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Author Topic: Underclocking the Sheevaplug  (Read 3487 times)
mos6510
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« on: March 29, 2010, 04:07:08 PM »

Hello all,

I've searched high and low, but this info eludes me.
How (if possible) do I underclock the CPU in a SheevaPlug?

I don't need 1.2GHz of grunt in my simple application (off grid, 3g connected web-cam and weather station). I need extra low power and cool running.
If I could get this thing running at 600MHz, I'd be happy. I'd even try a bit lower if I could.
I'm running Debian on it, and the kernel has CPUIdle, etc. enabled, but the current draw is still over 1A @5V. I know I've got a long way to go with techniques such as tickless kernels, etc, but for now I'd like to start by hard-coding the max clock speed if possible.
So, is it possible to underclock these Marvells (pun intended!)?

Really appreciate any hints,

Steve.
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MarkF
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« Reply #1 on: March 30, 2010, 02:21:25 PM »

The processor in a plug is a Marvell Kirkwood 88F6281.

Here is a link to the Marvell Kirkwood page.  According to the table, this processor can go down to 1 Ghz.  I *think* I remember reading somewhere that the clock speed used affects the speed of the memory that can be attached to the device as well so dropping the clock speed may not be the only thing you'd need to tweek.

Sorry, I don't have a better memory. Smiley
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Mark

mos6510
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« Reply #2 on: March 30, 2010, 02:43:44 PM »

Thanks Mark,

I'm looking at the specs now. Still lost to how I actually set the CPU freq. In days gone by I would change a crystal. Maybe I have to start looking for a resonator!
Hope my surface mount soldering skills are up to scratch ;-)

Actually I do see reference to a PLL locked CPU reference oscillator in the specs you linked to Mark. Might be a good start to see if it can be done in software somehow. If I work it out I will report back, but if anyone has any further hints, please do pass them on!

Thanks again,

Steve.
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peter a
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« Reply #3 on: March 30, 2010, 03:55:58 PM »

Or get something like the beagleboard , which uses less power anyway
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mos6510
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« Reply #4 on: March 30, 2010, 06:11:33 PM »

So far I've worked out that I need to set the following pins to the following values (this will run the ARM SoC at 1000MHz).

a) CPU_CLK Freq Select (a nibble made up of MPP[2],MPP[5],MPP[19] and MPP[10])
     Strapping this to 0x7 will run this clock at 1000MHz.

b) CPU_CLK to DDR_CLK ratio (a nibble made up of MPP[33],NF_ALE,NF_REn and NF_CLE)
     Straping this to 0x8 will set the ratio at 5:1.

c) CPU_CLK to CPU L2 Clock ratio (three bits made up of MPP[3],MPP[12] and NF_WEn)
     I'd strap this to 0x3 to give me a 3:1 ratio.

So this will probably give me the slowest Sheevaplug possible.

Does anyone know the following :-
a) Does the Sheevaplug have a firmware/software method of setting these registers (note that this would have to be implemented in some kind of logic cct controlled external to the Marvell SoC).
b) Alternatively, do these pins exist somewhere on the PCB for me do put a solder blob on?

This is the hardware forum. Lets see how many real hardware people are out there ;-)

Thanks in advance for any info.

Steve
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dattaway
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« Reply #5 on: March 31, 2010, 04:27:57 PM »

This looks like a kernel function we could stick in /proc/cpu ....something like the eeepc modules did.  They wrote a module that poked a few bytes into the PLL registers through the /proc interface.
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petervk
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« Reply #6 on: July 11, 2012, 04:35:32 PM »

b) Alternatively, do these pins exist somewhere on the PCB for me do put a solder blob on?


If anyone's interested I've got my Sheeva underclocked at 1GHz.  You need to be comfortable with hand soldering surface mount resistors (0402).
Pull off R212, R209 and R2.  Adding 4k7 across R3 will also drop your DDR clock down to 4:1.

Cheers,
Pete
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westyd1982
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« Reply #7 on: July 12, 2012, 09:17:31 AM »

Did underclocking reduce the power usage by much?  I run a SheevePlug and a Dockstar from a solar powered battery bank.  The Dockstar has its own built-in power supply, so you just connect to the batteries.  My focus on the SheevaPlug was building the most efficient power supply I could to get 5v.

b) Alternatively, do these pins exist somewhere on the PCB for me do put a solder blob on?


If anyone's interested I've got my Sheeva underclocked at 1GHz.  You need to be comfortable with hand soldering surface mount resistors (0402).
Pull off R212, R209 and R2.  Adding 4k7 across R3 will also drop your DDR clock down to 4:1.

Cheers,
Pete

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petervk
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« Reply #8 on: July 16, 2012, 06:36:00 PM »

I haven't measured the current, will try & get a chance to do so and post here.
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