Re: CPU frequency shifting "problems"

Jamie Lokier (
Fri, 28 Sep 2001 23:24:58 +0100

Eric W. Biederman wrote:
> > What does exist is the bus clock (well, a multiple of it, but you get the
> > idea), and that one is stable. I bet PCI devices don't like to be randomly
> > driven at frequencies "somewhere between 12 and 33MHz" depending on load ;)
> I doubt they would like it but it is perfectly legal (PCI spec..) to
> vary the pci clock, depending upon load.

Yes it is. Also, the PCI clock is frequency modulated to reduce
electrical interference. (Or on a more cynical note, to pass the
official emissions tests ;-)

However it's common practice to PLL to the PCI clock, for clock
distribution on a board, so varying the frequency must be done in a
strictly constrained fashion.

-- Jamie
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