SDRAM cas timing

[Verruckt]

first off, is the correct terminology "cas timing" or "cas latency"?

Anyways, I have PC133 CAS2 SDRAM, but it is set to cas3 through the bios and the mobo is running at 143 FSB.

Why does some RAM need to be set to CAS3 when overclocking? What is this whole CAS business, and what does changing it do?

If I run the mobo at 138 FSB might the RAM work at CAS2?

Which is better - CAS3 RAM, 143FSB and a 933 P3
or
CAS2 RAM, 138 FSB and a 900 P3?

...thanks

 

[medic]

first off, is the correct terminology "cas timing" or "cas latency"?
Simply put, CAS Latency is a number that refers to the ratio - rounded to the next higher whole number - between column access time and the clock cycle time. It is derived from dividing the column access time by the clock frequency, and raising the result to the next whole number.
Good info:http://sysopt.earthweb.com/articles/latency/

Anyways, I have PC133 CAS2 SDRAM, but it is set to cas3 through the bios and the mobo is running at 143 FSB. Why does some RAM need to be set to CAS3 when overclocking?

143MHz. is pretty damn good if it's stable! All ram needs to be set for CAS3 to clock higher than CAS2 will allow it. Higher CAS number equals higher MHz.

What is this whole CAS business, and what does changing it do?
See above.

If I run the mobo at 138 FSB might the RAM work at CAS2?
Quite possibly.

Which is better - CAS3 RAM, 143FSB and a 933 P3 or CAS2 RAM, 138FSB and a 900 P3?
Hmmmmm....try it and let us know!! I would guess that setting the CAS from 3 to 2 would be more beneficial than a 5MHz. FSB increase...it would also depend heavily on the type of benchmark you used.

 

[peemo]

Thanks CRV. Do you have any info on the relative impacts of adjusting CAS Latency, tRP - Time to switch between memory banks, and tRCD - Time between RAS and CAS access?

Benchmarks I have seen usually compare 2-2-2 vs 3-3-3 which makes a big difference:
http://www.tweakmax.com/html/cas/cas-1.cfm

http://www.tweakmax.com/html/cas/cas-4.cfm

My bios settings:
SDRAM RAS-to-CAS Delay = 3
SDRAM RAS Precharge Time = 2
SDRAM Cache Latency Time = 2
SDRAM Precharge Control = Disabled by default

 

[medic]

That TweakMax article you mentioned is one of very few on the subject, I sure would like to see more of these indepth comparisons of ram timings, the Q3 scores in this were interesting though:
Interesting conclusion:
So what do we learn from these tests? Well, faster isn't always better. For example, in the latest test we can see that 800MHz at Cas-2 is just as fast as running the CPU at 840Mhz, memory at Cas-3! This means that if your 600E maxes out at 834MHz (139x6), but your memory can't run Cas-2 at this speed, you will get better performance if you go down to 800MHz (133x6). Now you will even have a much greater memory bandwidth (MB/sec) than at 840MHz, as shown in Part I of this article, which may give you an even greater advantage at this lower speed, in some applications. So the info in this article might be useful for those people that things having the best possible performance is more important than having a cool bios startup screen showing a lot of MHz
Must be carefull though on setting agressive timings..I've had a few computers come in with black screen errors from people messing with delay and precharge settings.
I do have a format coming up, I might do a basic install with only a few benchmarking programs and play with the timings and get some numbers.

 

[Verruckt]

How do I know if the CAS setting is not working? Will the computer not POST or something?

I want to try to set it at CAS2, but I'm not sure how to tell if it is working...if the system POST's and loads windows fine, do I then have to do some stability checks, or is merely getting into windows(2000) proof that it works?

thanks...

 

[peemo]

Ooops! I pushed the setting on my RAM a bit too far. Tried for
2-2-2 and hung at post. Luckily I just turn the PC off and reboot into default/recovery bios settings using my Soyo 6BA+IV and adjust downwards. Even after doing this I couldn't start Q3. Just got a blank screen bfore the ID logo animation started. Reduced FSB to 100 and used memory timing to 3-3-3 to give it a rest and now I'm back up to 3-3-2 at 115 FSB. Wonder if I overstressed my RAM. Could I have burnt it a little around the edges?