuit to the little PCB that plugs into the shifter socket. But it sounds like a bit of a pain.
dhedberg wrote:
Wed Aug 29, 2018 9:15 pm
Awesome achievement! I'd be interested in buying one if the sync issues with opening the borders can be solved (I mostly use my ST to watch demos, so would hate to not have the possibility to watch old classics without switching back to the old Shifter).
Right now most demos don't run properly because the shifter model doesn't behave exactly like the ST shifter that most machines had. I think it does behave like a real ST shifter - but an early, uncommon version (the one that Ijor reverse-engineered). Changing the model is no problem, but I need a demo coder's help to find out where the difference lies.

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Re: Project: HDMI/DVI out for STFM

Post by exxos » Thu Aug 30, 2018 8:15 am

Smonson wrote:
Thu Aug 30, 2018 12:23 am
The problem is the FPGA needs a full digital clock signal, and the ST's board (mine, at least) only has the crystal, with part of the clock circuit completed inside the ST shifter. Maybe it's possible to add a clock generator circuit to the little PCB that plugs into the shifter socket. But it sounds like a bit of a pain.
The ST generates 32Mhz signal, can't you use that then ? All the shifter does is convert that to 16mhz for the MMU.
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Re: Project: HDMI/DVI out for STFM

Post by Smonson » Thu Aug 30, 2018 8:24 am

exxos wrote:
Thu Aug 30, 2018 8:15 am
The ST generates 32Mhz signal, can't you use that then ? All the shifter does is convert that to 16mhz for the MMU.
On mine it doesn't, the crystal connects to 2 pins on the shifter to create the resonant circuit, the clean digital signal is only inside the shifter. If you measure one of the pins off the crystal, you get a low-voltage sinewave.

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Re: Project: HDMI/DVI out for STFM

Post by exxos » Thu Aug 30, 2018 8:44 am

Smonson wrote:
Thu Aug 30, 2018 8:24 am
On mine it doesn't, the crystal connects to 2 pins on the shifter to create the resonant circuit, the clean digital signal is only inside the shifter. If you measure one of the pins off the crystal, you get a low-voltage sinewave.
oh wow, never seen the 32mhz gen "missing" before, what revision board is that ?
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Re: Project: HDMI/DVI out for STFM

Post by troed » Thu Aug 30, 2018 8:49 am

Smonson wrote:
Thu Aug 30, 2018 8:24 am
On mine it doesn't, the crystal connects to 2 pins on the shifter to create the resonant circuit, the clean digital signal is only inside the shifter. If you measure one of the pins off the crystal, you get a low-voltage sinewave.
I think this is unknown .. which Shifter do you have? All I know about and have seen get a proper 32.xMHz clock generated by the circuitry from outside of the Shifter. I agree the crystal itself isn't the full oscillator circuit.

XTL1 is the pin that clock goes to. I use it as a source clock in my doubleST GAL for the stock mode.

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Re: Project: HDMI/DVI out for STFM

Post by Smonson » Thu Aug 30, 2018 8:54 am

Ummm off the top of my head I couldn't tell you the board revision number - it's the one with 8 J-leaded drams underneath the power supply. Next time I have it opened up I'll try to remember to get a photo.

Troed, maybe mine does have a proper external clock and I misinterpreted what I saw. I can't remember which pin I measured to see the sinewave. For a machine with a full clock on pin 2, do you get a sinewave on pin 1?

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Re: Project: HDMI/DVI out for STFM

Post by troed » Thu Aug 30, 2018 8:59 am

Smonson wrote:
Thu Aug 30, 2018 8:54 am
Troed, maybe mine does have a proper external clock and I misinterpreted what I saw. I can't remember which pin I measured to see the sinewave. For a machine with a full clock on pin 2, do you get a sinewave on pin 1?
I'll check after work. Now the 32MHz clock is known to be "weak" so I wouldn't be surprised if it's not a perfect squarewave ... I think exxos has posted scope readings off it in one or a hundred of his booster threads :D :D :D

(but I've had no problem running my GAL off it with a 7 cm wire. Not to say it wasn't sensitive to outside disturbances though .. it was)

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Re: Project: HDMI/DVI out for STFM

Post by exxos » Thu Aug 30, 2018 9:03 am

gen.JPG
gen.JPG (31.72 KiB) Viewed 1183 times
32MHz gen is basically a analouge circuit , it outputs a crappy 1v p-p sine, probably with 2.5V offset as well. The shifter cleans that up and outputs 16MHz to the MMU.

You need to run the 32MHz line via a schmitt buffer. I had exactly same issues on my 40MHz STE booster. My GAL output was like 2-4V and didn't hit a logic low (rise and fall times way to slow even being like 2ns!) . The buffer thankfully solves that and gives a decent 0-5V wave for the CPU.
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