On Sun, Apr 27, 2014 at 5:37 PM, Charles Steinkuehler <[email protected]> wrote: > On 4/27/2014 5:30 PM, Britton Kerin wrote: >> Bad things happen when I try to power the beaglebone through the header >> pins. >> I'm doing this by connecting an external 5V supply to header P9 pin 5, >> and ground header P8 pin 1. >> >> I have the pull-downs for GPIO1_12:15 enabled in software. I believe >> this is >> done correctly: I've verifying that the pull-downs overcome a large >> external >> resistor pulling up, and that enabling the internal pull-up instead works >> as expected (my interface doesn't support no-pull so I can't test that). >> >> The IO pins stay right near 0V when I power the bone through the jack. >> When I >> power it through the the header as described above, the pins float up to >> 0.1 V or more, sometimes high enough to cause spurious high reads on the >> pins. > > How "large" is your external pull-up resistor? > > The AM335x pull-down current is specified to be between 51 and 210 uA, > so if you see 0.1V at the pin, that would put your external pull-up at > 15K to 63K. > > Be more specific on your exact circuit, the exact voltages you are > seeing, and all details related to operating state when you measure the
Sorry, I gave a wrong impression mentioning the external pull-up. Its not part of the circuit that behaves strangely, I just used it to verify that the software that enables the internal pull-ups is working right. There's nothing to the circuits under test. One uses the barrel connector, with a 5V 1A wall wart, the other uses power via header. Then I plug in a wire and check the voltage on GPIO1_14 at the other end. With the barrel connector its 0.014, with the header-fed power its about 0.08. I did notice that the power supply feeding the bone via the headers might be lightly underpowered: its coming from a 12 V 0.7 A wall ward that then runs though a DC-DC coverter that might be as low as 50% efficient for maybe only 4-5 W. The bone seems to run fine, but it is possible that slight underpower could somehow cause the IO pins not to pull down correctly? Britton -- For more options, visit http://beagleboard.org/discuss --- You received this message because you are subscribed to the Google Groups "BeagleBoard" group. To unsubscribe from this group and stop receiving emails from it, send an email to [email protected]. For more options, visit https://groups.google.com/d/optout.
