Project: HDMI/DVI out for STFM

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

Post by czietz » Thu Dec 06, 2018 10:43 am

I think tuxie's comment was more meant to measure the dynamic ground offset (not the DC offset). I.e. he was saying the same as me in viewtopic.php?f=29&t=330&start=360#p15179.

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exxos
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Re: Project: HDMI/DVI out for STFM

Post by exxos » Thu Dec 06, 2018 10:53 am

Smonson wrote:
Thu Dec 06, 2018 10:00 am
I measured a differential of around -4 millivolts between the FPGA board to the chassis ground (measured off the big electrolytic near the power supply). Not too bad considering the whole mod draws a lot of current (over 300mA).
That doesn't sound right.. With a scope across grounds you should see ringing and all sorts of chaos likely going +/- 1V or so.

This is a example of VCC rail.. (images taken a couple years ago)

n2.png
n2.png (2.08 KiB) Viewed 982 times

and a gnd spike..

n1.png
n1.png (1.53 KiB) Viewed 982 times

This was across the CPU pretty much, so like 1cm of track. Once it gets to like a foot long trace, welcome to crazy town!

I don't know how immune FPGA are, put some inputs can "latch up" if the voltage is to high or low on a input.
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Smonson
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Re: Project: HDMI/DVI out for STFM

Post by Smonson » Thu Dec 06, 2018 11:19 am

Oh, right. That was a DC offset. On the scope I saw occasional noise on the HDMI board ground of up to about +/- 650mV relative to PSU ground. But that shouldn't affect the level shifting buffer direction signal, I think, as it's localised to the mod board only.

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Re: Project: HDMI/DVI out for STFM

Post by exxos » Thu Dec 06, 2018 11:42 am

Smonson wrote:
Thu Dec 06, 2018 11:19 am
Oh, right. That was a DC offset. On the scope I saw occasional noise on the HDMI board ground of up to about +/- 650mV relative to PSU ground. But that shouldn't affect the level shifting buffer direction signal, I think, as it's localised to the mod board only.
As a mad idea, could you double the length of your ribbon cable on your board and try tests again ? Make sure PSU is at 5V or even 5.2V.
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Smonson
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Re: Project: HDMI/DVI out for STFM

Post by Smonson » Thu Dec 06, 2018 12:20 pm

Will do! ...soon, but not for a couple of nights, I have my work Christmas party tomorrow.

I can't actually set the voltage - my bench power supply can only supply 1.5A and the ST is just a bit too much for it. So I just have the ATX power supply which is fixed.

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sandord
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Re: Project: HDMI/DVI out for STFM

Post by sandord » Thu Dec 06, 2018 12:23 pm

Smonson wrote:
Thu Dec 06, 2018 12:20 pm
Will do! ...soon, but not for a couple of nights, I have my work Christmas party tomorrow.
Ah, be sure to bring your ST then and run this:


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Smonson
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Re: Project: HDMI/DVI out for STFM

Post by Smonson » Thu Dec 06, 2018 12:27 pm

Haha, I'm sure my boss would love it, and everyone under 30 would be confused!

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Re: Project: HDMI/DVI out for STFM

Post by ijor » Fri Dec 07, 2018 12:05 pm

So it is a powering issue indeed. Too bad Smonson can't yet reproduce it.

I still don't see however, how glitches, ringing, or gnd noise would produce the exact symptoms we see. According to the tests, at least for some of the readings, the bidirectional level shifter is not driving the bus at all. So whatever is happening, lasts at least for a full cycle. May be there is a combination of issues, or may be the tests are misleading somehow.

One thing that might make some sense, is that the FDIR signal is not being recognized as a high level. If the voltage level doesn't quite reach the minimum requirement for a high level (2V), the result might depend on the current VCC level of the chip. That might conceivable explain why it works with lower system 5V voltage. And the lower system voltage might not affect the FDIR signal itself because the FPGA power is regulated.

The FDIR signal looks quite healthy on Smonson's scope. But it might be different for some reason on other systems. So some additional scope measurements could be very useful. But they have to be done on a failing system and on a failing cycle. And scope ground should be connected at the daughter board.

A possible idea to confirm if the problem is the voltage level at the transceiver, could be to insert a diode between the chip VCC pin and the pad? Not sure how difficult would be to do that because of how the HDMI dual board is made.

May be there is a gnd problem at the transceiver that it's affecting? This chip has multiple gnd pins.
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Smonson
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Re: Project: HDMI/DVI out for STFM

Post by Smonson » Sat Dec 08, 2018 9:31 am

exxos wrote:
Thu Dec 06, 2018 11:42 am
As a mad idea, could you double the length of your ribbon cable on your board and try tests again ? Make sure PSU is at 5V or even 5.2V.
It worked perfectly for at least ten minutes with a 10cm cable (no bad reads) - when I increased it to 12cm it wouldn't even boot up once. Crashes immediately with garbage on the screen, or three bombs.

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Re: Project: HDMI/DVI out for STFM

Post by exxos » Sat Dec 08, 2018 10:11 am

Probably the 32MHz line gets to noisy with the longer cable. Annoying you couldn't replicate the fault. Will have another think.
4MB STFM 1.44 FD- VELOCE+ 020 STE - 4MB STE 32MHz - STFM 16MHz - STM - MEGA ST - Falcon 030 CT60 - Atari 2600 - Atari 7800 - Gigafile - SD Floppy Emulator - PeST - HxC - CosmosEx - Ultrasatan - various clutter

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