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Re: Frequency (cpu speed) control on AU1100

To: Dan Malek <dan@embeddededge.com>, Pete Popov <ppopov@mvista.com>
Subject: Re: Frequency (cpu speed) control on AU1100
From: Bob Lees <bob@diamond.demon.co.uk>
Date: Tue, 30 Mar 2004 23:38:08 +0100
Cc: linux-mips@linux-mips.org
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On Tuesday 30 March 2004 22:56, Dan Malek wrote:
> Bob Lees wrote:
>  > ....I suspect I am
> >
> > missing something somewhere, but I can't find any references to cpu speed
> > control for the MIPS processors, specically the au1x range.
>
> The Au1xxx has a PLL that multiplies the incoming 12 MHz clock up to the
> internal frequency.  Just be aware there are lots of peripheral clocks
> and bus clocks derived from this internal frequency.  There is code
> in the kernel power management to allow changing the frequency during
> operation of Linux, but I don't know how well it works today as I have
> not tested that for quite some time.
>
> Thanks.
>       -- Dan

Thanks Dan & Pete for the prompt response.  

I have tried the /proc/sys/pm/freq interface and by putting a bogomips calc 
into power.c, it appears to indicate a change in core frequency.  I think 
your caution may be well founded as I got input overruns on the serial 
console when I took the speed down to 84MHz, good character recognition 
though, so it was an input buffer speed issue.

Also I can see an approx 40-50mA change in current from 84 to 396MHz which 
indicates something is changing.  Supply is at 5 volts thru a simple switcher 
down to 3.3 volts on the Aurora board.  This is with nothing else running and 
an nfs filesystem.  As part of monitoring current I am seeing an anomoly: 
namely after boot is complete and system is quiesent, at apparently 396MHz, 
the current is 200mA, now after playing with the freq control the current at 
396MHz stabalises at around 250mA.  Verrry strange - any thoughts??

On another topic, what state is the IRDA driver in?  This is building from the 
patched 2.4.25 kernel on your site Dan.  And a big thank you for this source 
of a patched kernel and build tools.

Bob


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