How to change Bus speed for MPC8xx CPU

[Wolfgang Denk]

Hello,

in message <E19HMrf-0008UY-00.xol-mail-ru at f9.mail.ru> 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 at denx.de
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