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overclocked my 386 - w00t

M

Mike Chambers

Veteran Member
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Sep 2, 2006
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i was feeling a bit daring tonight. i've been sitting here all day getting Debian 3.0 Woody all set up on my 386DX 20 MHz beast here, and decided this was just a little too slow. i remembered i also have two other long-dead 386 mobos, and one was 40 MHz.

i desoldered the 40 MHz crystal on my working board, and replaced it with the 80 MHz crystal on the dead mobo. swapped the AMD 40 MHz CPU to it as well. also, something cool is that the dead board actually used a socket to go in between the mobo and crystal so i also moved the socket to the working board for easier toying around in the future. i wonder how quick i can get this thing while remaining stable with some faster crystals.. :)

booting back up with the new crystal into debian... huuuuge difference immediately. it's definitely about twice as fast. i'm feeling a little proud considering that the machine still works, lol.. so i felt the need to share.

does anybody have experience with crystal-swap overclocking? just how much boost can these AMD 386DX/40's take before they start becoming unstable? the chip isn't even lukewarm.

EDIT: i guess it's technically not a "crystal" - just read that these type are called "TTL clock oscilators"
 
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I'd be a bit cautious overclocking if the board you are using is an older model. I think the early VLSI chipsets were not always happy about running out of spec, and often times you will see the maximum rated speed stamped onto their surfaces. Not to mention, older boards might not allow you to adjust dividers that control operating speed of ISA or DMA, or insert extra wait states for DRAM and SRAM.

Though, it's always possible that your board and chipset are much later models, and your original 386DX-20 was put in there to create a budget system. In that case you shouldn't have a problem.

I recently performed a similar oscillator and socket procedure on a nice AMI 386SX cache motherboard that was running at 20MHz. In my case the "overclock" to 25MHz was fine because this board was offered with either 20 and 25MHz CPUs. The original CPU is not being overclocked, since it's being bypassed with a clip-on Cyrix upgrade. Because the BIOS lacks user options, I would not consider clocking this board to 33MHz.

If you're interested in pushing a board past 40MHz, I recommend using one of the very last models. Typically a very late 386 motherboard will infact use a 486 chipset, and shouldn't have any troubles handling 50MHz bus. While I've never done it myself, I have heard of people successfully pushing their am386DX-40s to 50MHz. I strongly recommend a heatsink if you try this. Quite honestly though, I don't know if it would be worth the trouble. While you will get a 25% boost in integer performance, don't be surprised if your FPU won't tolerate that kind of speed. In my experience overclocked FPUs often run very hot and return false results. Additionally, it is doubtful that you would see any gains in memory performance. Actually, it's quite likely that the wait states necessary to operate your memory stably at 50MHz will infact decrease throughput. I have played with the 50MHz bus quite a bit on 486 systems, and it usually turns out to be a big waste of time. If you really want to increase 386 system performance, you're much better off with a DRx2 or Blue Lightning upgrade chip.
 
Dood, you just got a 386 running faster than my 486DX-2!!! Nice!!
Click to expand...

I think you may have misread... CPU speed is half the oscillator speed. An 80mhz oscillator gets you 40mhz CPU speed. It would be quite a feat to get a 386 clocked up faster than a DX2 of any sort.

Anyway, how's the ISA speed and such? I'd presume it was running at 1/3 CPU speed, so did you find a way to change the ratio, or is it just running the bus at 13mhz now? And did the modification affect anything else, like the RTC or whatnot?
 
Old Thrashbarg said:
did the modification affect anything else, like the RTC or whatnot?
Click to expand...

... this reminded me of a malfunctioning Celeron board I found in the trash once. It only booted occasionally, and when it did, the RTC was ticking minutes by in seconds. I have no idea why, but I suspect it had problems...
 
Anonymous Coward said:
I'd be a bit cautious overclocking if the board you are using is an older model. I think the early VLSI chipsets were not always happy about running out of spec, and often times you will see the maximum rated speed stamped onto their surfaces. Not to mention, older boards might not allow you to adjust dividers that control operating speed of ISA or DMA, or insert extra wait states for DRAM and SRAM.

Though, it's always possible that your board and chipset are much later models, and your original 386DX-20 was put in there to create a budget system. In that case you shouldn't have a problem.

I recently performed a similar oscillator and socket procedure on a nice AMI 386SX cache motherboard that was running at 20MHz. In my case the "overclock" to 25MHz was fine because this board was offered with either 20 and 25MHz CPUs. The original CPU is not being overclocked, since it's being bypassed with a clip-on Cyrix upgrade. Because the BIOS lacks user options, I would not consider clocking this board to 33MHz.

If you're interested in pushing a board past 40MHz, I recommend using one of the very last models. Typically a very late 386 motherboard will infact use a 486 chipset, and shouldn't have any troubles handling 50MHz bus. While I've never done it myself, I have heard of people successfully pushing their am386DX-40s to 50MHz. I strongly recommend a heatsink if you try this. Quite honestly though, I don't know if it would be worth the trouble. While you will get a 25% boost in integer performance, don't be surprised if your FPU won't tolerate that kind of speed. In my experience overclocked FPUs often run very hot and return false results. Additionally, it is doubtful that you would see any gains in memory performance. Actually, it's quite likely that the wait states necessary to operate your memory stably at 50MHz will infact decrease throughput. I have played with the 50MHz bus quite a bit on 486 systems, and it usually turns out to be a big waste of time. If you really want to increase 386 system performance, you're much better off with a DRx2 or Blue Lightning upgrade chip.
Click to expand...

i believe the board is from 1989, possibly mfg. in 1990. it's an Everex 1830D:

http://irc.rubbermallet.org/th99/m/E-H/31568.htm

i haven't noticed any issues yet. in fact, when working on it i noticed that i had the wait state jumpers set to 5 and bumped it down to 4! no problems at all. there are 3 TTL oscillators on the board. the CPU one which i switched, a 16 MHz which i believe is for an 80387 if you install it, and a 14.31818 MHz right next to the VLSI chip and ISA bus - i am assuming it is clocked by that independent of the CPU speed.

even if the ISA bus were running a bit faster, maybe the VLSI chip wouldn't like it, but the cards i have installed i don't think would mind they're all not that ancient. a 3C509-TPB ethernet card, a diamond speedstar VGA (et4000 chipset), and a SIIG 16-bit IO/IDE/floppy controller from around 1996.
 
Old Thrashbarg said:
I think you may have misread... CPU speed is half the oscillator speed. An 80mhz oscillator gets you 40mhz CPU speed. It would be quite a feat to get a 386 clocked up faster than a DX2 of any sort.

Anyway, how's the ISA speed and such? I'd presume it was running at 1/3 CPU speed, so did you find a way to change the ratio, or is it just running the bus at 13mhz now? And did the modification affect anything else, like the RTC or whatnot?
Click to expand...

if it isn't governed by the crystal i mentioned in my last post, then it probably is and the cards and chip just happen to not mind. it's been running stable for over 24 hours now as-is in Debian 3.0. doing lots of processor intensive stuff, including a PHP rendering torture-test.:twisted:
 
Interesting. I was sorta eyeing one of those boards on eBay, wondering if it could be upgraded to a faster CPU... or rather, work reliably with an upgrade. Good to know that it will.
 
Old Thrashbarg said:
Interesting. I was sorta eyeing one of those boards on eBay, wondering if it could be upgraded to a faster CPU... or rather, work reliably with an upgrade. Good to know that it will.
Click to expand...

actually, to be honest i would reccommend against this motherboard. the performance is still awful overall because there are no sockets to install cache chips! i believe you can get a special board to put on it for extra RAM and some cache, but good luck finding one. other than that, it's pretty rock-solid.

oh and you can't install more than 8 MB RAM without that card. let me guess, you were looking at one being sold by "lamcsales"...

P.S. i'm posting this message from the 386 in question with ELinks. :)
 
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Hmm, looks like you have a mid model 386 board. Everex boards are generally known to be of pretty high quality, but as you mentioned it doesn't offer much in the way of cache or memory expansion. The C&T chipset was a pretty good choice for 286s, but I'm not very familiar with their 386 chipsets. What kind of BIOS is on this thing, are there any options you can set? My AMI 386 board with 1990 BIOS has almost nothing to play with.

It is my guess that your ISA cards are currently running way out of spec. The 14.31MHz oscillator you mentioned provides the OSC signal for ISA cards that use it, but as far as I know the ISA bus always runs synchronous to the CPU (at least I've yet to see an asynchronous ISA bus). I'm not sure what Everex decided to use as an ISA clock divider, but I think the OP who guessed 3 is probably right. That would mean that your cards are likely now operating at 15MHz. A lot of later model ISA cards can cope with that speed, but certain NICs, many SCSI controllers and sound cards will freak out.

If you want to invest in a new 386 motherboard, here's what you might consider:

-ability to expand to 16 or 32mb directly on the motherboard
-somewhere between 64kb and 256kb cache
-opti, forex, ali or eteq chipset (there might be other good ones too, but these are some that I've used)
-PGA sockets for 386 and 387 chips (weitek is always nice too)
-AMI BIOS is always nice, though sometimes Phoenix is good too. Avoid DTK BIOS.
-A way to hook up external CMOS battery
-Support for 40MHz CPUs (DLC support is always nice too)
-If possible get a board manufactured by AMI, Micron, Everex or Mylex (AMI Baby Screamer is a good one)

Try to avoid paying too much too. The boards on ebay are generally a ripoff, and a lot of them have leaky CMOS batteries. I recently bought a board on ebay where the battery leaked and dissolved a few traces. Be prepared to desolder the battery, clean with isopropyl alcohol, and patch up bad spots.
 
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Old Thrashbarg said:
Ah, right, I was thinking it did have cache... maybe it was a different board I was thinking of, then.

In any case, the memory expansion thingies aren't particularly hard to find. Still doesn't seem to add any cache, though.
Click to expand...

thanks for the link! thinking about getting it, but i might as well just go for a new motherboard really. decisions, decisions.. lack of cache really gives a nasty performance decrease.
 
Yeah, all things considered it would probably be a better option to just upgrade the whole board.

One thing that might be worth looking at, is the Everex EV-1816 from the same seller. It's another one of those that only officially supports 20 or 25mhz, but it seems that can be worked around fairly easily. It does have cache... up to 256KB if Statson is correct, and it's cheap.
 
That EV-1816 is a weird board. It has sockets for cache both on the motherboard and the memory expansion card.

I think you'd be better off with a board more like this one:

http://cgi.ebay.com/Motherboard-w-I...emQQptZLH_DefaultDomain_0?hash=item48372c2969

I don't necessarily agree with the price, but it looks like a good board. You'll notice that the battery is a bit leaky.

If you're patient, you will eventually find a similar board with a better price.


*EDIT*

What do you think about this one? Looks good to me.

http://www.weirdstuff.com/cgi-bin/item/15015

http://stason.org/TULARC/pc/motherboards/C/COMPUTREND-SYSTEMS-INC-486-CONTAQ-386-MS-3124.html
 
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Now that you guys posted it, I was following up some leads this weekend and found home garage junk sale. Picked up TWO NOS never assembled Jameco flip top cases. Then I asked if they had any other stuff and she pointed out pack to the family trash pile. Buried in the leaves and mud, literally, were two complete XT systems and a complete 386/486 convertable system where the MB will take either a 386 or 486 chip. I remember those when they were new. All boxs are complete with HD. The XT's I can save but would have to strip out the 386-486 board and totally clean and double check it first. Right now it has an AMD 386 in it.

I ran a 386-40 AMD at 48mhz back in 1995 with 8mg ram and an external cash chip set. Running dos and WFWG 3.11 I set up a boot file that booted from dos, made a ram drive then copied a working of 3.11 into the ram drive then booted the 3.11. The system ran every bit as fast as a 486-33 486-40. If I modified the 3.11 I would have to save copy it back to HD before turning off the power.

Z


Mike Chambers said:
i believe the board is from 1989, possibly mfg. in 1990. it's an Everex 1830D:

http://irc.rubbermallet.org/th99/m/E-H/31568.htm

i haven't noticed any issues yet. in fact, when working on it i noticed that i had the wait state jumpers set to 5 and bumped it down to 4! no problems at all. there are 3 TTL oscillators on the board. the CPU one which i switched, a 16 MHz which i believe is for an 80387 if you install it, and a 14.31818 MHz right next to the VLSI chip and ISA bus - i am assuming it is clocked by that independent of the CPU speed.

even if the ISA bus were running a bit faster, maybe the VLSI chip wouldn't like it, but the cards i have installed i don't think would mind they're all not that ancient. a 3C509-TPB ethernet card, a diamond speedstar VGA (et4000 chipset), and a SIIG 16-bit IO/IDE/floppy controller from around 1996.
Click to expand...
 
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