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Mixing memory (same DDR2-667 & CAS 5, but different other timings)

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June 26, 2010 11:25:12 PM

1) What performance will I get if I mix modules with the same DDR2-667 and CAS 5 timings but possibly different tRCD, tRP, and/or tRAS timings?

2) And will dual-channel operate with the 2 modules within each pair the same, but the two pairs different from each other (in size and/or some timings)?

3) And will the setup likely be stable in a situation where I have no control of motherboard voltages or timing?

I am planning to upgrade an HP m8120n pavilion media center PC.

[currently fully as original: Intel Q6600 processor, ASUS "P5BW-LA" / HP "Basswood3G-UL8E" motherboard (P965-based, but doesn't seem to correspond to any of the boards in ASUS P965 archive, so probably a partial custom job for HP), 3 GB DRAM as 2 x 1 GB + 2 x 0.5 GB, DDR2 667 MHz, PC2 5300, 1.8 Volt Samsung chips, 1 GB modules have chips on both sides, 0.5 GB modules have chips on only one side; HP specs say board will support 2 GB modules, 8 GB total with 64-bit OS].

Not overclocked nor overvoltaged in any way, and BIOS doesn't seem to allow any such changes. (Doesn't matter; fast enough --- but I don't want to lose any significant amount of speed, or cause instability, when I increase RAM).

CPU-Z reports all of existing DRAM is 5-5-5-15-20 at 667 MHz.

I would like to upgrade DRAM to 6 GB (in conjunction with an install of Windows7 64), by retaining existing 2 x 1 GB, and replacing the 2 x 0.5 GB modules with 2 x 2GB ones.

Looking at Newegg's DDR2-667 2 x 2GB memory offerings, I see several with CAS 5, but no info on other timings. Corsair's has 5-5-5-18 timing, so its tRAS is clearly different and 20% slower than what I have now. The Mushkin does have the same exact timing and voltage as what I now have, but it has a heatsink, which increases the thickness. I could probably get away with a heatsink if I'm only replacing 2 modules. However, based on recommendations I have read, I'm inclined to choose Kingston or Crucial --- but standard modules from these only list the CAS timing, and no data sheet available on their company web sites. Kingston's DDR2-800 HyperX (KHX6400D2K2/4G) might be a possibility, because supposedly its SPD is programmed as 667MHz 5-5-5-15 to ensure bootup, but it's tested at 800MHz at 1.85 volts, and I wonder if it'll run properly at 667 1.8 v. Also, their datasheet is still labelled "preliminary" and dated 08/09/07, and they haven't replied to my email technical inquiry, which doesn't inspire confidence.

Thanks, in advance, for any answers or suggestions.
June 26, 2010 11:35:33 PM

I think if you mix things up you have to use the lowest settings across all sticks. Likely to be the same with multiple banks, lowest common timings.

Anyone else?
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June 26, 2010 11:37:30 PM

If memory is rated 800@1.85v, 667@1.8v might well work. Best find 1.8v sticks though, or check you can return them if they don't pass memtest etc..
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