SheevaPlug Car Stuff

Introduction

The plugcomputer by Marvell is a fantastic, fantastic, device.

It's a small, low-power device running a real Linux OS. Easy to develop for, easy to use. Everyone should get four.

I specifically wanted to be able to power one in my car, for running obdgpslogger. Along the way, I was curious about the exact power usage of the plug. This webpage covers both these things.

I have two plugs, now; one is the default one manufactured by globalscale, and one is the newer one by ionics. For this exercise, I modified the globalscale plug since it has more space inside to futz with, the plug prongs aren't moulded into the device itself [so less modding the plastic case], and it has the SD slot built in [I'm logging to an SD card with obdgpslogger, although there's no reason I couldn't use a USB thumb drive, or the internal flash]

The ionics plug is the smaller one. Note that the SD slot and the slave USB port come as a separate JTAG device. These photos don't do justice, though; the ionics plug is literally about half the size of the globalscale one, and much sleeker and sexier.

Software

OBDGPSLogger builds and runs great on the Sheeva.

If you want to cross-compile, cmake [the build system I use] supports cross-compiling via a "Toolchain" file. In your obdgpslogger code checkout, look at the Toolchain-sheeva.cmake file, it contains comments on how to build using it. View Toolchain-sheeva.cmake here

If you wish to compile on the plug itself, that's even easier; install the following packages, and you're good to go [build using the normal build instructions]:

apt-get install build-essential make g++ cmake libgps-dev gpsd

Hardware

Credit where credit's due, I mainly followed instructions from here: hacking the sheeva plug's power. One thing I did differently was adding an extra wire from the 5V output to power an external USB hub, but otherwise it's mostly the same.

I bought a Carnetix CNX-P5V power supply from mp3car.com, and a cheap USB hub that runs on 5v from best buy. The PSU is a really easy-to-use 12V to 5V regulator and PSU.

So, to get started, undo the four screws under the little rubber feet, and pull the plug apart. The power plug that goes into the mainboard of the plug just pulls right out. The main PSU just pops right out, and is *huge* compared to the P5V

Snip off the plug from the PSU. Read the official CNX-P5V documentation, but for our purposes the short version is:

1. Connect the single black wire with an eye hook to a thick black negative wire. [Snip off the eye hook]
2. Snip the connector off the pair of red wires, and the connector off the green/black pair. Cut off the black wire from the green/black pair, near the device.
3. Solder the pair of red wires, and the green wire, all to a nice thick chunk of positive wire.

Those two thick wires [one red, one black] will stick out the end of the plug and attach to your car's power

1. The output wire pair should be connected to the plug you snipped off, red to red and black to black.
2. As mentioned, I threw an extra wire into the mix, so I could power a 5V device outside of the plug

The extra 5V out can be seen here. Note that originally I put on some pretty short wires. Don't do that; I had to de-solder and resolder longer ones on there, after putting the plug back together and realising what I'd done.

Similar to Paul, I grabbed a bit of scrap metal and a couple nylon ties, to make sure the cables wouldn't accidentally pull out.

All I did to power the hub was buy a $1.99 piece from radio shack. Radio shack sell wallwart PSUs and separate connectors so you can use the base PSU to power just about any device. Find the connector that fits your USB hub, and solder it on there

Power

The question is, how many electricities does the plug drink, post-op?

Short version: fully cranked up, with an OBDII dongle, a GPS puck, and the USB hub all plugged in, it draws about 6W, and the power supply is at 65C [149F]. That's absurdly hot, and it explicitly needs a heatsink if you're drawing that much.

65C and .51A on the 12V end:

There are some other interesting data points for power. I measured a few things at the 12V end of the device [measured, 12.2V] and a couple things at the 5V end of the device [measured, 5.1V]. Numbers for current presented to two significant figures [the useful limit of my cheap multimeter]

Status	12.2V / PSU Input	5.1V / PSU Output
Everything plugged in, including USB devices, and sampling	0.51A/6.2W
Same but with GPS unplugged	0.48A/5.9W
Running with no USB devices		0.58A/3.0W
After running "shutdown -h now", USB devices still plugged in	0.36A/4.4W
After running "shutdown -h now", USB devices removed	0.29A/3.5W	0.58A/3.0W
Unplugging ethernet from shut down plug	0.21A/2.6W	0.45A/2.3W [fluctuating]

You can mostly draw your own conclusions, but the salient ones for me:

• The P5V PSU burns between 1/4 and 1/2 watts depending on how busy it is
• I didn't measure with the CPU going fast, but the plug seems to draw a similar amount of power before and after shutting down the OS
• Just having the ethernet port plugged in burns 3/4 W. That's a lot