Question about applying voltage to an unpowered BBB.

Alright, I know that there are a lot of discussions on here about this topic, and I have read through them, but I am still a bit confused about this line in the manual:
NOTE: DO NOT APPLY VOLTAGE TO ANY I/O PIN WHEN POWER IS NOT SUPPLIED TO THE BOARD. IT WILL DAMAGE THE PROCESSOR AND VOID THE WARRANTY.
NO PINS ARE TO BE DRIVEN UNTIL AFTER THE SYS_RESET LINE GOES HIGH.

I am working on a project that will involve me using multiple sensors with the BBB that are powered from another source. One of those sensors is a GPS that spits out data as long as it is powered, but the others only send data after they have received a request. Here are my questions:

  1. Based on that statement, it is my understanding that if the GPS I’m using is powered and the BBB is not, then the BBB will be damaged. Is this accurate?
  2. What if the board and all the sensors are powered from the same source controlled by a single switch? Will this damage the board when I flip on the switch?
  3. What if I power all the sensors from the board itself so that it is impossible for them to receive power before the board does? Will this damage the board?

I know it is a bit confusing. If you do, it will blow up the processor. It is a function of the processor.

I suggest you read the datasheet on the processor as it relates to power sequencing.

http://www.ti.com/product/AM3358/technicaldocuments

Until the 3.3V rail comes up, nothing on your board can be powered up.Even if it is from the same source. Again, it is a matter of power sequencing. All the rails must come up and power down in a particular sequence. Violate that sequence, then you are asking for trouble.

Power your circuit off of the VDD_3V3B rail or use the VDD_3V3 rail to enable the power on any external circuitry that connects to a pin on the processor.

Gerald

Alright, I know that there are a lot of discussions on here about this
topic, and I have read through them, but I am still a bit confused about
this line in the manual:
NOTE: DO NOT APPLY VOLTAGE TO ANY I/O PIN WHEN POWER IS NOT SUPPLIED TO THE
BOARD. IT WILL DAMAGE THE PROCESSOR AND VOID THE WARRANTY.
NO PINS ARE TO BE DRIVEN UNTIL AFTER THE SYS_RESET LINE GOES HIGH.

I am working on a project that will involve me using multiple sensors with
the BBB that are powered from another source. One of those sensors is a GPS
that spits out data as long as it is powered, but the others only send data
after they have received a request. Here are my questions:

Opinions follow:

1. Based on that statement, it is my understanding that if the GPS I'm
using is powered and the BBB is not, then the BBB will be damaged. Is this
accurate?

That's what it says.

2. What if the board and all the sensors are powered from the same source
controlled by a single switch? Will this damage the board when I flip on
the switch?

The problem could be if the sensor power comes up before the board is
fully powered up, and that would be a problem.

3. What if I power all the sensors from the board itself so that it is
impossible for them to receive power before the board does? Will this
damage the board?

Better solution, and is only based on the board's capability of
supplying the power. This is how the capes operate, IIRC.

Another possibility is to have external power for the sensors
controlled by either a relay or a solid state switch, biased so that
the switch is off unless a voltage is applied to the switch control
pin.

This allows the sensors to be shut down for lower consumption if
needed (or if useful), but does give you rather complete power control
over the sensors. I'd use a bidirectional driver with separate power
lines (normally used to do level translation). IIRC those chips do
not drive with power off on one side. This not only protects the
processor, but gives you some additional buffering.

Harvey