Clock / Timebase / Bus Frequencies Help

[Richard Whitlock]

Scott,
Thanks for that - you're right - redboot has the wrong crystal frequency 
in the cdl for our board.

Cheers,


Richard.

Scott Wood wrote:
> On Mon, Aug 18, 2008 at 07:52:12AM -0400, richw at netcomuk.co.uk wrote:
>   
>> We've got an 8347 based board very similar to the A&M asp8347. Core clock
>> is 400MHz. Bus clock is 266666666Hz.
>> According to the data sheet for the 8347, the decrementer clock runs at a
>> quarter of the rate of the bus clock. I have two questions:
>> In arch/powerpc/boot/redboot-83xx.c, the timebase clock is passed to
>> dt_fixup_cpu_clocks() as bi_busfreq / 16. If I leave it like this, my
>> system clock runs approximately 4 times too fast. 
>> Can anyone point me in the direction of an explanation for the div by 16
>> rather than 4?
>>     
>
> It's a bug, which I pointed out here:
> http://ozlabs.org/pipermail/linuxppc-dev/2008-June/058704.html
>
>   
>> If I change the call to dt_fixup_cpu_clocks so that bi_busfreq/4 is passed
>> in, then the clock runs more accurately. However, its still not correct.
>> This gives a decrementer frequency of 66666666Hz, but if I hard code the
>> value to 66000000Hz, the clock runs accurately.
>> Can anyone shed any light on why the value passed in by the boot loader
>> (redboot) seems to be inaccurate.
>>     
>
> Redboot probably has the wrong crystal frequency hardcoded.
>
> -Scott
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