A couple of CPU pins are connected to the VRM's voltage selection pins. A slotket's jumpers can override these signals, but obviously only with voltages the VRM actually supports.

smtkr wrote on 2022-06-08, 02:01:

I've been looking at slotkets and a common feature of them is a jumper set (or dip-switch set) that is configured for a specific voltage setting. One of the settings is typically auto. This is probably common knowledge, but how does the auto setting work?
Is it
1. The motherboard polls the CPU for a voltage. The CPU returns the voltage it is configured for. The motherboard supplies that voltage.

I think [1] comes closest.
During bootup the BIOS checks the voltage register of the cpu and 'communicates' that value to the motherboard's voltage regulator.
The motherboard (or slotket) can have jumpers or a switch that enables overriding the cpu register value.
And more sophisticated motherboards can have a BIOS that allows overriding the cpu register value by the user via software.

I looked it up in the datasheet today while I was taking a break (see attached).

Based on that, I guess the slotkets mask the VID pins if you select a voltage and pass the pins through if you put the slotket on auto.

I don't have anything to test, but that makes sense. Thanks for the replies. I've satisfied my curiosity.

Oh that’s clever. If I’m understanding this right, instead of jumpers, they just use the cpu as it’s own programming card of sorts?

And if you intercept those pins you can change the vrm controller to other settings.

Nice.

Designing a tweaker slocket might be a fun project to tackle some day down the road. Got my hands full for now though. No time soon.

Sphere478 wrote on 2022-06-09, 06:43:

Designing a tweaker slocket might be a fun project to tackle some day down the road. Got my hands full for now though. No time soon.

Having followed a number of threads on the subject here leads me to believe that voltage tweaking on slotkets is not much of a priority requirement. Many if not most Coppermine motherboards from the popular manufacturers come with BIOS versions that support voltage adjustments made by user.
The two requirements that clearly stand out and are not out of the box available on most slotkets are:
--On board voltage regulator fed by external power source (molex => psu) that covers the 1.3volt to 2.0volt range required by Coppermine cpu's. This allows Coppermine cpu's to be run on older motherboards that do not provide voltages lower than 1.8volt.
--Support for Tualatin cpu's on Coppermine dedicated motherboards.

Are you tempted ? 😀

PARKE wrote on 2022-06-09, 17:01:

Having followed a number of threads on the subject here leads me to believe that voltage tweaking on slotkets is not much of a p […]
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Sphere478 wrote on 2022-06-09, 06:43:

Designing a tweaker slocket might be a fun project to tackle some day down the road. Got my hands full for now though. No time soon.

Having followed a number of threads on the subject here leads me to believe that voltage tweaking on slotkets is not much of a priority requirement. Many if not most Coppermine motherboards from the popular manufacturers come with BIOS versions that support voltage adjustments made by user.
The two requirements that clearly stand out and are not out of the box available on most slotkets are:
--On board voltage regulator fed by external power source (molex => psu) that covers the 1.3volt to 2.0volt range required by Coppermine cpu's. This allows Coppermine cpu's to be run on older motherboards that do not provide voltages lower than 1.8volt.
--Support for Tualatin cpu's on Coppermine dedicated motherboards.

Are you tempted ? 😀

I am, I’ve kinda been itching to make a dual tualitin build.

One of my ideas was to try and shoehorn two into that dual slot 1 tyan mobo that uses simms hahaha

Perfect platform for developing a slot 1 to tualitin adapter.

But alas this is something I don’t have time for atm. Need to finish the projects I’ve already started.

You are a mad scientist 😁

There is old style, YOU set it, for things like Super Socket 7.
New style like in LGA775, where the CPU uses the FID pins to request a FSB clock and the VID pins to request a voltage, plus additional dynamic selection according to speedstep (set multiplier and voltage)
Now when the transition was made, I'm not 100% sure, I guess it began with Slot 1

The spec sheet for the Pentium Pro mentions automatic selection of voltage on page 16:
https://www.dexsilicium.com/Intel_PentiumPro.pdf

The VID is really 5-bit with VRM regulator chips that are used with P2/P3/SS7, but that doc only shows 4, I guess because the upper bit had lost relevance by then.
With a 5-bit VID the upper bit locks you into a range above or below 2.0V. If you're using jumpers it can be a good idea to put a red jumper on that bit to remind yourself it's the one that kills CPUs.

I have a slot-1 board where I absent mindedly installed a row of 2.5V capacitors on Vcore, forgetting that 2.8V Pentium-2s exist. That board uses auto voltage, so if a 2.8V Klamath was installed it would blow the caps.
I've thought about hardwiring the upper bit on that board so it can't go over 2V. Kind of a band-aid but I don't think I care if it can actually run Klamaths.

IIRC I tried a interesting test out of pure fun where I tried the lowest voltage a Mendocino could POST with, and got it as low as 1.35v. (tried 1.3v as well but it flat out refused to beep at that point)

Which makes me wonder if Klamaths may be able to tolerate 2.2v Vcore, kinda the same voltage some K6s run at. This is something I'll be looking into if I can find a mainboard that lets me adjust Vcore.