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How to: Clock Skew Can Save Your Overclock And Memory

post #1 of 10
Thread Starter 
Reason For This FAQ
Are you letting the clock skew adjustment sit at the factory default of "Auto" because you don't know what it does and what to do with it? If so, you are missing an important piece to the overclocking puzzle. Not all mobos have this adjustment - but if you've got it, you need to know how to use it.

Overview
When timing problems occur between CPU and memory, your rig becomes unstable. If this happens while overclocking, you back off a bit thinking you reached your cpu/memory limits. If it happens during the initial boot of a new build you curse your bad luck and get ready to RMA your high-priced memory and/or mobo. One last adjustment you need to try before either of these actions, that might just save the day, is adjusting your clock skew. Timing issues can occur between any of the many buses in your computer, but it is a real problem when it occurs amidst the ultra high speed chatter of the cpu, memory controller and memory channels. Mobo manufactures know this and some provide settings in the bios to allow you to fine tune the memory clock and compensate for these timing issues. According to ASUS, you may be able to maintain a stable 10 to 15% performance pop by adjusting clock skew. It has also been reported that mystery compatibility problems between mobos and high quality memory modules such as Corsair and OCZ were fixed by adjusting skews.

First - My Standard Bloodthirsty CYA
I am not an electrical engineer and according to my wife of 36 years not even particularly smart or good-looking. If you follow these instructions and break your mobo or any part of your rig causing dire consequences to your bank account and/or sexual performance - it is entirely your problem and all I owe you is my sympathy. I am just a doofus like you messing everyday with a perfectly good machine discovering new ways to improve or break it.

Don't Care To Get Down and Dirty?
For overclockers with A.D.D. you can skip to the bottom line at the end of the FAQ for the adjustments. For others with an inquiring mind, read on for a fuller explanation of how transmission timing issues in buses occur and how clock skew works.

Excruciating Detail In Non-Technical English about What Is Happening
As any overclocker knows, the different components of a computer operate at different speeds called frequencies. It is these that we adjust to maximize the efficiency of our computers. One function of any component is to adjust the speed of any communication it receives to match it own. When the speed of a component overwhelms another's ability to compensate - problems occur. For you old-timers, think Lucy and Ethel working the assembly line in the candy factory

Every component communicates to others through its bus. Most buses (SATA being a notable exception) are connected to other buses using paired transmission lines; each line is physically a wire or circuit. One transmission line in a pair carries data only, and the other control information such as what type of data is being transmitted, signal requests and acknowledgments.

Most of the time these lines are in sync with each other, so that for example; the memory controller knows how to process the data it received from the CPU on the data line, because of the description of the data it simultaneously received from the CPU on the control line. However, when we increase the already extreme speeds of the CPU or memory, we enhance the likelihood that these two lines become out-of-sync. This happens for the simple reason that the lines are of different length and composition, although the control information and data are in sync when they enter the two lines, they do not come out at the same time.

One of the jobs of the memory controller is to resynchronize the information it receives from the CPU. It delays whichever information is received first until the other arrives. The amount of delay is called “setup/hold time.” It accomplishes this by sending the earlier received data on an electronic run-around-the-block called a "feedback loop." The hope is that by the time the first data has returned from its delaying run, the other paired data will have arrived. If the lines get too far out of sync, the memory controller does not have sufficient setup/hold time to reconnect the data. It ends up with two bits of data from one line and one from the other. When this happens, the memory controller gets confused as to what data goes with what description and has a functional breakdown.

Minuscule adjustments or “skewing” the speed of the memory clock changes the setup/hold time. By entering a delay value into the bios clock skew setting we slow the memory clock. This slows the first data a little bit on it's run-around-the block, thereby increasing the likelihood that it's paired data will appear.

As a result, by increasing the setup/hold time (slowing the memory clock) we may be able to run stable overclocks at higher overall frequencies. This is because we are giving the memory controller more time to resolve transmission timing issues that become more severe when we run at higher frequencies. If our overclock is already stable, decreasing the setup/hold time (speeding up the memory clock) won’t give much of a performance increase. This is because we are making adjustments in trillionths of a second, and a trillionth here and a trillionth there, still doesn’t add up to much. But if advancing restores peace and harmony between the CPU and that pricey memory you just bought, than you will be glad you read this FAQ.

If you have a mathematical background or really want to know more about clock skew – here is a link that describes this effect in mind-boggling technical detail: Optimize Timing Margins For Your High-Speed Interface

The Bottom Line Adjustments:
Most overclockers leave clock skew adjustments at their factory default of “Auto.” The problem is that when overclocking at higher frequencies, the auto control does not adjust precisely enough to keep your overclock from crashing, hence a manual setting is often needed. Also, "Auto" cannot compensate sufficiently to resolve some memory compatibility problems.

Manual adjustments for clock skew are here for ASUS and DFI mobos:

ASUS mobos:

ASUS mobos allow you to tune each memory bank separately. Very handy, since each bank has its own transmission path and need different skews to be fully optimized.

In some ASUS mobos the bios adjustment is referred to as “DDR Clock Skew.” There is one for “Bank A” and another for “Bank B”. In other mobos they are referred to as “AI Clock Skew for Channel A” and another for “Channel B”.

Regardless of how they are labeled the choices are the same: “Auto”, “Normal” and a series of “Advance” (speed up clock - reduce setup/hold time) and “Delay” (slow down clock - increase setup/hold time) adjustments from 150ps to 900ps. These are not large adjustments - one picosecond (ps) is one trillionth of a second. Light travels only 3mm in one picosecond.

I don’t have any specific recommendations as to settings because variances between components are too wide. However, a 1 or 2 ps adjustment is not going to do much. Think in increments of 50 or 100ps starting at 100 or 200ps. It’s tough to find solid info on this adjustment (a cynical no-thanks to ASUS) but I did find this reference from Tony at bleedingedge that seems to be a good and reasonable approach. I took the liberty of including it. He was tuning his A8R32 MVP mobo. Here is what he wrote:

“For channel A I found normal or a slight delay worked best, for DDR600 delay 150ps

For channel B I found advancing worked well, 300ps for DDR600.

Now, you must tune and test with 1 dimm, tune the skew and get the ram stable as high as you can in memtest86+. Then lower the clock a little and remove the dimm, add the second dimm and test the same and tweak the skew for the second channel. Channel A is closest to the CPU and B furthest away. Do NOT alter any memory timings; only tune the skew under the config system frequency/voltage section. Once you have a base line knowing what each dimm is stable to, re enter bios and set 5 fsb lower than the lowest stable clock from the 2 dimms tested. Switch off and add the second dimm back to its slot, reboot and re enter memtest...check if it’s still stable.”

DFI mobos:

“DQS Skew Control” = “Auto”, “Increase Skew” (speed up clock - decrease setup/holdtime), “Decrease Skew” (slow down clock - increase setup/hold time).

"DQS Skew Value"= 0-255 in 1.0 increments. This is the value that is increased or decreased when you set the DQS Skew Control. Begin adjustments in the 50-255 range.

Conclusion
I was as many overclockers content to let Clock Skew ride at Auto because I did not understand how it worked. I wrote this FAQ simply because internet searches yielded a lot of references to promotional materials lauding its value - but not much as to how to actually use it. After much research I now know that it can be a vital adjustment and is an important tool in the overclocker's tool chest. I’d like to apologize to all electrical engineers if I killed any technical details in the writing of this FAQ, but I wrote this for the non-technical overclockers – like me.
Edited by woop - 5/22/11 at 8:35pm
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post #2 of 10
pretty useful guide for those, like me, who thought that they had all the variables of an overclock covered rep+
    
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post #3 of 10
VERY nicely done. If I could give you 100 Reps I would
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post #4 of 10
Nice Faq--Well Written
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post #5 of 10
Nice !

Can it be done with Gigabyte Motherboards? can't seem locate it in mine..
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post #6 of 10
Thread Starter 
Sorry - afraid I don't know. I went through the Gigabyte web site and didn't find anything. Maybe a Gigabye owner will chime in.
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post #7 of 10
Well researched/written.........hell, I gave you rep for attitude alone and having been married to the same woman for 31yrs.
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post #8 of 10
Very nice. I haven't actually seen that option in my MSI K8N Neo4 or ASUS A8N-SLI Premium BIOS, but I know A64 tweaker has the option, and that it increased stability with my Corsair XMS Platinum RAM. Thanks!!
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post #9 of 10
Nice faq - rep+. Now I have yet another thing that I know I really should do, but am too lazy to do. <G>
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post #10 of 10
I'm still lost....
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