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RAM Speed Mismatch Problem?


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I recently bought two 1GB sticks of Corsair "Value Select" DDR400 RAM to go in my machine. The product code is VS1GB400C3.


I already had two 512MB sticks of Crucial 333mhz RAM in the other two slots. I made some enquiries before buying the new RAM, and was told that it was OK to put 400mhz RAM with 333mhz - the 400 would simply run at the lower speed. I got the 400 because it was a good deal, and because I really wanted 1GB sticks and have had some God-awful problems with 1GB sticks of 333 (long story).


My Motherboard is an Intel D875PBZ.


My CPU is a P4 2.4Ghz with 533mhz FSB.


OK, this is what happened:


I put the new RAM in the two spare slots (the matching ones to run in dual channel mode). When the machine booted, I got the following error message:


"SERIAL PRESENCE DETECT (SPD) device data missing or inconclusive. Properly programmed SPD device data is required for reliable operation. DDR333mhz memory assumed at slowest timings.


Press Any Key To Continue..."


The machine completed booting, but was very flakey. Some things were really fast, some really slow. The mouse was jumpy and erratic. I inserted a CD and opened the CD window in explorer, but none of the files on it showed up. I tried to create a new folder on the hard drive and it crashed. (This machine has NEVER crashed, in the few months I've had it).


I rebooted and the error message didn't come up again, but all the same flakey behaviour did.


To try and shed some light on it, I removed the Crucial RAM, and put the new RAM alone in it's place. Bingo - no more problems. The BIOS shows that it is running at 333mhz, but I presume that's because of the 533mhz FSB of the CPU (?) Anyway, the machine seems stable (though I haven't had it up for long yet).


So what's the story? Is there something wrong with the SPD of the RAM? I've had a hunt around this excellent site and found out (a) that this RAM is guaranteed compatible with my mobo, and (b) that all Corsair RAM has SPD. So why the error message?


Can I use these two lots of RAM together confidently, and if so, how? If not, why not?


If necessary, would it be possible to exchange the two sticks for ones that can co-exist with the Crucial? I don't mind if they're 333mhz (since they're going to run at that anyway), but there's this whole hoopla about high-density and low-density 333mhz that gave me grief for months before I got to the bottom of it. They need to work!


Many thanks for any help you can give me.

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Here is my guess about what is going on:


The VS modules you have probably only have SPD programming for CL3. The part number only guarantees operation at 400MHz CL3. The SPD is probably not programmed for CL2.5 333MHz or CL2 266MHz operation. From other posts, I think Corsair's argument is that they use 'generic' RAM for these modules. Therefore, it would complicate things for them to add full SPD support.


On the other hand, the Crucial modules are 333MHz. Industry standard 333Mhz modules do not include CL3 support. They only guarantee CL2.5 support. It is possible that CL2 support might be programmed (but not likely).


So what is happening is that the BIOS cannot find a solution using the SPD's. The Corsair only supports CL3 and the Crucial does not support CL3. Hence the error message. I think the error message only comes up once after the memory configuration is changed.


The board then tries a lowest common denominator solution, probably 333Mhz CL3. Apparently, the Crucial cannot live with this. This is quite possible if the Crucial is more than one year old. If the Crucial is newer than that, it should work at CL3. In that case, 266MHz CL2.5 might be needed because of a loading issue.


If you had a non-Intel board, you could manually adjust the timings to 333Mhz CL2.5. Both the Corsair and the Crucial would probably work together at this setting (or 266MHz CL2.5 if necessary). However, Intel boards do not allow manual memory timings.


You could verify this theory by using a hardware information tool that shows/decodes the full SPD contents of both module brands.


So I am afraid (IMHO) you have to choose either the Crucial (333MHz CL2.5) or the Corsair (333MHZ CL3), but not both. Maybe one of the gurus or RAM GUY can find a solution you will like better.


Mixing memory is dangerous business! Unfortunately, the Intel board does not give any tools that you could use to help.

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  • Corsair Employees

Hello Pin_one, and thanks for your help! And you are correct.

And Pin1_Refresh, there are a few things that you have been told that are correct, but there are exceptions to every rule. And many of the MB's today will not post unless all of the modules are exactly matched. So Pin_one is correct. And you can run DDR400 memory in a system that only supports DDR333 so the answer you were given is correct as well. But, the SPD of all modules would need to be exactly matched for the MB to be able to decode them. If they are different at a given speed you will get the error that you see! You can try and set the timings manually to Cass 2.5-3-3-7 and see if the system will post, but the VS1GB400 modules are not rated at that speed so I do not know if it will be stable. I would suggest you just use our modules for best performance and compatibility!

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Thanks both of you for your speedy replies.


I run several machines so I'll probably just find a home for the Crucial somewhere else. Or even if I have to sell it on, I probably won't lose too much as it has a good name.


Man, memory is so much more complicated than it used to be in the PIII days. I seem to have been going through this stuff for about six months and there's always some little hidden away compatibility issue that turns around and bites me in the bum.


C'est la vie.

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In the PIII days, Intel ruled. The 440BX was king. They wrote and released the PC-66/PC-100 memory specs and everyone had to follow.


Then Intel lost their way (another story!) and AMD / JEDEC brought us DDR. Since there are fewer AMD people checking everything for 'compliance' and more companies participate in developing AMD chipset platforms, experimentation started to flourish. This led to PC-2700 and PC-3200.


The experimenters focused on faster systems to sell NOW!, not backward and forward compatibility. The standards that JEDEC approved 'after the fact' reflect this, especially in PC-3200.


Left to their devices, it is unlikely JEDEC would have ever extended DDR1 beyond 266MHz (2-2-2).


I can say from some personal experience that it is impossible to program a module SPD following any set of consistent rules (or even exception to rules) that will boot with optimal performance in every motherboard. If one follows the 'official' rules, some board BIOS versions might not even boot at all! Mixing memory makes finding a solution even worse. The haphazard way that high speed DDR came to market is largely responsible for this.


So we are left with DDR1 memory systems that are much faster than many 'experts' predicted possible a few years ago. Industry-standard DDR2 has to achieve its 3rd generation (PC2-5300) to consistently beat it in PC's. But these memory systems are not as 'robust' on the compatibility front as most consumers expect.


Are we better off? You be the judge! (And you are allowed to give different answers when you are building/upgrading your PC than when you are playing games with it.)


One man's opinion....End of editorial!

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