First, I think the waveforms you are displaying do not look like they originate from the power rails. I suspect you have a scope probe where the ground wire and probe tip are too long and you are picking up radiated noise from other sources. Start by making the scope ground very short. I’m talking about 1/4 inch or less and measure the power rails again. The power rails use parallel power planes, and array of low ESR decoupling capacitors and several bulk capacitors. The design is very good and that is why these waveforms look suspect.
Regards, John > On Jul 6, 2016, at 7:13 PM, Joshua Collins <[email protected]> wrote: > > Hi Robert, > > I thought I had checked for power supply issues and not found anything, but I > checked again and found problems. Our cape board powers the beaglebone > through the VDD_5V pins on the header. > > Here are the VDD_5V (blue) and VDD_3V3B (yellow) rails when the beaglebone is > powered from our custom cape, captured at a point during flashing: > > > > <https://lh3.googleusercontent.com/-EBhCzNStAqA/V322CKFxxjI/AAAAAAAAAKo/iLdc--L8XlAkpezfA97WvPimrM6djzLvACLcB/s1600/VDD_5VandVCC_3V3BSpike3.png> > > <https://lh3.googleusercontent.com/--5ZnqjQBwPE/V322FNIMPFI/AAAAAAAAAKs/i-t4gxU0UuYtlRq427FYuJP1XSEo9KevwCLcB/s1600/VDD_5VandVCC_3V3BSpike4.png> > So, we've found a problem. > > But here is what those same rails look like when the beaglebone is powered by > a 5A lab supply through the barrel jack, without our custom cape connected - > it's interesting that there are still significant (albeit much smaller) > fluctuations. Are they large enough to be a problem? I wonder whether the > beaglebone has enough local capacitance to handle the eMMC's power demands. > > > <https://lh3.googleusercontent.com/-qKEWPJOYYQs/V322TTKacRI/AAAAAAAAAKw/LQpcLiOtvU0C3AXJAP7OmFBO9Y8YFlxaQCLcB/s1600/BenchSupplyVDD_5VandVCC_3V3BSpike3.png> > > <https://lh3.googleusercontent.com/-Mkjpi_c7DAk/V322W7WQYbI/AAAAAAAAAK0/XyDGRYQoaIYYle0mWiYRWc8EDBdY9gSawCLcB/s1600/BenchSupplyVDD_5VandVCC_3V3BSpike4.png> > > <https://lh3.googleusercontent.com/-mV_zjIGnWmg/V322aAzd15I/AAAAAAAAAK4/-eHHLUlfY7whIkh7xglC_fbW7wfqkx_4gCLcB/s1600/BenchSupplyVDD_5VandVCC_3V3BSpike5.png> > > > <https://lh3.googleusercontent.com/-2Va53Bhhbkw/V322dKWg9RI/AAAAAAAAAK8/cmt0Ki7HN54eUsuthwxgGlPqQvP0iVGvgCLcB/s1600/BenchSupplyVDD_5VandVCC_3V3BSpike6.png> > > Thank you for your help, > Joshua Collins > > -- > For more options, visit http://beagleboard.org/discuss > <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] > <mailto:[email protected]>. > To view this discussion on the web visit > https://groups.google.com/d/msgid/beagleboard/815d709d-9b51-495b-a8b9-c29b213bd0b7%40googlegroups.com > > <https://groups.google.com/d/msgid/beagleboard/815d709d-9b51-495b-a8b9-c29b213bd0b7%40googlegroups.com?utm_medium=email&utm_source=footer>. > For more options, visit https://groups.google.com/d/optout > <https://groups.google.com/d/optout>. -- 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]. To view this discussion on the web visit https://groups.google.com/d/msgid/beagleboard/5D1B2731-19B7-4C4A-9CF8-E16FDE155640%40gmail.com. For more options, visit https://groups.google.com/d/optout.
