Changing the frequency of a 386

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Changing the frequency of a 386

Postby fitzpatr » 2017-12-02 @ 04:41

Hi everyone,

I've got a Motherboard with a PQFP-132 AMD 386DX-40 and a socketed DIP-14 80MHz oscillator (originally zip-tied). I've purchased a batch of 66.6666MHz and 50MHz oscillators with the intent of being able to adjust the clock frequency to 33.3333MHz and 25MHz for games, specifically the notoriously speed-sensitive Wing Commander.

My questions are:
Other than changing the oscillator, for a normal 386 motherboard would anything else be required to get the CPU and motherboard to clock down?

Is the motherboard able to detect the frequency at which the CPU is running and/or its designed frequency?

I can post pictures of the board tomorrow or the day after, if needed or interested.

http://stason.org/TULARC/pc/motherboards/M/MAGITRONICS-386-A-B341H.html

Thanks!
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Re: Changing the frequency of a 386

Postby sd_entertainmnt » 2017-12-02 @ 05:59

I have done this mod to use a faster 386 by changing a crystal that was soldered, I don't see why it wouldn't work to go with a slower one.
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Re: Changing the frequency of a 386

Postby Jo22 » 2017-12-02 @ 06:25

1) I agree to what sd_entertainmnt said. In addition, some mainboards can insert wait states or
halve the system's clock rate. This also makes things slower (sometimes also used for turbo buttons).
2) Yes, the BIOS is often able to detect the frequency at which the system is running at.
Some of the 386/486 hybrid chipsets may even include a BIOS that is able to to read the CPU ID
feature found in later 486 processors. It will then display the detected model (486DX2-66 at 66MHz, etc).
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Re: Changing the frequency of a 386

Postby JidaiGeki » 2017-12-02 @ 07:14

Having just played through Wing Commander on a 386DX-40 a week or so ago, I can tell you that you likely don't need to slow it down at all from 40 MHz for WC; in fact there were times during some dogfights when I wanted a few more MHz! Same goes for WCII.
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Re: Changing the frequency of a 386

Postby fitzpatr » 2017-12-04 @ 02:56

JidaiGeki wrote:Having just played through Wing Commander on a 386DX-40 a week or so ago, I can tell you that you likely don't need to slow it down at all from 40 MHz for WC; in fact there were times during some dogfights when I wanted a few more MHz! Same goes for WCII.


I'm having a hard time telling where, exactly, it is supposed to be running at. Judging by the opening cinematic, a DX-25 or 33, or SX-33 match the music the best.

Thank you to all for the input.
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Re: Changing the frequency of a 386

Postby Anonymous Coward » 2017-12-04 @ 04:06

You don't need to do anything else to slow down the 386 CPU, but you will need to make adjustments to memory timings and clock dividers in the BIOS to make sure other devices on the motherboard aren't underperforming.
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Re: Changing the frequency of a 386

Postby feipoa » 2017-12-04 @ 17:10

Are you using a UMC 481/482-based motherboard?

What are the symptoms which led you to believe that you need to slow down your system?

Did you already use the turbo button in the non-turbo state to ensure that this speed-down is not sufficient for your application?

By the way, I've tested oscillators below 12 MHz which still appear to work, but boot-up is particularly slow.
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Re: Changing the frequency of a 386

Postby fitzpatr » 2017-12-04 @ 18:22

feipoa wrote:Are you using a UMC 481/482-based motherboard?

What are the symptoms which led you to believe that you need to slow down your system?

Did you already use the turbo button in the non-turbo state to ensure that this speed-down is not sufficient for your application?

By the way, I've tested oscillators below 12 MHz which still appear to work, but boot-up is particularly slow.


No, I'm not experiencing any particular symptoms at the present time. I had originally bought a 386DX-33 under the assumption that I could change speed by changing the processor as with modern machines. I didn't even install it before I realized how the operating frequency was actually determined.

You are correct that it is UMC 481/482 based motherboard. I've attached a picture of it (or will after resizing it). It was in my father-in-law's storage area, and it did have some battery leakage before I got to it. Fortunately it was fairly minor, and the board is still fully functional.

I'm waiting on a case before I assemble a system around it, but I have all of the parts ready.

I'll be using a Diamond Speedstar64 ISA (Cirrus Logic CL-GD5434), Sound Blaster Pro 2 CT1600, an Goldstar Prime2 controller and possibly a Conner CP-30064H 61MB Hard Drive if it passes muster.
Attachments
Magitronics 386DX-40.jpg
Magitronics AM386DX-40 Motherboard
UMC 481 / 482
Last edited by fitzpatr on 2017-12-04 @ 20:22, edited 1 time in total.
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Re: Changing the frequency of a 386

Postby carlostex » 2017-12-04 @ 20:05

I have a similar board, and i'm now using MR. BIOS instead of the original AMI. I found that my turbo button stopped working but i can now use CTRL ALT - and + for turbo off/on and CTRL ALT SHIFT - and + to disable/enable cache.

I'm pretty happy with it.
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Re: Changing the frequency of a 386

Postby feipoa » 2017-12-05 @ 02:19

Oh, nice that you don't have the soldered-on 386 PQFP. I've never seen that UMC 482 chipset with such small font.

You definately want to use the MR BIOS bios with this motherboard as the AMI BIOS for this chipset is slow. It seems like MR BIOS did something which allowed the RAM to be run at much less wait states. I found that to run it at 0 WS (as set in the BIOS), I needed to use 60 ns DRAM. Has anyone else experience this? Oddly, I found that not all my 60 ns RAM would work with 0 WS and some required 1 WS (which is the case for 70 ns DRAM). I wondered if this could be because the responses time of some RAM increased with decades of use.

I highly recommend you scrape away all of the blue corrosion and rinse these areas with vinegar. Then wash away the vinegar, rinse with ISO, then dry. The blue corrosion travels along the copper if not removed removed and neutralised (e.g. with vinegar).

I use the Diamond Speedstar 64 (GD5434) in my system. Nice fast card. Are you able to use yours with the graphic card's jumper set to 0 WS? Do you use Windows 3.11 with your board? I found that for stability in Windows 3.11, when the Diamond drivers are loaded, I needed to increase MR BIOS's 16-bit ISA wait-state from 1 WS to 2 WS. Luckily, this did not alter the results of benchmarks. This trick doesn't fare so well when using faster clock-doubled/tripled CPUs, like the BL3.

Carlostex: I too discovered the inability to use the turbo button, which was rather upsetting, but as you pointed out, you can use CTRL SHIFT options.
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Re: Changing the frequency of a 386

Postby CkRtech » 2017-12-05 @ 02:36

feipoa wrote:I highly recommend you scrape away all of the blue corrosion and rinse these areas with vinegar. Then wash away the vinegar, rinse with ISO, then dry. The blue corrosion travels along the copper if not removed removed and neutralised (e.g. with vinegar).

Seconded. Get it neutralized, scrubbed, removed, rinsed and dried properly.
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Re: Changing the frequency of a 386

Postby fitzpatr » 2017-12-05 @ 06:16

Funny, I was examining the board tonight and thinking that that would be necessary with this as well.

I tried it on a recovered Amiga 2000, but it was too late to prevent the damage to that board.

Thank you to both of you.

I haven't yet played around with the 5434. It's on my list of things to do.
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