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I'm trying to create a cape that will safely power my BeagleBone Black from my car's electrical system. The design I am using is based on the one from this answer. The circuit there suggests a simple voltage divider to connect the ignition switch to a GPIO on the Raspberry Pi, which could then detect when the car is turned off.

I believe this should work for the BeagleBone Black as well, except the documentation says very clearly that "NO PINS ARE TO BE DRIVEN UNTIL AFTER THE SYS_RESET LINE GOES HIGH." (p. 95)

Now, I'm relatively new to electronics. I'm doing my best to research, so please bear with me if I'm way off on this.

I did some searching, an it seems what I need is a tri-state buffer to feed the ignition signal through. Am I right about that? And would something like the NL17SZ126 be the best tool for the job? Or should I be doing something completely different?

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  • \$\begingroup\$ Absolutely the right idea, although I'd prefer something with multiple buffers in one package (in case you want to connect more things) and possibly a larger package if you're hand-soldering it. \$\endgroup\$ – pjc50 Oct 9 '14 at 20:35
  • \$\begingroup\$ @pjc50 Thanks. It was surprisingly hard to find useful information about this, so I'm glad to know I'm on the right track. \$\endgroup\$ – Nick Oct 10 '14 at 0:06
  • \$\begingroup\$ Does it absolutely, positively HAVE TO supply power via cape connection? It would be a heck of a lot easier to have a 5V 2A regulator feed the power plug (jack) on the side. You could even mount the regulator on the cape - just keep it isolated from any other cape circuitry. \$\endgroup\$ – Alan Campbell Oct 24 '14 at 8:43
  • \$\begingroup\$ @AlanCampbell Well, no, it could feed through the jack, but that's not really the hard part. There are a couple expansion pins that I could feed from a 5V DC-DC converter to power the board (and in fact I am), but the issue is protecting the SD card and eMMC against corruption. The idea here is to provide a way for the board to shut down safely before power is cut. Presumably, that means the board needs some sort of signal when it should start the shut down process. I don't really see any way around that. \$\endgroup\$ – Nick Oct 24 '14 at 18:33
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For multiple signals a bus switch like the TI SN54CBTD3384 http://www.ti.com/product/sn54cbtd3384 would be a good option. It has DIP package which will be easily solderable. Power it off the 5V supply to the beaglebone (which means that if the beaglebone isn't powered then signals don't get through and then connect the reset to the enable pin. This part will also level translate to prevent any voltage greater than 3.3V getting to the beaglebone.

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OK, the big issue is shutting down... Someone With Raspberry Pi had similar issue. They built a power supply with supercapacitors, effectively creating an UPS (uninterruptible Power Supply).
The article was on the Hackaday web page.

For your version, you would need to detect power loss, and initiate a write of all variables to memory. The system can then shut down (or start up) as and when it pleases.

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  • \$\begingroup\$ This is the plan, roughly (though rather than supercapacitors, power from the always-connected battery will be temporarily used). See the first link in the question for details. The issue I was asking about, though, is that the "power present" signal needs to remain disconnected until the board is ready. I probably could have made my title a bit more descriptive. \$\endgroup\$ – Nick Oct 25 '14 at 2:01

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