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Please Advise! Cas Latency confusion! :(

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February 13, 2007 9:17:52 PM

Am planning on getting e4300 and overclocking. I need 2gb RAM however confusion lies in whether to get 2GB ddr2 800 with CAS latency 5-5-5-12

2GB (2x1GB) CorsairTwinX XMS2, DDR2 PC2-6400 (800), 240 Pins, Non-ECC Unbuffered, CAS 5-5-5-12

or 2GB ddr2 667 with CAS latency 4-4-4-12.

2Gb (2X1Gb) Corsair TwinX XMS2, DDR2 PC5400 (667), 240 Pin, Non-ECC Unbuffered, CAS 4-4-4-12

Price difference is only £10. I understand that lower CAS is better but then if the frequency is higher of the RAM is greater then this is better? Which do you advise?

Thanks

-Ohwoojin-
February 13, 2007 10:10:31 PM

Ok remember all the CAS is is a number of clock cycles.
Chips are all about timing.

Usually, the lower the timing, the better off you are. However this can change depending on what you're shooting for. Of course, if you max out the RAM, you can get a higher CPU overclock on the 800Mhz RAM, right?

So first lets see how the RAM stacks up to the other:
CAS of 5 clocks. Now what's a clock cycle? 1/(frequency of RAM)
So 5/800Mhz = 6.25ns.
(yeah I know its not REALLY 800Mhz, but we're making it easy, plus we're dealing with relative time so it doesn't matter if its 400 or 800)

The CAS 4 of 667Mhz is 4/667Mhz = 6ns

So you can see that the 667Mhz RAM has a slightly faster refresh rate than the 800Mhz RAM, right? 6.25ns > 6ns

Now what difference does that make on overall performance? Not much. you're not going to all of a sudden have a supercomputer between the 2 RAMs there.

You have to decide how far you're going to push your overclock. If you're going to push to FSB400Mhz, then yeah, the 800Mhz RAM will do it for you.

If you're not sure, you can always try to find out how the CAS on the 800Mhz RAM if you run it at 667Mhz. It might be 4, same as the other ones. In that case, the 800Mhz makes more sense because it gives you greater flexibility later if you want to o'c more.
If you don't care about o'c'ing that much, or want a minimal overclock, might as well save the 10 pounds now. I'm assuming you're not going to go over FSB333Mhz else you would have picked the 800Mhz RAM.
February 13, 2007 10:41:49 PM

Thanks very much for your post. It was very useful. Unfortuantely I have to ask a really basic question but if I dont ask Ill never know. So CAS is responsible for the overall refresh rate. Right...Now as the FSB goes up the frequency of the RAM goes up at an equal rate. right?

Now, what is the difference between the number of clock cycles and the the memory frequency (clock speed?).

I think the 800 is better for me. For £10 the extra flexibility is good. The next model up has 4-4-4-12 CAS but is an extra 30 odd £. I know this is probably a hard question but what would that higher refresh rate show in performance?


Thanks
Related resources
February 13, 2007 10:52:53 PM

Thought about it some more and then I saw this:

"Please also use RAM that’s rated:
-DDR2-667 4-4-4-xx (good for ~400Mhz*)
-DDR2-800 5-5-5-xx (good for ~410Mhz*)
-DDR2-800 4-4-4-xx (good for 500Mhz+*) ->Best for E6300/E6400
-DDR2-1066 5-5-5-xx (good for 530Mhz+*) "

So if CAS latency is just responsible for the refresh rate of the RAM how come it affects the FSB able to be reached as the DDR2 800 with the lower CAS can reach 500 as opposed to the DDR with the higher CAS only being able to reach 410MHZ FSB.

Sorry if I missed your point....
February 13, 2007 11:12:38 PM

Hi ohwoojin,
I think I understand your questions, but I've got to run right now. I'll try to get back to you later tonight.
:) 

I'll throw this out and answer w/ a more detailed response later:
We know that the lower the CAS is, the better. That means that the chip can perform at higher speeds. CAS & the speed affect the stability of the system. If you have a lower CAS, you can overclock more (of course this assumes that your motherboard and CPU can take it).
February 14, 2007 4:32:28 AM

Okay.
Lets start off by saying that a memory module's rated speed doesn't necessarily mean that that is the maximum speed it can go. So a DDR2-800Mhz 4-4-4-12 module can go higher than 800Mhz.

So how does that work?
Each chip has a finite limit on it. We'll call it the chip's timing limit. At its base level, a chip is made of gates. These gates have a minimum time that they need to process information. (This is a somewhat high level, I don't want to get too technical.) Let's say that a chip has a limit of 10ns. That's the time it needs to be stable and to process information correctly. Each chip is different. One chip in the batch might be 10ns. The next might be 11ns. You never know.

So a 10ns chip can run a number of configurations using CAS and speed. Remember that they are related. x CAS clocks = x/speed.

Example:
Our chip is limited at 10ns.
So we have a number of configurations to get that maximum speed.
We can do CAS 4 and FSB400Mhz and that makes 4 clocks/400Mhz = 10ns. We also can do CAS 5 and FSB500Mhz and that makes 5 clocks/500Mhz = 10ns.
And CAS 3 and FSB300Mhz also makes 10ns.
At CAS 3, if you go over FSB300Mhz, you get instability.

So now you're thinking "why don't we do CAS 10 and 1Ghz FSB!!!?"
And this brings in the determining factors. Voltage. Power. Can the chip take that much voltage? Probably not. It might blow up. Can the CPU take that w/ its multiplier? Probably not. It might blow up too!

So when people bring down their CAS, they're trying to get to that chip timing limit. Same thing when they raise their frequencies. Once you go past that chip limit though, your data is probably bad, which leads to unstable systems, etc. So when you overclock, you're trying to push the chip limits on your memory, your motherboard, AND your CPU. When any of those can't take it anymore, you run into instability.

Now you're probably wondering why memory manufacturer's don't rate the memory higher. Well, I can offer this explanation. Memory modules have to last a certain time. Usually they last a long long time w/o overclocking. And that makes good press, because dying memory after 1 year makes people mad. When you overclock, you stress your components, decreasing their life span. So manufacturers rate their memory at a point where the memory can last a long time. Also its usually rated at a specific voltage, etc.

Note on the 4-4-4-xx at 667Mhz in wusy's guide, they're set at 5-5-5-12 in the instructions. Another way to get a better overclock is to raise the CAS and then you have a little more room for more frequency. Note that CAS 4 on 667Mhz is 6ns, and CAS 5 at 800Mhz is 6ns - on the RAM's specified rating of 6ns.

As for CAS 4 to 5.... you probably won't ever know the difference if you change it and don't do anything else (e.g. overclock more). Maybe a few % points in benchmarks, but hardly any effect in real-life.

EDIT: Fix math miscalcaulation: CAS 4 on a 667Mhz speed is 4/667Mhz = 6ns, not 5ns.
February 14, 2007 8:31:14 AM

Ah. I see now. Thanks for your time.

So surely the 5-5-5-12 DDR2 800 is better than the 4-4-4-12 DDR 2 667? The price difference is not that large so surely this is the better RAM as they are manufactured both by Corsair?

For my planned build I do not need to increase the FSB by too much, to about 333MHZ. I know this is can be achieved by either of those two RAM but surely it would involve running the DDR2 667 above its specified frequency? Lowering its life? Im sure it could handle this by as you said by increasing the CAS timings but the DDR2 800 with higher CAS would run without needing to be overclocked so therefore run more stabily/happily etc?
February 14, 2007 5:53:31 PM

Yeah so its sounding like the 5-5-5-12 DDR2 - 800Mhz will probably run CAS 4 at 667Mhz. Can't bet 100% on it, but it might be probable.

If I were in your position, and the 800Mhz RAM is only 10 pounds more, I'd just go for it. It gives you more headroom if you need it, plus a better chance of stability at 333Mhz. You could probalby push easily to a little higher w/ the 800Mhz RAM.

At this point, you're pretty much running the RAM at its specified life. You might have to raise voltage, etc. It could lead to decreased lifespan of the components. Can't say for sure, but its a risk. Of course the more you o'c, the more risk. But generally the risk is pretty small - the reduced lifespan of the components will probably still last past whenever you upgrade next.
February 15, 2007 10:03:26 AM

Thanks again. Ok I've decided to go for the DDR2 800 as for the above reasons. Im not planning to do MASSIVE overclocking. Components are supposedly designed to last 7-10 years. My mild overclock may lower that buy a year or so but to be honest I will probably upgrade in a few years so no problem.
February 15, 2007 5:24:30 PM

Great, glad to help. have fun building your computer! Now I want a new one. :p 
!