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CAS Latency

C

coolego1

Member
I was reading an article (can't remember where) that said that CAS Latency was only defined in integral values and was rounded up, meaning that 2.5 latency, without OC'ing runs as 3 latency... Correct me if I'm wrong. I'm looking into an AMD Athlon64 3000+ for a CPU and some OCZ Performance Series RAM with 2.5-3-3-6 latency...
 
Uh, I have never ever seen anything like that. DDR memory has cas timings that run from 2.0 to 3.0 to 4.0 depending on the speed of the ddr memory. ddr2 memory has higher latency due to the higher speeds and starts at 4 but they are already planning on 3.5 CAS latency! About time ddr 800 CAS 2.5 memory comes out AMD will release the ddr2 integrated memory controller! The higher latency is why ddr2 at 400(200ddr2) is the same speed if not slower than ddr memory at 400 (200ddr)
 
Sounds like FUD to me. If it "rounds up" already then why would there be both a CAS 2.5 & 3.0 setting?
 
my Corsair Value RAM is CAS 2.5, I'm running 2 sticks of 512, and lovin' it 🙂 my memory is like WAY up thur on AIDA 32's relative benchmark 🙂
 
I don't buy the part of "rounding up". DDR transmits on both the rising and falling edge of the clock signal, so I'd think (assuming we start with the request - is that the right term? oh well - on the rising edge)
So, if the data arrives:
two full cycles later, on the rising edge, you've got CAS2
two full cycles later, on the falling edge, CAS2.5
three cycles later, CAS3

I'm not sure if I explained that well.. a timing diagram would make things much easier :->
 
OK, I just had to make sure that spending the extra $$ on some 2.5 was really worth it. By the way, is a 2.5-4-4-8 worth $25 over a 2.5-3-3-6 ? I don't know how important the other numbers(after the first) really are...
 
Timings aren't very significant to A64 gaming performance so I'd suggest buying economy ram unless you intend to overclock, then I would go high frequency over tight timing.
 
I am not overclocking.

The question remains about the second and third numbers. Actually I just want to know if some 2.5-4-4-8 is better than some 3-3-3... The reason I'm so concerned about price is that I'm buying 2 sticks of 512MB so any money I can save is very good because it counts double.
 
Originally posted by: coolego1
I am not overclocking.

The question remains about the second and third numbers. Actually I just want to know if some 2.5-4-4-8 is better than some 3-3-3... The reason I'm so concerned about price is that I'm buying 2 sticks of 512MB so any money I can save is very good because it counts double.
Click to expand...
The performance difference between them is virtually non-existant, buy the less expensive ram.
 
However, the AMD 64's are more latency sensitive due to the integrated memory controller but the two rams you are talking about will not be great. 1 or 2 fps. MMMMM 1000 FPS in Quake 3. My goal (sarcasm)
 
There is a difference for me between 2.5 and 3 on my system on benchmark.... Anyways go for the cheaper stuff. Most of the timmings have little to no significant performance benifits.
 
Originally posted by: jediknight
I don't buy the part of "rounding up". DDR transmits on both the rising and falling edge of the clock signal, so I'd think (assuming we start with the request - is that the right term? oh well - on the rising edge)
So, if the data arrives:
two full cycles later, on the rising edge, you've got CAS2
two full cycles later, on the falling edge, CAS2.5
three cycles later, CAS3

I'm not sure if I explained that well.. a timing diagram would make things much easier :->
Click to expand...

As far as i know that is the reason, DDR memory works on two edges and due to the cas latency is the most important timing value sometimes the system performs better. In my own case i've reached better results with cas2 than cas2.5
 
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