Quote Originally Posted by GAR View Post
I was wondering the same thing.......anyone??
i found this

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, such as ASUS and DFI, 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.

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.