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Re: [PATCH] hp-timing for ppc32/64


> > Note that some CPUs like the 970 can have an externally clocked
> > timebase. Apple uses this feature to make the CPU immune to bus/cpu
> > frequency slewing, they use a 33Mhz clock for that. 
> > 
> The 970 does not implement a alternate timebase. So for 970 the timebase 
> is the highest frequency counter available. 

I didn't mean an alternate timebase ... I was saying that the 970 does
implement the main timebase, but use the option of having it clocked
externally instead of as a divider of the bus clock (this is an option
that can be toggled in a HID register iirc, in which cas the TB_EN pin
becomes the timebase input).

> It seems that Apple choose 33MHz to meet the minimum (slowest)CPU-clock / 
> 32 timebase. But that was their choice. The IBM hardware seems to be 
> holding to the CPU_clock / 8 timebase (including 970 based JS20).

No, I think the 33Mhz is due to an HW upper limit when clocking the
timebase externally. IBM doesn't use external clocking.
 


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