Sphere478 wrote on 2024-03-30, 18:21:

You would need a floating power supply and to tie one of the legs to negative of the chip. Probably the +5 of the new floating p […]
Show full quote

kingcake wrote on 2024-03-30, 15:24:

feipoa wrote on 2024-03-30, 01:21:

As pshipkov mentioned, I need 5.15 V going to the SXL2 for stable operation at 90 MHz. I only need 3.85 V for stable operation at 80 MHz.

That should read 37 C on VRM heatsink, steady-state, with only the linear reg. I've fixed it now.

Wire your regulator to +12V and +5V to get 7V Vin. You'll need an amp of load on your +5V power supply rail to ensure it doesn't try to sink current. A 5R power resistor would work fine hooked up to a molex connector. This will drastically cut your linear regulator's heat load.

You would need a floating power supply and to tie one of the legs to negative of the chip. Probably the +5 of the new floating psu

It can be done. I thought of it earlier but it’s kinda silly when you can just buck down from
+12v into some capacitors

Oh definitely. I was just saying it's a possible hack to avoid interposer redesign for a one off type thing.

kingcake, if I wanted lower VRM or CPU temps, I'd us my $1 in-line buck. Interposer redesign not needed. Without it, 35-37 C on the VRM and 27 C to the CPU is nothing to frown upon. Nonetheless, I took some additional measurements at steady state, 90 MHz.

Linear MIC29302WT only (no in-line buck)
5.15 V to CPU
VRM w/fan = 35.4 C
CPU w/fan = 27 C
VRM w/out fan = 43 C
only CPU fan, CPU = still 27 C
The 30 mm 3.2 cfm fan seems to help the VRM temp quite a bit, but not having it doesn't harm the CPU temp.

Buck at 6.3 V, Linear MIC29302WT at 5.15 V
VRM w/fan = 22.4 C
CPU w/fan = 23 C
VRM w/out fan = 23.5 C
only CPU fan, CPU = 23 C
So, if I use the buck, I don't need the VRM fan and the CPU runs 4 C cooler. Conclusion, the buck would only be needed if the CPU temperature once inside a case becomes a problem for stable operation.

Some other numbers from the benchtop:

DRAM = 50.2 C in the centre of 8 sticks
GD-5434 = 32 C
Northbridge w/heatsink = 26.9 C
SRAM = 29.9 C
FPU while running Quake for 8 minutes = 22.2 C
Buck regulator = 30.5 C

The DRAM seems a bit warm, but I'm using 9-chip modules here. This board did not like my 3-chip pieces at 45 MHz. Should I blow the DRAM with a fan?

By the way, there was another of these SXL2-50 chips with flush dongle from Improve-It that sold for $250. I was surprised to see that it sold for that price. https://www.ebay.com/itm/326071534917

I can tell these are collectors, otherwise the SXL2-66 + adaptor is the cheaper path to a much better place.
Of course, many of these threads are not visible from top level search engines, so good chance many people are just not aware.

Have you been able to stress the CPU @ 90 MHz against a sustained synthetic benchmark to ensure complete stability?

feipoa wrote on 2024-03-30, 23:55:

By the way, there was another of these SXL2-50 chips with flush dongle from Improve-It that sold for $250. I was surprised to see that it sold for that price. https://www.ebay.com/itm/326071534917

That seems like a rather silly amount to pay given how easy it would be to get a regular CPU and make one of those circuits just from visually looking at one of them. It seems like it wouldn't take much to re-create one of those circuits (although I could be wrong).