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DDR2 667Mhz CLass 5 vs CL6

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June 29, 2012 2:57:37 PM

Hi,
I see several threads already on this subject but if you don't mind I would like to ask again.

Currently have 2GB RAM (2x1GB modules) in a Dell Latitude D630 which I have recently installed Win 7 on. Someone informed me that this machine could benefit with 2GB more RAM (2x2GB modules). The Chipset/BIOS in this machine only allow it to clock @ 667MHz. The Crucial website memory advisor tool recommends 800MHz RAM - perhaps they are unaware Dell won't allow that.

Amazon sells Crucial 200-Pin SODIMM DDR2 PC2-5300 CL=5 Unbuffered NON-ECC DDR2-667 1.8V 256 Megx64 2 GBx2 4 GB Kit Memory Modules & Crucial 4GB PC2-6400 2GBx2 200-Pin CL6 Unbuffered DDR2-800 SODIMM RAM Kit for virtually the same price. I have read that 800MHZ RAM would be more stable while down clocked to 667MHz but the 667MHz RAM has a Class 5 latency.

Which set should I purchase if any?

thanks

More about : ddr2 667mhz class cl6

a c 347 } Memory
June 29, 2012 3:07:35 PM

Whatever the Default RAM Frequency is for your CPU then that's the fastest Dell or any OEM BIOS is going to allow. DDR2 800MHz will down-clock to DDR2 667MHz and the lower the CAS per Frequency then the faster the kit -- DO NOT MIX: Frequency, CAS nor especially Voltage. Yes 2GB for XP and 4GB of Windows 7 are the minimums I recommend.

As far as which kit -- flip a coin.

CAS vs Frequency:
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a b } Memory
June 29, 2012 3:09:12 PM

If RAM is listed on Crucial's online tool then that means they have tested it and gotten it to work before, so the ones on there will work, although they may require some configuration.

The CL actually stands for CAS Latency, something technical if there is any confusion about what that means.

Anyway, 667 @5 is the same as 800 @6. It is the same speed. The higher the MHZ speed the higher the CL is. It is measured in cycles of the processor not in real time, so a processor that cycles faster means it can get more cycles in the same amount of real time.

You can tell by dividing the 667 by 5 = 133 and the 800 divided by 6 = 133.
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a c 347 } Memory
June 29, 2012 3:24:03 PM

CAS is a measure in ns nanoseconds (billionths of a second). The DDR2-800MHz will down-clock to same overal 'speed' as the DDR2-667MHz kit.

The calculation of CAS latency for the Chart above is CAS latency (ns) = (CAS / Frequency (MHz)) × 1000
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June 29, 2012 3:28:26 PM

OK. Thank you both. The chart is very helpful & the info on higher MHz & higher CL is great. Will commit to memory - no pun intended.
Guess will buy the PC2-6400 Kit, it is a couple $$ cheaper.

thanks
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a c 139 } Memory
June 29, 2012 3:35:02 PM

jaquith said:
Whatever the Default RAM Frequency is for your CPU then that's the fastest Dell or any OEM BIOS is going to allow. DDR2 800MHz will down-clock to DDR2 667MHz and the lower the CAS per Frequency then the faster the kit -- DO NOT MIX: Frequency, CAS nor especially Voltage. Yes 2GB for XP and 4GB of Windows 7 are the minimums I recommend.

As far as which kit -- flip a coin.

CAS vs Frequency:
http://i1013.photobucket.com/albums/af254/Jaquith/CAS_Frequncy_Chart.png


I hate to nitpick but the DDR transfer rate is twice the IO bus frequency, not twice the core memory frequency. There's another ratio between the IO Bus frequency and the core memory frequency, for DDR it is 1:1, for DDR2 it is 2:1, and for DDR3 it is 4:1. Thus, DDR-400 / PC-3200, DDR2-800 / PC2-6400 and DDR3-1600 / PC3-12800 all have core memory frequencies of 200Mhz with IO bus speeds of 200Mhz, 400Mhz and 800Mhz respectively. Even top of the line DDR3-3000 only has a core memory clock of 375Mhz
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June 29, 2012 4:04:41 PM

Best answer selected by snoots.
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a c 347 } Memory
June 29, 2012 4:50:30 PM

Pinhedd said:
I hate to nitpick but the DDR transfer rate is twice the IO bus frequency, not twice the core memory frequency. There's another ratio between the IO Bus frequency and the core memory frequency, for DDR it is 1:1, for DDR2 it is 2:1, and for DDR3 it is 4:1. Thus, DDR-400 / PC-3200, DDR2-800 / PC2-6400 and DDR3-1600 / PC3-12800 all have core memory frequencies of 200Mhz with IO bus speeds of 200Mhz, 400Mhz and 800Mhz respectively. Even top of the line DDR3-3000 only has a core memory clock of 375Mhz

Not to nit pick either, I never said anything to the contrary. Though on Intel the '1:1' doesn't pertain here if the BIOS is limiting the RAM to DDR2-667; the OP's Intel® Core™2 Duo Processor T7250 (2M Cache, 2.00 GHz, 800 MHz FSB) therefore the ratio is 3:5 not 1:1. Further, you're mixing two different things into one: (1) I/O bus clock - is the actual clock speed of the RAM, (2) 1:1 refers to the ratio of the CPU's (rated) FSB : I/O bus clock of the RAM. I've seen this same issue with Dell and other OEM BIOS many times.

OP:
Quote:
The Chipset/BIOS in this machine only allow it to clock @ 667MHz.

ME:
Quote:
Whatever the Default RAM Frequency is for your CPU then that's the fastest.


Looking up the Intel Core 2 Duo T7250 - http://ark.intel.com/products/31728/Intel-Core2-Duo-Pro...(2M-Cache-2_00-GHz-800-MHz-FSB) ; it has a FSB of 800MHz so DDR2-800 will work but Dell is apparently setting (limiting) the RAM to DDR2-667 (3:5).

Dell's Specs "Min: 512MB DDR2 shared 667MHz; Max: 4GB DDR2 shared 667MHz" - http://www.dell.com/downloads/ap/products/latit/LAT_D63...

Example of a Dell:
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a c 139 } Memory
June 29, 2012 6:03:52 PM

That's the Front Side Bus, not the memory IO bus. The IO bus clock is not the speed of the ram modules. DDR3 makes 2 transfers per IO clock and there are 4 IO clocks per module clock
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