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"AT Bus Clock" bios question

N

nippyjun

Diamond Member
In my new mobo's bios there is the setting for the AT Bus Clock. Is this the same thing as setting the speed of the AGP clock based on the CPU clock speed? So if I'm running my CPU at a bus speed of 133 then if I select the AT Bus Clock setting of CLK1/2 then the AGP clock would be at 66? Or is the AT bus a different thing?
 
The AT bus clock can either be forced to stay in spec (8.something MHz) or tied to the PCI bus at a 1/4 ratio. If your bios has an AUTO option, use that, or else use the 8.something option. Not that it really matters these days.

Modus
 
The AT bus runs your ISA slots, serial ports, parallel port, floppy controller, real time clock, and a few other miscellaneous legacy devices.

Modus
 
The default setting for mine is CLK2/4. Should I just leave it there even if I overclock the system?

Anyone know???
 
well, if you don't have ISA cards installed in your system, most likely you don't need to change it. but if you OC your FSB a lot, some ISA cards might go crazy, and you'll need to adjust it to a fixed freqency or a slower ratio. (fried my SB16 this way 😛 )
 
Strange, 2/4clk sounds like the AGP speed, or could be the PCI speed if you have a Celeron. But "AT bus" should indeed be the ISA one, and 2/4 or 1/2 was never the right multiplier for the ISA bus; PCI started out at 25MHz or 33MHz depending on your FSB, so the ISA bus was 1/3 or 1/4. Umm, I think you better read the manual. Sorry!
 
here's what the manual says:

"AT bus Clock: Select the speed of the AT bus in terms of a fraction of the CPU clock speed, or at the fixed speed of 7.16 MHz."

The choices are: 7.16, clk2/2, clk2/3, clk2/4(default), clk2/5, clk2/6

Any ideas about what this actually is?
 
Ah. Maybe. I wonder if clk2 is the PCI clock. Then PCI clock /4 would be the default. 7.16MHz would be the original ISA bus spec. Normal PCI /4 is 8.33MHz. You might change it if you wanted the ISA to go faster (many work at PCI/3, 11.11MHz), or you pushed up your PCI clock (e.g. PCI at 40MHz, use PCI/5 to get 8MHz). Some stuff like the keyboard and the speaker maybe still runs off the ISA clock, even if you don't have ISA slots, though maybe there's a version of your mobo which does have ISA slots?
 
DoctorBooze, I don't have any isa slots on my mobo. And I don't have the ability to change the PCI clock. It's weird that the default is 2/4.
 
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