G5 timebase_frequency

Benjamin Herrenschmidt benh at kernel.crashing.org
Sun Feb 8 17:45:19 EST 2004

On Sun, 2004-02-08 at 09:40, Michal Ostrowski wrote:
> I've noticed that timebase_frequency on a G5 appears to be defined by OF
> to be 33.3333MHz. OTOH, the specs for the 970 claim that timebase has a
> frequency of 1/8 that of the CPU clock rate.  So, on a 2.0GHz processor
> timebase would be 250MHz.
> Which one of these, if any is correct?
> My guess would be to say 250MHz, but my experiments show that a clock
> based on timebase, assuming that rate, appears to be about 10% slow.

10% Only ? :)

It's really 33Mhz as OF says. AFAIK, the HID0[19] bit is set by the
firmware causing the timebase to be clocked on the rising edge of
the TBEN input, thus the timebase is externally clocked. This allows
a stable timebase when slewing the cpu & bus clocks.


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