[Tfug] Interchangeability of VRM's?

[Adrian]

On Monday 10 December 2012 19:16, Bexley Hall wrote:

> > More or less, there are 4 (or was that 5?) digital lines that select the
> > output voltage.
>
> Are they "static"?  Or, can the processor vary them while
> executing?  (i.e., are the 4/5 signals effectively hardwired
> to a specific processor-specific configuration?)

They could be changed dynamically, but presumably the processor would have to 
be halted during the change to prevent corruption. On a user-configurable 
board it could typically only be done from the BIOS setup screen. Usually it 
was set on POST before the CPU was active.

> So, they are more a function (responsibility) of the motherboard
> and *not* the processor.  I.e., the processor manufacturer expects
> your "board" to provide power "per this specification" and you
> (the MB manufacturer) are free to implement that however seems
> best for your goals

Yes, the motherboard manufacturer is responsible for meeting the 
specifications defined by the processor manufacturer. VRMs made it easy at 
the time as if one CPU required one core voltage and a later/faster CPU 
required a different core voltage, changing the core voltage profiles was as 
simple as updating the firmware.

Back in the day (PII/PIII/Xeon) these new "low power" CPUs required a 
precisely regulated ~1-2V direct core voltage with surges up to/over 100A, 
instead of the direct 3.3V or 5V power supply the 386/486/Pentium chips ran. 
Using VRMs allowed the motherboard manufacturer to skip the design/validation 
intensive task of a custom core voltage regulator that would work at a 
particular core voltage that might be needed later (making an easy swap for a 
later processor if need), while simultaneously eliminating the board space 
required by building vertically.

Typically what would happen is that at power-on, the baseboard management 
controller would start first (a 8081 or PIC type embedded microprocessor). 
The BMC would apply partial power to the CPUs, but not the core. At this 
point the BMC could read out the CPU ID via the I2C bus. After completing 
other house-keeping functions, the BMC would set the digital lines to the VRM 
based on its CPU ID lookup table, and then enable the VRMs, at which point 
the CPUs are "on". Alternatively, you could have the VRM lines held with 
pull-up/down resistors at power-on and then later have the CPU twiddle them 
with high-impedance output buffer circuits... but I suspect it was more often 
the former.

VRMs where mostly used in higher-end and server motherboards. My old SC450NX 
quad-PIII Xeon server had 6 VRMs onboard, 1 for each processor core and 1 
each shared between CPU-pair cache. Here's is a blurb from my eBay auctions 
way back in Sep 2001 when I was selling alot of them:

==========================
 This auction is for a VXI/Celestica model 073-20742-90 VRM. These are 12V 
input, VRM spec. 8.2/8.3 compliant and work with PPro, Xeon II/III, PII, and 
slot 1 PIII processors. Don't be fooled by other people selling 8.1 spec and 
saying the VRMs will work in everything, they don't. PIII require 8.2+ (PIII 
socket 370 require spec 8.4, P4 require spec 9.0+ and will not work with 
these VRMs). Xeon II/III require 8.2+ (usually 8.3+ in muli-processor 
boards). I personally use these in SC450NX "Sitka" boards with no problems.

 These VRMs are clean pulls, all have been thoroughly checked and load tested. 
Guaranteed not DOA and in good condition. You can get a copy of the PDF spec 
sheet <073-20742-90.pdf> here. There are a mix of manufacturing revisions 
with date codes of late 98 to early 2000, that means the VRM you get may 
differ slightly in appearance from the picture, though all are the same model 
and are functionally identical. I can most likely ship matched sets if 
requested.

 Note: VRMs come in both 5V and 12V varieties, these are 12V VRMs. Your 
motherboard must support the 12V type for these to work. Many earlier PPro 
motherboards and some PII/PIII motherboards only support 5V VRMs. The 
following use 5V only and are known NOT to work:
ALR Quad6 and Revolution 6x6
ALR 7200/8200
Intel PR440FX
HP Kayak
Some HP Intergraph

 Please check your motherboard documentation or ask before bidding.
==========================


Adrian