AT Bus Clock

Common Options : 7.16MHz, CLK/2, CLK/3, CLK/4, CLK/5, CLK/6

Quick Review

This BIOS feature allows you to select the ISA bus' clock speed. The chipset actually generates the ISA bus clock by dividing the PCI clock. Hence, the available settings of CLK/2, CLK/3, CLK/4, CLK/5 and CLK/6.

There is also the fixed speed of 7.16MHz which is derived by dividing the reference clock generator speed of 14.318MHz by a factor of two.

As you can see, the setting of CLK/4 yields an ISA bus speed of 8.33MHz which is the maximum speed allowed by the official ISA specifications. However, you can choose to overclock the ISA by by selecting the settings of CLK/3 and CLK/2 which yield clock speeds of 11.11MHz and 16.67MHz respectively.

Overclocking the ISA bus greatly improves its performance. Therefore, it is recommended that you try to use the faster settings if possible. However, while newer ISA cards are capable of running at this 'out-of-spec' speed, older ones may not work properly at this speed.

If your ISA cards fail to work properly, then you should select the setting of CLK/4 or 7.16MHz. That will keep the ISA bus within specifications.

Please note that the calculations and recommendations above were based on a 33MHz PCI bus clock. If you are overclocking your PCI bus, please take the increased PCI clock speed into account!

If all this is confusing and you want to play safe, select the setting of 7.16MHz. That is the fail-safe setting because it will set the ISA bus to run at a fixed speed of 7.16MHz, irrespective of the PCI bus speed.