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I just updated the BIOS for the first time on my Intel d975xbx2 to BIOS 2809. After the BIOS update completed, in the "Event Log" I saw the error: "SPD Tolerance (1)".

My RAM is Patriot 4GB(2 x 2GB) DDR2 800, and I only had 1 stick in at the time of update (b/c I have yet to install Vista).

Anyone have any clues as to what "SPD Tolerance" means? I thought there'd be at least SOME documentation about this, but Google only has 2 hits...

I've seen it a few times on older machines, SPD if I remember correctly is Serial Presence Detect. In your case probably ram speed timings detected incorrectly. Go into bios and check the memory timings, if their incorrect set them manually to the manufacturers recommended timings and try that.

Thanks, I think I understand. I guess it comes down to 2 questions:

1. Is it OK to run memory at less than the manufacturer's specified voltage? i.e. my Patriot memory works at 1.84v but spec'd at 2.0v, but memtest86+ running for 24hrs with 1.84v on both modules returns no errors. Would it be safer for stability to adjust it to 2.0v, or is the less heat from 1.84v actually safer?

2. If I only get the SPD Tolerance error during a BIOS upgrade, and not during POST (as Intel says below), this must mean that there's nothing to really worry about?

Some more observations for any others having this issue:

One oddity I noticed is that when I set the BIOS to its defaults and immediately check my memory timings, it's set at 533MHz with 4-4-4-13 timings. When I reboot, this is suddenly corrected to 800MHz at 5-5-5-14. The memory is rated at 2.0v, but detected at 1.84v--memtest86+ testing for over 24 hrs is stable at 1.84v.

It doesn't say it in any d975xbx2 manual, but here (http://download.intel.com/support/mothe ... ver_13.pdf) under "Event Name" it says:

"Bad SPD tolerance - Some fields of the DIMM SPD may not be supported, but could be tolerated by the Memory Reference Code. When Error Is Caught: POST"

Apparently, no Patriot memory (or even Corsair for that matter) is officially tested on the d975xbx2 to work:
http://www.intel.com/support/motherboar ... 026616.htm

Some Corsair users have experienced the same error after a BIOS upgrade with the d975xbx2:
http://forums.guru3d.com/showthread.php?t=212193

So I am guessing that the SPD "standard" is not yet perfected across all systems. However, Intel's "Memory Reference Code" apparently managed to configure my memory to meet most of Patriot's specifications.

Most performance DDR modules have several SPD profiles.

I believe the DDR800 specification states modules should be 1.8v with a latency of 5ns.

Therefore to ensure compatibilty all memory modules will have the default profile set to 5-5-5-15 (or there abouts) so that the module will run correctly at 1.8v.

I'f you have performance modules you need to select the second profile for optimum performance. However you will need to increase the voltage to 2v or whatevers specified.