# Measuring power consumption on BeagleBoard XM with i2c tools

Hi,

I have instaled Ubuntu 11.10 on BB-XM and I measured power consumption while running a video decode.

First I configured MADC with i2c (as said on: https://groups.google.com/forum/?fromgroups#!topic/beagleboard/8PqigpF1VEk by horse_dung)

i2cset -y -f 1 0x4a 0x00 0x01
i2cset -y -f 1 0x48 0xbb 0x08

i2cset -y -f 1 0x4a 0x06 0x28
i2cset -y -f 1 0x4a 0x07 0x00
i2cset -y -f 1 0x4a 0x08 0x28
i2cset -y -f 1 0x4a 0x09 0x00

# start read and get results

i2cset -y -f 1 0x4a 0x12 0x20
i2cget -y -f 1 0x4a 0x3d w
i2cget -y -f 1 0x4a 0x41 w

In example, I got two values
0xc5c0
0xc940

then I turned it on decimal and divided by 64 and then by 1024 (because it is 10-bit res.). Multiplying then by 2.5 I got the voltages (dynamic range of the ADCs are 2.5V)
50624/64-> /1024 → *2.5
51520/64-> /1024 → *2.5

as said on the xm manual, that value is 46% of the voltage
so, multiplied by 2.17, I got those values:

4.19
4.264

to get the drop of current, I made (value2 - value1)/0.1R.

(4.264 - 4.19)/0.1 = 0,741699 → 741mA

Finally, with P = V * I
I get, for example
P = 5 * 0,741 = ~3,7 W

1. Well, is that process correct?

2. The values I get, by looping the (#start read and get results) process in a 0.5s with cpufreq-set at 300 MHz and then at 800MHz
shows minimal diference. Somebody made similar tests to compare the results?

3. In the loop with a video decode on ARM get Average drop current at 614mA.
Comparing with values on Beagle C4, where the read values are

(Dec/64) (V - 5V)

That means ~2.9x more drop current on XM than C4, in that example. Is it because the USB hub integrated on the XM board?

I would appreciate some coments on that, and if someone done similar tests or to confirm those values…

Yes, the HUB and the Ethernet port on the HUB takes a lot of power. If you are not using it, you can turn it off.

Gerald

Hi Gerald,

thanks for the reply. I'm using some USB ports, so I can't turn it off.

But in xM manual, chapter 7.4 says

"Please keep in mind, that this current reading does not include any
current consumed by
the USB HUB, USB ports, or the Expansion headers."

So, that means that those values are associated only to the processor
measurement?

Yes. It only measure current into the TPS65910. Any current that the HUB or devices on the HUB are not included in these numbers along with expansion headers.

Gerald