Overclocking by Front Side Bus (FSB)
The AMD Athlon EV6 Bus is a new one for the overclocker market. Since the new bus is a DDR based system this becomes harder to overclock, as it is more susceptible to signal degrading and EMI interference. Just about all motherboard manufactures supply some way for you to overclock your front side bus, either by dipswitches, jumper caps, jumperless setup in the BIOS or software jumper setting using programs such as softFSB, the only problem is how far can you get it. AMD Irongate chipsets are one of the hardest chipsets to overclock, usually the maximum stable obtainable speed is around 105MHz, this is a very little increase. VIA KX133 are a little better with the max being around 115MHz, this is a bit higher but nothing compared to what can be achieved on a P6 bus, The Epox EP-7KXA holds the record with making 115Mhz FSB stable. VIA KT133 however is somewhat temperamental at times, some boards can't get past 105 while some get whopping high 120Mhz, this is possibly due to the board layout. ABIT KT7-RAID holds this crown with a 122Mhz FSB reached.
When you overclock your front side bus you also overclock the may preferable you have in your pc. The speeds of the other devices like ram and buses such as the AGP and PCI buses are obtained by using the front side bus and then dividing it down. 66Mhz is the default speed of the AGP bus, to achieve this the Front side bus on the EV6 is 100MHz. To get 66Mhz a 2/3 AGP divider is applied to get 66Mhz, same applies for the PCI bus only its 33Mhz so at 100Mhz a 1/3 divider is used. E.g. say you change your front side bus to 105MHz, then the AGP speed is 2/3 of 105 which is 70MHz, not a lot over 66Mhz but when you keep going so does the amount you overclock your devices, so when overclocking by front side bus you have to make sure all your preferable on the AGP and PCI bus (including the hard disks) can handle the increased speeds. Ram however is different, just about all ram companies produce high quality ram designed for the overclocker, the best example of this is PC133 SDRAM, when overclocking the front side bus over 100Mhz you need either good quality PC100 SDRAM or PC133 SDRAM that has been rated to do up to and over 133MHz. Another twist is the new PC150 SDRAM that's out, this is designed to do up to and over 150Mhz, this can come in handy for people out there with the KX and KT133 chipsets. Since the VIA chipsets uses a Asynchronous memory clock you can have your ram run faster than the front side bus by adding the PCI clock speed to the front side bus, at 100MHz the PCI clock is 33Mhz, so 100MHz FSB + 33Mhz PCI Clock= 133Mhz Ram clock. But when overclocking the front side bus and wanting to have the + PCI clock option on you need good quality PC133 or PC150 ram. E.g. if you have your front side bus at 105Mhz then your PCI clock is 1/3 of 105 which is 35Mhz, so at 105MHz with the + PCI clock option on its 105Mhz FSB + 35Mhz PCI clock you get 140MHz ram clock. On VIA chipset based Athlon boards overclocking via front side bus is pretty limited as the VIA chipsets are more unstable. When we used the ABIT KT7 RAID for FSB's over 117MHz the AGP system became unstable with distortions on the screen and the AGP speed had to be changed back to 2x.
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