X-bit labs

Fastest Pentium 4 Platform: Performance of i925, i915, i875 and i865 with DDR2-533, DDR2-400, DDR533 and DDR400 SDRAM (page 3)

Category: Chipsets

by Ilya Gavrichenkov

[ 08/04/2004 | 12:38 PM ]

Overclocking

A great drawback of the current i925/i915-based mainboards in comparison with boards on the older chipsets is in the problems you encounter trying to overclock the CPU. The problems take root in the support of the new peripheral bus (PCI Express), which is clocked not like the PCI and AGP used to be. Many i925/i915-based mainboards offer you the option of setting the carrier frequency of the PCI Express bus independently in the BIOS, but this option turns to be ineffective.

As a result, increasing the FSB clock rate (and that’s the way you do overclocking) also leads to a higher PCI Express frequency, causing numerous problems. Particularly, PCI Express x16 graphics cards are the first to stop working at overclocking, since many of them are fastidious about the clock rate of the bus. Then, don’t also forget that Intel uses a bus called Direct Media Interface (DMI) to link the North and South Bridges of its new chipsets, and this bus is in fact an analog of PCI Express x4. Thus, the growth of the FSB frequency on modern mainboards provokes an appropriate growth of the clock rate of the PCI Express x16 bus as well as of the DMI bus and then of all the busses that are realized through the South Bridge. That is, the clock rate of PCI Express x1 grows and the frequency at which all IDE controllers work increases, too. That’s why you may render your hard disk drive (or any other peripheral) non-operational trying to overclock the CPU on a mainboard based on Intel’s new chipset.

Thus, it’s not so easy to do overclocking with the new chipsets. In fact, average mainboards on the i925/i915 start having problems at FSB frequencies of about 220-230MHz. On this background, the i925X-based products from ABIT and ASUS stand prominent – they achieve better results at overclocking. The engineering teams of these two companies found a way to control the multiplier of the carrier frequency of the PCI Express bus. As a result, you can raise the FSB clock rate much higher at overclocking – the record is about 280MHz. To achieve this result, you need to use special components, though. For example, graphics cards on ATI’s GPUs only.

Well, the overclocker shouldn’t spurn i925/i915-based boards yet. Yes, they are not as good at overclocking as their i875/i865-based predecessors, but they may improve. Speaking about the impossibility of independent clocking of the PCI Express bus on modern i925/i915-based mainboards, the engineers forget to add “it’s impossible to do that by modifying the mainboard’s BIOS”. I think the overclocking problems may be solved with a hardware modification of the mainboards.

So, it is possible that i925/i915-based mainboards will be perfected within the next few weeks to become fully suitable for overclocking. As a minimum for that, the R&D departments of the mainboard makers should do some work plus new clock generators should come out that would support independent clocking of PCI Express. Right now, they use the ICS954119 generator, which cannot clock the PCI Express bus asynchronously.

I’m inclined to put the overclocking problems into the category of “childhood diseases”, rather than fatal errors of the new chipsets. However, I can now only give two recommendations to overclockers: wait for the second wave of mainboards on the new chipsets or go for the i875/i865.