Increasing the FSB

The method of overclocking that we are most used to, courtesy of the multiplier locked Intel CPUs that we’ve had for such a long time, is accomplished by increasing the FSB frequency.  Since the CPU derives its clock speed from the FSB frequency (CPU Clock = FSB x Clock Multiplier), increasing the FSB frequency will increase the CPU clock speed and thus overclock the CPU.

The major benefit of this method is that the overclocking can generally be accomplished without even taking the case off of your computer.  Most motherboards allow for the FSB frequency to be adjusted in the BIOS setup utility (generally under the Chipset Features/Advanced Chipset setup or its own special Frequency Setup section) so with a few keystrokes you can be off and running at your overclocked speed.

The first misconception that must be cleared up is that the Athlon’s FSB actually runs at 100MHz and not at 200MHz.  The EV6 bus transfers on both the rising and falling edges of the clock (instead of just the rising) and thus the effective transfer rate over the EV6 bus is equal to that of a 200MHz FSB, but the actual FSB frequency is 100MHz.

The downside to increasing the FSB frequency, as we briefly mentioned before, is the fact that most of the components on the motherboard derive their operating frequencies from the FSB frequency and increasing the FSB frequency will, in turn, increase their frequencies as well. 

For example, the PCI bus operates at either 1/2 1/3, or 1/4 the speed of the FSB.  The 32-bit PCI bus specification calls for an operating frequency of 33MHz, and if your FSB is set to 100MHz then, by using the 1/3 divider, the PCI runs at the appropriate 33MHz setting.  Most PCI devices have no problem running at frequencies up to 37.5MHz, and there are usually very few problems until you get above a PCI bus frequency of 40MHz.  If you take into account the fact that there is a ¼ PCI divider, achieving a PCI bus frequency greater than 40MHz would require you to have an FSB frequency greater than 160MHz. 

The AGP bus is designed to run at 66MHz and operates on a fraction of the FSB frequency.  This fraction is determined by the North Bridge of a chipset, which in the case of the AMD 750 chipset is the AMD 751 North Bridge controller.  The 751 only supports the 2/3 and 1/1 AGP clock dividers, and most AGP cards have a problem working at frequencies above 83MHz.  This means that for most AGP cards, a FSB frequency too far above 124MHz would be difficult to pull off.

The memory bus on the Athlon is currently designed to run at 100MHz; however, the tolerance level of the memory bus is normally dependent on the memory you use.  For example if you use PC133 memory that is rated for operation at 133MHz, then you should not have any problem if your memory bus is running at 133MHz. 

The Athlon’s EV6 system bus is designed to run at speeds of up to 200MHz, and according to those that we have spoken to that have a part in designing the future EV6 based motherboards, reaching that 200MHz goal isn’t a major problem, it is mainly an issue of implementing it on the motherboard rather than tweaking the chipset.  So increasing the FSB frequency shouldn’t be limited by the EV6 bus anytime soon, although the motherboard may be the limiting factor in this case.

With that said, what frequencies are available for use with the current Athlon platform?  The FIC SD11 was the first readily available Athlon motherboard to offer support for FSB frequencies other than 100MHz, with its 125MHz and 133MHz FSB settings.  Unfortunately, neither of those settings were of any use since they would not boot at all.  The real overclocker came with the ASUS K7M, which supports FSB frequencies from 100MHz to up 125MHz in 1MHz increments. 

From our tests as well as the reports of hundreds of K7M owners that have been posting to online message boards and newsgroups, the consensus seems to be that 110MHz is the maximum FSB frequency the K7M will run at with any Athlon. 

Two Methods to Overclocking the Athlon Increasing the FSB - Content Creation Performance

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