A Lisa Inside An FPGA

[slewis1962]

Alex, just wondering how many amps your usb-c circuitry can handle. Is this going to do any damage in the long term? I don't have a lot of experience designing boards as far as power rails go.

*EDIT*
Alex, I looked up the 12v boost converter and see that it can supply 10A so I guess that answers my question.

[sigma7]

the 12v boost converter ... can supply 10A

The TPS61178 datasheet says the switching current limit is 8A +/- 1.4A ( +/-17% !) when the current limit setting resistor is 80K6. I suspect Alex's 86K6 value is close enough to consider it the same. However, I think this is not the output current limit of the circuit... if the duty cycle of the current switch were 75% then the average current would be more like 6A, and I wonder if this limit applies to the switching current at the source voltage (5V), so the overall circuit would be making 30W, or 2.5A at 12V. Perhaps (presumably?) Alex used the WEBENCH® Power Designer and may be able to recall what performance it predicted?

slewis1962's measurements suggest 2.9 Amps more draw from the overall +5 supply when the Twiggy is in use, so +14.5W and thus maybe ~1A from the 12V regulator is enough. When I look at the 12V on my drive, I see substantial spikes, but that may be completely due to my power source, or maybe the Twiggy breakout board would benefit from a big cap.

theory that uninitialised PRAM is in a random, invalid state; it's not all zeroes

I've found this depends... contents at powerup seems random/unpredictable in general, but may be consistent for a specific part. IIRC, I helped someone repair an I/O Board and after replacing the SRAM the all-zero bytes power-up problem revealed itself on a consistent basis. They had another I/O Board, so I suggested mixing the parts between the boards to get around the issue. I don't recall much about it, so I may be imagining things again.

[AlexTheCat123]

The TPS61178 datasheet says the switching current limit is 8A +/- 1.4A ( +/-17% !) when the current limit setting resistor is 80K6. I suspect Alex's 86K6 value is close enough to consider it the same. However, I think this is not the output current limit of the circuit... if the duty cycle of the current switch were 75% then the average current would be more like 6A, and I wonder if this limit applies to the switching current at the source voltage (5V), so the overall circuit would be making 30W, or 2.5A at 12V. Perhaps (presumably?) Alex used the WEBENCH® Power Designer and may be able to recall what performance it predicted?

I wasn't aware of the WEBENCH Power Designer when I did the first one or two regulators (which included the 12V one), so I did the calculations by hand based on the info in the datasheet. That was NOT fun, so I soon discovered WEBENCH and used it for the rest of them. But I know I went for 2A on the 12V regulator at an absolute minimum, and perhaps more than that. I was trying to match or beat the specs of the original Lisa PSU, so I know that I didn't go below that.

[slewis1962]

Here is my video showing the LisaFPGA board with dual Twiggy drives. Let me know what you think.

[AlexTheCat123]

Here is my video showing the LisaFPGA board with dual Twiggy drives. Let me know what you think.

YouTube is saying that the video is private.

[sigma7]

dual Twiggy drives. Let me know what you think.

Looks like it works, fantastic!

I see that at timestamp 0:51, with both drives running, the 5V power supply says 6.49 Amps

[AlexTheCat123]

Looks like it works, fantastic!

How were you able to see it? The video still shows up as private for me.

[sigma7]

Looks like it works, fantastic!

How were you able to see it? The video still shows up as private for me.

I have no idea. It plays fine embedded in the post, and I can play the link in YouTube in a separate tab too. I'm not logged-in to Google/YouTube is the only thing that comes to mind as a potential difference.

[AlexTheCat123]

I have no idea. It plays fine embedded in the post, and I can play the link in YouTube in a separate tab too. I'm not logged-in to Google/YouTube is the only thing that comes to mind as a potential difference.

Strange. I can't play it embedded, in a separate tab, in a separate logged-out browser, or anywhere. I guess it doesn't matter as long as we know that the Twiggies work, although I would like to see them!

[slewis1962]

Here it is again. I changed some settings. Hopefully this will play now. If not I'll delete and re-upload. I first published it through iMovie and accidentally set it to private. I published it and then changed to public at that time. Maybe that messed something up.

[slewis1962]

dual Twiggy drives. Let me know what you think.

Looks like it works, fantastic!

I see that at timestamp 0:51, with both drives running, the 5V power supply says 6.49 Amps

I actually saw around 7.5 amps the other day while I was testing with both drives. I noticed with one drive connected it was showing right at 1 amp when the drive was idle and with both drives connected around 1.5 amp while both drives are idle.

[slewis1962]

If that doesn't work you can try searching YouTube for "Dual Twiggy drives on a LisaFPGA board"

[slewis1962]

I re-uploaded it just to be sure it is viewable by everyone.

[ried]

Awesome! Congrats!