Computing stuff tied to the physical world

Carambola 2 power consumption

In Hardware, Linux on May 7, 2013 at 00:01

The Carambola 2 mentioned yesterday is based on a SoC design which uses amazingly little power – considering that it’s running a full Linux-based OpenWrt setup.

There are a couple of ways to measure power consumption. If all you’re after is the average power on idle, then all you need to do is insert a multimeter in the power supply line and set it in the appropriate milliamp range. Wait a minute or so for the system to start up, and you’ll see that the Carambola 2 draws about 72 mA @ 5V, i.e. roughly a third of a watt.

If you have a lab power supply, you can simply read the power consumption on its display.

But given an oscilloscope, it’s actually much more informative to see what the power consumption graph is, i.e. over time. This will show the startup power use and also allows seeing more detail, since these systems often periodically cycle through different activities.

The setup for “seeing” power consumption is always the same: just insert a small resistor in series with the “Device Under Test”, and measure the voltage drop over that resistor:

JC's Grid, page 51

Except that in this case, we need to use a smaller resistor to keep the voltage drop within bounds. Given that the expected currents will be over 100 mA, a 100 Ω resistor would completely mess up the setup. I found a 0.1 Ω SMD resistor in my lab supplies, so that’s what I used – mounting it on a 2-pin header for convenience:


With 0.1 Ω, a 100 mA current produces a voltage of 10 mV. This should have a negligible effect on the power supplied to the Carambola 2 (a 1 Ω resistor should also work fine).

Here’s the result on the scope – white is the default setup, yellow is with WiFi enabled:


Sure takes all the guesswork out of what the power consumption is doing on startup, eh?

  1. Are those measurements done on the devboard? Also, did you disable the ethernet ports via the ethernet port status register? When I did that on the Carambola1, this saved me about 30mA.

    • Yes, devboard, no ethernet ports not disabled. Is this documented somewhere? I’d be very interested to see how low we can get this – 30 mA less would be nice!

      What I also did not try was to just remove the lan/wan entries in /etc/config/network – maybe that’s also enough to keep the ports disabled?

  2. I don’t think that it’s sufficient to put down the entries – on Carambola1, I was in fact required to do the following line:

    mmio 0x101100c0 0x08002000 
    mmio 0x101100c0 0x08002001
    mmio 0x101100c0 0x08002002
    mmio 0x101100c0 0x08002003
    mmio 0x101100c0 0x08002004

    That’s posted somewhere on the carambola forums, and it will most likely not work on Carambola2 with the same registers. Unfortunately, I don’t have my Carambola2 yet, so I haven’t done any research on that so far.

Comments are closed.