Re: Re[4]: How to change Bus speed for MPC8xx CPU

Hello,

in message [off-list ref] you wrote:

You r right I've posted same message to
embedded list. I though this is better place for this topic.
Ok, I can get configuration of
CPU 100M
Bus 50M
Or
CPU 80M
Bus 40M
In the worst case second is ok for me, but
I have a luck of performance in my system and do not
want to loose 20% of CPU speed.

There is no other choice.

So I want to find a way how to get
CPU 100M
Bus 40M

You cannot. The MPC8xx CPUs gives you two options: you can select the
EBDF bits in SCCR either for a divider factor of 1 (bus clock ==  CPU
clock) or 2 (bus clock = CPU clock / 2).

No other modes are supported on this platform.

Also I'm driving a PPC Bus via number of boards in the system
and 40M can be problematic. So probably I'll need to
downgrade it to 30/35M.

In this case you have to run the CPU at 60 or 70 MHz, resp.

This is reason I can't just use multiplier but have to
find a way control bus speed by myself.

You cannot. Bus clock and CPU clock depend on  each  other,  and  the
only  option is a divider factor of 1 or 2. See the documentation for
the clocks and the SCCR register in the UM.

Best regards,

Wolfgang Denk

--
Software Engineering:  Embedded and Realtime Systems,  Embedded Linux
Phone: (+49)-8142-4596-87  Fax: (+49)-8142-4596-88  Email: wd@denx.de
Nothing ever becomes real till it is experienced -- even a proverb is
no proverb to you till your life has illustrated it.     - John Keats

** Sent via the linuxppc-embedded mail list. See http://lists.linuxppc.org/