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Mainboard oscillators - are they used to set the FSB speed?

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First post, by retro games 100

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I notice when I replace an 80MHz oscillator with a 66MHz oscillator on a 386 mobo, the CPU speed is reduced from 40MHz to 33MHz. Is this because the mobo's FSB speed is dictated by the oscillator? If that is the case, then when I set a jumper/shunt configuration on a more modern board (such as a slot 1 board), or adjust the FSB value in the BIOS set up area on an even more modern board, is this action telling some kind of "programmable" osciallator what speed to set the FSB?

If that is the case, then what I would like to have is an osciallotor with a "speed wheel" embedded in to it, which looks like a volume wheel found on an old 8-bit ISA based sound card. I could then turn this speed wheel, to increase or decrease the speed of the mainboard.

I wonder - is it possible to adjust the FSB in "real time"? If not, then the wheel's position could be "locked" before the PC is switched on. If any of this is possible, I wouldn't need a handful of different speed oscillators for my 386 mobo, and have to change it everytime I want the board to run either faster or slower.

Reply 1 of 6, by Mau1wurf1977

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I think what you're saying is correct. Just that the "speed wheel" is likely a chip / IC and talks digital...

FSB can be adjusted in real time, I had this on a 486DX4-100 as a kid. I routed the FSB and CPU multi to the front of the case (silver switches). However going from certain settings to some settings would freeze the machine. Others would work fine while playing a game, which was pretty cool.

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Reply 2 of 6, by 5u3

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In the early '90s mainboard producers started to use variable PLL chips instead of one or multiple fixed oscillators.

I remember reading magazine adverts for exactly the thing retro games 100 described, it was a device which could be installed in a 3.5" or 5.25" drive bay, had some controls and a LED MHz display on the front, and connected to the mainboard via the osciallator socket. There even was a test of this thing in a german computer magazine (sorry, don't remember which) where it was critisized for causing instability, most likely caused by the long and not very well shielded cable connection between the mainboard and the "external oscillator".

Btw, real-time switching of the FSB always crashes my 486. 😒

Reply 3 of 6, by Dant

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Mau1wurf1977 wrote:

I think what you're saying is correct. Just that the "speed wheel" is likely a chip / IC and talks digital...

FSB can be adjusted in real time, I had this on a 486DX4-100 as a kid. I routed the FSB and CPU multi to the front of the case (silver switches). However going from certain settings to some settings would freeze the machine. Others would work fine while playing a game, which was pretty cool.

Hmmm.... Having some experience with game console overclocking, couldn't you fix the freezing issues by (if you can) soldering a switch onto the CPU halt line and breaking the connection before changing FSB speeds?

Reply 5 of 6, by Mau1wurf1977

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MatthewBrian wrote:

Some newer motherboards has a feature exactly what you described.

http://ru.msi.com/html/popup/MB/feature/oc_dial/index.html

That is pretty cool!

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