Underclocking the XO 1.75

[James Cameron]

On Thu, Jun 14, 2012 at 10:30:15PM +0200, Arne Babenhauserheide wrote:
> Dear OLPC hackers,
> 
> I read the underclocking information on the custom bootloader page??
> und wondered how to achieve the same with the 1.75. I read in the
> hardware specifications, that the processor should be able to go down
> to ~400 M??z, and I???d like to get it down as far as possible.
> 
> Can I adapt the commands to achieve that?

No.

> More specifically: How can I get the XO 1.75 to run at the lowest
> possible CPU and Bus speed?

Ensure you are on Q4D16 (or later).

Type pj4-100mhz to run the core at the lowest CPU speed allowed by
Open Firmware.  This configures the core to use the main PLL (800MHz)
with a by-eight divider.

Other configurations include pj4-200mhz , pj4-400mhz , and pj4-800mhz .

Type .fuses and press enter to see the maximum certified operating
frequency set in the CPU by the manufacturer Marvell.

To make a change permanent, edit the olpc.fth file in the boot
directory of the first partition of the internal storage, and add the
line after the first line.

I'm not sure about Linux suspend and resume.  I suspect kernel support
is needed.

As for "Bus" speed, what bus are you talking about?

http://tracker.coreboot.org/trac/openfirmware/browser/cpu/arm/mmp2/clocks.fth
is the source code in Open Firmware for the clock frequency change
functions.

To display the currently configured speed, type .cpu-speed and press
enter.  This is also shown by banner .

-- 
James Cameron
http://quozl.linux.org.au/