How to know when signals can be sent to gpio pins on boot?

The documentation for BBB says all over the place not to apply signals to the GPIO pins until the BBB is powered up. What is the best way to know when that occurs? Is there a signal or pin to watch for going high or low that can be used to set up some simple logic to inhibit my input devices until the BBB is ready?

3.3v rail on expansion header vdd_3v3exp.



But , I tested the pin 3 (3.3 v) in the P9 header and it appears inmediately when the BBB power on. Then, Is it neccesary wait for this signal, if is present always ?

Correct, the issue, if you apply a signal to any pin before 3.3v you
will toast the cpu..

the sys_boot pins have other issues related to the bootrom pin
scanning on bootup...



Then will be neccesary to install a switch between the GPIO/ analog signals of the BBB and the external signals( analogs or digitals)? Maybe a CD4066 using pin 3 (3.3 v) of the BBB to control (open or close) all IC pins ?

I learned it int the VLSI chip design courses that transmission gates

are about the ugliest thing wrt signal overvoltage, so that they need

their own protection circuits. It is safe to assume that this has been

done for the 4066. But have you seen its timing data at 3V3?

Slow as molasse. No. Slower. Much slower.

I use 74LVC or LV or other buffer/gate to drive the lines to the latch-up

endangered thing, and I'd feed them from the BBB 3V3 in this case.

That takes virtually no power. The inputs of these gates are usually

safe to 6V or so. For slowish signals it is usually enough to use a series

resistor, 1KOhm or so, instead of the gate. That limits the current that

could trigger the latch-up to safe values. That passed design reviews

even in space projects where they are in continuous panic about latch up.



the 74LVC looks like a good solution. Wondering whether that is a capability that is worth building into the BBB?

Thank you

I apologize if I don´t understand well.

As I mentioned before, I will need pass analog inputs which vary from 0 to 5 Vdc only, they will change slowly.

Do you believe is neccesary yet to use the 74LVC series? I do the question because is a bit hard to find in the local stores these IC’s.

However, Could you recommend me one in particular ( 74 LVC) . I was revising the information of Texas Instrument , but I don´t find a IC as switch ( equivalent to CD4066) in this series.
How long time will be neccesary to wait from the BBB turning on until the external signals can be connected to it ?



That'll blow the ADC, which is only rated to 1.8v.. Stick something
like a resistor divider to scale that down..


Yes , I will use an voltage divider.