exxos's Atari, Amiga & retro forum

Can you measure the voltage from the PSU 0v to 0v on the HDMI connector ( or somewhere as far up the board as possible) ?

exxos wrote: 06 Dec 2018 00:19 Can you measure the voltage from the PSU 0v to 0v on the HDMI connector ( or somewhere as far up the board as possible) ?

I did not get to this last night. CPU failed on my PC. Am not doing well with several PSU issues last month and now CPU failure this month.

I should get time to look at this tonight.

Sorry to hear about your CPU Icky. That really sucks.

Also I ran my machine on an ATX PSU at 5.06v for over fifteen minutes, but unfortunately didn't experience a single bad read. Ah, well.

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).

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 https://www.exxosforum.co.uk/forum/viewt ... 360#p15179.

Smonson wrote: 06 Dec 2018 10:00 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)


and a gnd spike..


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.

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.

Smonson wrote: 06 Dec 2018 11:19 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.

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.

Smonson wrote: 06 Dec 2018 12:20 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: