Problems with AM386DX-40 chip

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Problems with AM386DX-40 chip

Postby Gahhhrrrlic » 2017-12-05 @ 00:49

Hello all. I have an old 386DX-33 system with all original equipment in it but I wanted to upgrade to 40 MHz so I bought an AM386DX/DXL-40 chip and swapped it. The swap went fine, computer turned on no problem but at the POST screen it identified the cpu is a 80386 with no suffixes (just like before) so I could not tell from the screen that anything had changed. I put a diagnostic util on a floppy and ran it and it reported back that the cpu was running at 16 MHz. Then I went into the BIOS, found that it was set up to boot in slow speed mode so I changed that and checked the speed again... 33 MHz. Why isn't it running at 40 MHz? Did I do something wrong or is there I step I have yet to do? My mobo is this one:

http://www.elhvb.com/mobokive/Archive/B ... c-40ch.JPG

Thank you for the help.
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Re: Problems with AM386DX-40 chip

Postby Scali » 2017-12-05 @ 01:05

Motherboards from that era were hardwired for a single clockspeed. You can clearly see a 66 MHz crystal installed there.
This is divided by 2 to generate the 33 MHz clock.
This crystal has to be replaced by an 80 MHz one to make the CPU run at 40 MHz (but I can't guarantee that it will work properly, the motherboard and chipset have to be designed for it).

It wasn't until the 486 era that chipsets would have integrated clock generators, and you could select which speed they generated with a few jumpers.
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Re: Problems with AM386DX-40 chip

Postby Gahhhrrrlic » 2017-12-05 @ 01:45

But if it was the 486 era that allowed it and the socket on my mobo clearly allows chip replacement, what would be the point? To swap an SX with a DX? So if I get the 80 MHz, should it work in theory?

Also I have a 40MHz 387 floating point unit on the way in the mail. Does this mean it won't work now because it's not synchronous with the CPU speed? Oh despair...
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Re: Problems with AM386DX-40 chip

Postby Koltoroc » 2017-12-05 @ 02:47

Boards at that time were usually sold with the CPU already installed by the vendor (and thus the correct oscillator) or in complete systems. while there was no easy upgrade path at that time, from a manufacturing standpoint it was easier to just produce a board that could be used for as many different chips and just let the CPU and oscillator be the variable parts, either populated by request or by the vendor themselves. Also, later there were "Overdrive" CPUs released that were specifically designed as upgrades for existing 386 CPUs.

BTW, you can't switch between 386 SX and DX, the difference is the DX has a full 32bit Bus, while the SX only has a 16bit one. Advantage of the SX was, that manufacturers could use 286 chipsets or complete boards with minor to no redesign.

The 387 will likely run at 33Mhz as well, I believe they derive the clock from the same oscillator.
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Re: Problems with AM386DX-40 chip

Postby Gahhhrrrlic » 2017-12-05 @ 03:32

Thank you.

Hmm.. perhaps I should stick with the 33 mhz combo for a while and maybe 1 day I can mess with the crystal.

BTW, were AMD and Intel chips clocked at the same speed identical in performance or was 1 superior to the other?
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Re: Problems with AM386DX-40 chip

Postby fitzpatr » 2017-12-05 @ 03:58

http://www.vogons.org/viewtopic.php?f=46&t=57263

I had just been discussing this topic in another thread.

Oscillators are very inexpensive on eBay or elsewhere. You would want a DIP-14 one of twice the frequency at which the CPU operates. In your case, you would need to replace the 66.6666MHz oscillator with an 80.0000MHz oscillator.

If you are lucky, the oscillator will be socketed and it will be easy to swap.
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Re: Problems with AM386DX-40 chip

Postby Gahhhrrrlic » 2017-12-05 @ 06:05

Either that or I'll have to dig it out of the PCB ;) Oh well, if it was easy there'd be no satisfaction in it, would there?
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