I think the VDD_3V3B rail is deliberately held off until the unit has started booting, and has already read the boot instructions. It is not a rise time issue. I suspect it was done deliberately to help with the "don't drive the rest of the pins" issue.
A little 5V to 3.3V regulator to power your LVC "input" IC will work nicely. Provided that it is powered at the same time the BBB is. That is, use VDD_5V if powered from the power jack. Relative to power appearing at the power jack: SYS_5V is delayed 50 ms. VDD_3V3B is delayed 100 ms. SYS_RESETn is delayed ~130 ms. I have not measured power supply sequencing when powered by USB. Or if you are dealing with both I and O on the boot pins, use something like a 74CBTLV3126 bus switch / transmission gate to isolate your circuitry from the BBB while it is booting. Just make sure the bus-switch / transmission-gate is powered while the BBB is booting. This issue only comes up about once a week here on the forum. :-) --- Graham -- 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.
