Feb 182013
 

A few days ago I was playing about with my Raspberry Pi Model A. I wanted to measure the power consumption, since I started a thread on the Pi forums that talks all about this.

This threw up an interesting problem because I wanted to measure the power consumption of the Pi both with and without my wifi dongle. But if I pull out the dongle, I’m not sure it will reconnect when I re-attach it, and then I won’t be able to communicate with it (I was using it headless – no monitor/keyboard). Eventually it hit me.

Delayed shutdown to the rescue

One solution was to to measure the power consumption with the dongle in, then
sudo shutdown -h 5
which tells the system to shut down in 5 minutes. So then I pulled the dongle out and measured power consumption again. Then waited for it to shut down, which it did.

Great. It all went according to plan and the Pi model A uses 0.81 Watts with the Edimax wifi dongle in and 0.51 Watts without it. This is at idle. (Model B uses 2 Watts).

Ok let’s start again then

So then I plugged the wifi dongle back in, without pulling the power cable, and I was pleasantly surprised to see the Pi boot all by itself. I knew it could be made to boot by shorting header P6 (if fitted) but this was a new one to me. Bring your Pi back from suspend by plugging in a USB wifi dongle. Nice. :) I haven’t seen this documented anywhere either.

Only Rev 2 model B and model A

This doesn’t work with Rev 1 Pis. It works with Model As and Rev 2 Model Bs, which, as far as I know, use the same board.

And here’s a little video to prove it

This will probably work with other USB devices as well, but I haven’t tried others yet. Leave a comment if you make it work with other devices ;)

  10 Responses to “Wake up your Pi with WiFi dongle”

  1. I’ve seen possibly related behaviour. My Pi reboots if I plug in a specific wifi dongle. It’s fine if it’s already plugged in when I turn it on!

    • Thinking about it, that used to happen on the Rev 1 Pi I bridged the polyfuses on (that one’s on permanent weather-station duty – it’s the NoSPI one now). People in the Pi forums talked about using a 1 Ohm resistor instead of a ‘zero’.

    • I was gonna say I think this is a side effect rather than a feature. I think its because plugging in USB devices causes a surge which resets the Pi.

      • I think you’re right it does Daniel, but it’s interesting that it still happens when the pi is shut down. :) I’m sure I remember seeing a thread about low Ohm resistors curing the “reset when hotplug problem”.

        Another point to note is there’s a wide disparity between current consumption of different dongles. My kbd/mouse dongle doesn’t cause a reset.

        Edimax wifi uses ~ 0.06A. Dynamode wifi uses 0.16A (and gets hot).

  2. may i know what case is you using? link?

  3. I guess you won’t be able to use this trick on the B+ or forthcoming A+ Pis ;-)

  4. Absolutely wonderful !
    This statement above is not correct:
    “I plugged the wifi dongle back in, without pulling the power cable, and I was pleasantly surprised to see the Pi boot all by itself. ”
    Doesn’t reboot, just restart the WiFi interface!

    My pi is running a long range test, already amost 72 hours, controlled by SSH. At some time last night the network interface got off.
    No response to ping.
    I followed the tip to remove the dongle for 15 seconds. Lo and behold, the application did NOT stop and the network returned to activity!

    Many many THANKS

    • This statement above is not correct:
      “I plugged the wifi dongle back in, without pulling the power cable, and I was pleasantly surprised to see the Pi boot all by itself. ”
      Doesn’t reboot, just restart the WiFi interface!

      Actually yes it is. Look at the video. The date of this blog post is 18 Feb 2013 it was done using an original model A Pi.

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