Re: PCI Clock speed.

["John R Pierce" <pierce@hogranch.com>]

Dominick writes...


>Is there a way to determine the PCI clock speed for a given system. I
>have a board that has a timer which is incremented once per PCI clock.
>To determine the PCI clock speed, I calculated the number of ticks in
>this timer over a period of 5 seconds which was clocked by the PC timer
>and then did a divide to determine the frequency.  If I didn't have this
>timer, I would not have been able to determine this frequency.
>
>In addition, if I didn't have this timer, I probably wouldn't need to
>know the PCI clock rate.  But that's a different subject.   It seems
>that there should be a system call in the PCI bios that shall give this
>information.


'Fraid not.  To start with, its not necessarily a constant.  Various power
down states can slow the bus clocks down, the spec allows DC to 33MHz.  Some
advanced systems have been rumored to use a dithered clock to minimized RF
emissions, this is well within bus specs.  The only requirement is the
minimum high and low time.

-jrp