[U-Boot-Users] 859/866 clock issues

Wolfgang Denk wd at denx.de
Mon Oct 20 20:49:46 CEST 2003


in message <20031020181719.GA5121 at orca.internal.zumanetworks.com> you wrote:
> > > 1) measure_gclk() is very inaccurate. Better to use the plprcr and
> > > CFG_8XX_XIN (which MUST BE 10MHz for that cpu) to derive the "real" bus
> > > clock.
> > 
> > What do you mean by "very inaccurate" - the intention of the function
> > (as documented) is to measure the "Approx. GCLK frequency in Hz". How
> > bad is "very inaccurate" in your case?
> 2-3%

Well, 2...3% means you get 52 instead of 50 MHz? I don't  think  this
is  "very inaccurate" - it could be improved by dedicationg more time
to the test, but IMHO this is sufficient.

> the error of using plprcr + XIN is 0%, and given we know XIN is 10MHz
> for the 859/866 we are using that now.

But I don't want to  make  too  many  assumptions  on  "known"  clock
frequencies.  I've seen more than one board where the PLL would _not_
lock on the intended clock frequency. It is  very  nice  if  you  can
_see_  that  the board is running at 25 MHz even when it's configured
for and assumed to be running at 50. This is  the  whole  reason  for
this test.

Actually, I don't care about a 5%  error  here.  If  you  do,  please
disable the function and #define CONFIG_8xx_GCLK_FREQ as you would do
on any board without a 32768 Hz oscillator.

> > > 2) the 859/866 does NOT support external speeds higher than 66MHz,
> > > so if cpu_clk is greater, EBDF01 MUST BE USED IN CFG_SCCR
> > 
> > I think this is the case for ALL MPC8xx processors  so  far.  I  have
> > never seen one which would allow a bus clock > 66 MHz.
> > I don't see why a patch should be  needed  here.  This  is  a  simple
> > matter of getting your board config file right, isn't it?
> yes, but somebody really should add a note to the readme :)

Ummm... I disagree. It seems obvious to me that you  should  not  run
your  processor  out  of specifications, and it is not the purpose of
the U-Boot README file to duplicate Motorola's processor specs.

Best regards,

Wolfgang Denk

Software Engineering:  Embedded and Realtime Systems,  Embedded Linux
Phone: (+49)-8142-4596-87  Fax: (+49)-8142-4596-88  Email: wd at denx.de
If I had to live my life again,  I'd  make  the  same  mistakes, only
sooner.                                          -- Tallulah Bankhead

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