Can you underlock DDR2-6400 to DDR2-4200?

October 15, 2006 3:20 AM

I want to upgrade my pc's memory, but I noticed 6400 is the same price as 4200, and I can use the ram to in a faster system later. Does faster ram (400mhz) 6400 underlock to (233mhz) 4200? Gskill has 2 gigs for 187 on newegg, 5-5-5-15, why my system ram is ddr2-4200 4-4-4-12.
posted by IronWolve to computers & internet (9 comments total)

The fast ram will underclock just fine but it won't work because the timing's are different (5-5-5-15 vs 4-4-4-12 is way off).

I would highly reccomend going to the crucial website and getting memory that is verified for your particular motherboard, i have had bad luck with random budget ram off newegg in the past.
posted by sophist at 3:57 AM on October 15, 2006

It will underclock just fine, and it will work without you having to do anything clever, because your BIOS will read the SPD on the RAM module to tell it what timings to use. However, it will end up running 5-5-5-15 timings even at the lower clock speed, giving it marginally slower performance than the 4-4-4-12 RAM you already have.

You might be able to tell your BIOS to use faster timing with the new RAM, and it might work. Latency timings are specified based on the RAM's specified clock speed, and if you feed it a slower clock, the required latencies correspond to fewer clocks. 5 clocks at 400MHz take the same time as a whisker under 3 clocks at 233MHz, so you might even get away with 3-3-3-9 timing for the new card. But if you've got a mix of old and new cards, and you're setting these timings by hand, set them to whatever the slowest cards need: in your case, 4-4-4-12.

If you do choose to faff about with hand-set memory timings, just make sure you can run Memtest86+ overnight with no errors before you actually boot an operating system on the machine.
posted by flabdablet at 5:55 AM on October 15, 2006

Actually, the SPD stores timing data in nanoseconds, not clock cycles. The system will know that fewer latency cycles are needed at lower clock rates.
posted by ryanrs at 7:18 AM on October 15, 2006

Even better then. Thanks for the correction.
posted by flabdablet at 7:22 AM on October 15, 2006

What happens if you run two pieces of ram, each with different timings? If IronWolve put the new in with the old?
posted by davy at 8:38 AM on October 15, 2006

Each module has its own SPD, and each module's memory range gets its own timings.
posted by flabdablet at 8:52 AM on October 15, 2006

No. Generally, all modules must run at the same clock rate and CAS latency. These settings will be determined by the slowest module in the system. The Intel 955X and 975X chipsets work this way, as do all Athlons and Opterons.

If you know of a chipset with support for independent per-module or per-channel timings, please post a link.
posted by ryanrs at 11:30 AM on October 15, 2006

Man, I'm not doing well here; it seems that much of what I've learned about RAM is wrong. Thanks for the education, ryanrs.
posted by flabdablet at 8:43 PM on October 15, 2006

I bought the PC2-6400, worked fine, in fact it had settings for slower speeds. So it dropped from 400mhz to 266mhz.

So, while it did 400mhz at 5/5/15, it did 266 at 4/4/10, the ram I had in the box did 4/4/12 so it all synced up nicely.

So, Buying the faster ram at a lower price, was a good deal, and no issues. Its was OCZ marked ram on sale, so it was 50 bux off, 190 bux for 2 gigs at newegg.
posted by IronWolve at 4:13 PM on October 21, 2006