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 […]
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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).

This is a test-fit of my design.

The double-sided 132-pin socket fits beautifully in the motherboard and PCB, with a ~2mm vertical gap.

The 168-pin press-fit fits the CPU, but PCB needs a redesign. Drills are 0.05mm too small.

I discovered the PCB should also be 2mm thick so the sockets sit flush on both sides. And I'll be adding the new flush circuit to make things easier.

Flush circuit on a 386sx/486SXL(C)2:
After testing a new 486SXL(C)2 386sx system I have questions about the BIOS knowing whether it's a 486SLC/DLC CPU and doing something with the 486 flush instruction "INVD" (https://www.felixcloutier.com/x86/invd). On the 386sx board with the Flush pin not connected to anything, and BARB disabled, Flush enabled... the cache worked. The board has hidden refresh and does not operate HLDA during normal memory refresh. When using another BIOS, the CPU was recognised as "486" not "486SLC" and the cache didn't work.

In any case, if the circuit on the Improve Technologies interposer covers 90% of cases and is better than hooking up MEMW, then I'll redesign it that way, with a solder jumper.

MikeSG wrote on 2024-05-02, 04:43:

On the 386sx board with the Flush pin not connected to anything, and BARB disabled, Flush enabled... the cache worked. The board has hidden refresh and does not operate HLDA during normal memory refresh. When using another BIOS, the CPU was recognised as "486" not "486SLC" and the cache didn't work.

According to my old notes, this was a common occurrence.

For motherboards which did not have the FLUSH# pin connected to anything, all I had to do was ensure that the full range of memory wasn't set to non-cacheable, e.g. using cyrix.exe -i1 -i2, etc. On other boards (still without FLUSH# connected), I may just need to enable the flush pin with cyrix -f, and again, ensure -i1 -i2 set.

The majority of 386 boards I've encountered did not have the double NAND gate flush circuit connected to FLUSH# (although some did) and had FLUSH# not connected, yet worked with L1 cache by setting cyrix -f -i1 -i2. Some of these 386 boards pre-date the release of the DLC by a year or two, and presumably, were not be L1 aware.

For instances in which I had to use the BARB method, it was because of a DMA based SCSI controller, like the AHA-1542C.

Yeah, might be a good idea to have a jumper dis-connectable option for the Improve-It flush schema on your design, while not eliminating the MEMW# option.

Can you describe the BARB method, I may try it on my system with the scsi controller. It’s been sitting on the shelf, need to get it out again.

MikeSG wrote on 2024-05-02, 04:43:

On the 386sx board with the Flush pin not connected to anything, and BARB disabled, Flush enabled... the cache worked.

of course, the problem was never cache not working, the problem was working cache glitching out other parts of the system like ISA DMA. So was floppy working reliably with cache enabled on that system?

One of the tests I like to do is play a (downsampled) mp3 file in Windows, while trying to access the floppy drive. Presumably, this tests floppy DMA, sound DMA, and FPU interaction in one shot.

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

Just out of curiosity: is this dongle already fully analyzed?
I got hold of a board with exactly this cpu/dongle and as it pretty rare to see in the wild I was wondering if is worth to analyze in detail.

feipoa wrote on Yesterday, 11:27:

One of the tests I like to do is play a (downsampled) mp3 file in Windows, while trying to access the floppy drive. Presumably, this tests floppy DMA, sound DMA, and FPU interaction in one shot.

.MP3s often use fixed-point arithmetic for compression, and any players (and compressors) from the from that era will almost certainly not use floating-point operations.

rasz_pl wrote on Yesterday, 10:37:

[...] was floppy working reliably with cache enabled on that system?

I tested a few file copies from a floppy and it works. I use "Cyrix.exe -cd -b- -f -m". I don't have much set up to test with at the moment..

Have you tried -m instead of -i?

Skorbin wrote on Yesterday, 11:40:

Just out of curiosity: is this dongle already fully analyzed?
I got hold of a board with exactly this cpu/dongle and as it pretty rare to see in the wild I was wondering if is worth to analyze in detail.

It's worth it - could you label the connections of the two large ICs? The traces are hard to follow when they go underneath.

AlaricD wrote on Yesterday, 14:07:

.MP3s often use fixed-point arithmetic for compression, and any players (and compressors) from the from that era will almost certainly not use floating-point operations.

feipoa wrote on Yesterday, 11:27:

I didn't mention an era for the mp3 player I'm using. If I remove the FPU, the player won't play the mp3 file very well. It tries, but there will be 10 second gaps of silence.

As a follow-up to my previous comment, AlaricD, could you let me know which mp3 players do not use the FPU? I would like to try them out on my SXL2 systems.

I was running a few tests on my SXL2-66 system with mp3 files.

Win 3.11
WinPlay3 v2.3beta5 16-bit (1994-1997)
WinPlay 3 will spit out an error if you don't have an FPU installed:
"Sorry, WinPlay3 requires a 486 Processor with a build in FPU"
Yes, it has gramatical errors.

Win NT 3.51
WinPlay3 v2.3beta5 32-bit (1994-1997)
Curious how WinPlay3 will actually attempt to play the mp3 file without an FPU installed, but the gaps between sound samples makes it unplayable. Install an FPU, and it can do 64 kbps, mono, 11 KHz. There was a sound skip at about 38 seconds into playback.

Win NT 3.51
Winamp v1.60RC5 (1997)
This plays a little better than WinPlay3, I think, because you can set the decoding priority to HIGH rather than NORMAL. WinPlay3 didn't have this option. The 38 second skip doesn't exist here, but don't put your song on repeat unless you want to manually reset the system. I did not bother trying to use Winamp without the FPU installed.