Raspberry Surprise (My experiences with the Pi so far)


6 posts
by gardrek » Tue Jun 26, 2012 11:55 pm
Hello, all!
I just got my Pi recently, and it's pretty cool. But I have been having some trouble getting up and running. For instance: I bought a PS/2 to USB adapter (as I talked about in this forum post: http://www.raspberrypi.org/phpBB3/viewtopic.php?f=24&t=3391&p=45148#p45148), but no keyboard I've tried has worked with it yet. I'm not sure, maybe it's a driver thing?
Also, today I bought a cheap USB keyboard to test with the Pi (This one: http://www.microcenter.com/single_product_results.phtml?product_id=0373546, cheapest they had), and it doesn't work either. Difference is, this one causes a kernel panic whenever I plug it in. Very strange.
I also purchased this USB Hub (pictured): http://www.microcenter.com/single_product_results.phtml?product_id=0360454
Image
This was the strangest experience of all. I plugged the USB hub into the (unplugged) Pi, and the Pi just started up! Is it supposed to get power this way? Is this a feature of the USB Hub? Should I be concerned, or is this normal?
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by Nr90 » Wed Jun 27, 2012 12:05 am
The USB feeding back current has been found by others as well. It happens on poorly designed hubs. No need to worry AFAIK.

Make sure your keyboard uses as little power as possible (no flashy lights, USB hubs etc)
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by gardrek » Wed Jun 27, 2012 12:09 am
Nr90 wrote:The USB feeding back current has been found by others as well. It happens on poorly designed hubs. No need to worry AFAIK.


Is it good practice to power the Pi solely from this feedback?
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by Nr90 » Wed Jun 27, 2012 12:11 am
I dont know tbh.
I doubt it'll supply enough current without triggering the fuse.

You could search this forum, there are a couple of other threads on the subject.
I think Dom (on of the designers) had some interesting posts on the subject.
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by gardrek » Wed Jun 27, 2012 12:27 am
Originally I was going to try powering it by plugging a microUSB cable into the powered hub. Is there any reason that shouldn't work? Also, do you know any way to stop the feedback?
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by jbeale » Wed Jun 27, 2012 1:17 am
The R-Pi has fuses on the power output through the two USB ports limiting the current on each one to about 140 mA. Since the R-Pi needs about 550 to 700 mA when running at full tilt, the system will lock up if you try to power it only by back-feeding current through the USB output. You can prevent the backfeeding by taping over the +5V pin on the USB connector going to the powered hub before connecting it. But you may not need to, it should not cause a problem so long as you have +5V power normally supplied, and yes you can supply it from that same powered hub using a microUSB cable. As long as the hub can provide enough current!

USB keyboard causing a kernel panic: apparently, some keyboards do this! Also, what are you using to power your Pi? One thing that causes a panic is a voltage drop, so make sure your power supply is adequate. See also:
viewtopic.php?f=28&t=9163
viewtopic.php?f=28&t=6893
http://elinux.org/R-Pi_Troubleshooting# ... r_problems
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