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
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