Hi Frederic: You could make the 74HC541 work, since it has a CMOS input and should not load the BBB during boot, provided that it has power supplied the entire time the BBB is booting.
I was thinking more of something like the 74CBTLV3126 bus switch which would disconnect your existing circuits from the BBB, rather than buffer them like the 74HC541. In both cases, the switching should be done from the 3.3 V bus, but you can not power the ICs from the 3.3V bus as you mentioned. As I remember, on the BBB, the 3.3V bus rail on the P9 connector does not power up until AFTER the BBB has completed booting, so if you power the buffer or isolation switch ICs from the 3.3V rail, the ESD protection diodes in the IC will clamp the boot lines to ground, preventing the BBB from booting. You will need to power the ICs, whichever you choose to use, from a separate 3.3V regulator that provides power anytime the BBB has power applied. I think the +5V rail is powered anytime the BBB is powered, so it could drive the separate +3.3V regulator. A few Voltage measurements are appropriate, before you finish your re-design. --- Graham == On Tue, Aug 18, 2015 at 12:19 PM, Frédéric <f...@gbiloba.org> wrote: > Le Tuesday 11 August 2015, Graham a écrit : > > > If you look at the diagrams in the System Reference Manual, the BBB uses > > 100K Ohm pull up and pull down resistors to tell the processor how to > > boot. So any load low enough to cause a line with 100K Ohm pull up or > > pull down to change logic state will cause boot problems. I would > > estimate that any load, less than 200 K Ohms can cause this problem, > > even though it is an "Input". Also, inputs connected to unpowered ICs > > will clamp a line to ground, because of the ESD diodes in the IC. > > > > So, either use powered buffer ICs that have only a CMOS input load, or a > > transmission gate as you described. I think you can switch the > > transmission gate using the 3.3V rail coming out of the BBB, since it > > does not come up until the unit is finished booting. Do not put an > > unpowered transmission gate on the GPIO lines, because of the ESD > > clamping I described. > > Thanks for the explanations! Didn't know about this ESD clamping > problem... > > I started to modify my cape, adding 4x 74HC541; can you confirm they are > OK regarding the inputs? They will be powered up from the 3V3 rail. > > -- > Frédéric > > -- > For more options, visit http://beagleboard.org/discuss > --- > You received this message because you are subscribed to a topic in the > Google Groups "BeagleBoard" group. > To unsubscribe from this topic, visit > https://groups.google.com/d/topic/beagleboard/ndlZtL9E1u4/unsubscribe. > To unsubscribe from this group and all its topics, send an email to > beagleboard+unsubscr...@googlegroups.com. > For more options, visit 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 beagleboard+unsubscr...@googlegroups.com. For more options, visit https://groups.google.com/d/optout.
