[coreboot] How to change clockchip frequency?

Oskar Enoksson enok at lysator.liu.se
Wed Nov 2 17:26:15 CET 2011

On 11/02/2011 04:31 PM, Marc Jones wrote:
> On Wed, Nov 2, 2011 at 6:37 AM, Oskar Enoksson<enok at lysator.liu.se>  wrote:
>> I have a chip called CDC960 on my motherboard. It is responsible for
>> generating the bus frequency for the cpu, the pci bus etc. I downloaded the
>> datasheet and noticed that I can change the base frequency by +/-10% by
>> sending an smbus command to the cdc960.
>> When I sent the smbus commands from a running linux the system froze for a
>> few seconds and then rebooted. But after reboot the frequency was indeed
>> changed (overclocked) and everything worked just fine.
>> I tried to add the smbus command to romstage.c in my coreboot, but that
>> didn't work. The boot process just hanged immediately after the smbus
>> command had been sent.
>> I guess the running CPU (or something else in the system) cannot handle the
>> frequency change and needs to be reset immediately after the command is
>> sent. Perhaps some watchdog under Linux resets the system after a few
>> seconds of irresponsiveness. But what is the recommended way in coreboot?
> It would be hard to tell why Linux resets, but you can look in the
> southbridge for a watchdog. You should also read the CPU spec about
> changing clock speed etc.
Thanks. There is a watchdog in the southbridge (amd 8111) and it could 
be set up to reset the system after a specified number of seconds, but 
it requires memory mapped I/O to work and I'm not sure how to do that in 

Do you (or someone else) think it's a doable way? If so I'll give it a 
try ...

More information about the coreboot mailing list