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RE: Real PCI Clock Rates



also note that "33MHz" systems can have the clock change
dynamically, even stop. My PC bios has an option to
enable/disable "spread specturm" PCI clocking. This means
you can't use PLL type ("0 nS delay") clock chips to
receive the clock. "66MHz" systems have to have a fixed
frequency, so this isn't a problem.

At 02:40 PM 10/22/99 -0400, you wrote:
>>I'm wondering whether I should
>>expect to see less than 33MHz in actual practice.
>
>The crude answer is, yes, you should "expect" to see less than 33MHz (down
>to 0Hz), or less than 66MHz (33-66MHz).  If you decided not to expect slower
>clock rates and designed something that doesn't accommodate them, it would
>not comply with PCI requirements.
>
>Will you actually see such reduced clock rates?  Yes.  My poor older
>Pentium-class PC at home runs the PCI bus at something like 25MHz or 30MHz
>(with the CPU that's installed in it) because it's synchronous with the CPU
>clock and there isn't a convenient ratio at 33MHz.  Laptops *might* throttle
>the clock down in quiet mode, but I don't know for sure if any actually do.
>Because 66MHz timing is very tight, it would not surprise me if they find it
>necessary to run somewhat slower.
>
>Regards,
>Andy
>
>
>