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Whats the deal with PCI CLOCK 33.33Mhz??

Last response: in Overclocking
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March 18, 2006 2:51:03 AM

Hello.

SO i have a moderate overclock of 2.25 Ghz on my 3200 venice. vcore 1.35

should i set the PCI synchronization of something to 33.33? or leave it as auto?

Whats the difference?
March 18, 2006 3:16:00 AM

If your PCI clock gets over 37.5 MHz expect some issues with some cards plugged into it.

The PCI bus speed can be a ratio, or it can be locked as 33.33 MHz.

It is generally better to lock it at 33.33 MHz as if it raises to much you'll have issues.

The SATA bus, AGP / PCIe x16, and HTT buses can all also be affected if they are out of spec by more than 12.5%, for SATA even less (+/-3% is best).

However some SATA controllers are not affected by bus speed changes, others are, etc, etc

Also if you overclock the FSB to the 'next divider' the PCI bus will go down to 33.33 MHz again.

eg:

:arrow: 100 FSB / 3 = 33.33 MHz
:arrow: 115 FSB / 3 = 38.33 MHz (too high)
:arrow: 133 FSB / 4 = 33.33 MHz again (so stable, assuming the CPU can handle a +33% overclock.

The same rule tends to apply when going from 133 to 166, 166 to 200, etc, the divider raises by one every 33.33 MHz, to keep PCI bus stable.

133 to 166 is only a +25% CPU overclock
166 to 200 is only a +20% CPU overclock
....and so on.

Some chipsets / mainboards may not be affected by it, and just lock the PCI bus to 33.33 MHz.

It dates back to the origin of the PCI bus, back in the Pentium 60 and 66, and keeps everything in sync nicely.

Having things in sync nicely may let you overclock more than running the PCI bus async, or it may not..... there are many variables to consider.
!