When to power sensors connected to the BeagleBone Black?

[Matthew Witherwax]

As part of the Control of Mobile Robotics class offered through Cousera, the students are building robots based around the BeagleBone Black.  The robot has 5 infrared distance sensors and 2 reflectance sensors used for odometry.  These sensors are connected via voltage dividers to pins 33, 35, 36, 37, 38, 39, and 49 of P9, and are powered from the same power supply used to power the BeagleBone Black.  The System Manual has a rather explicit warning:

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.

My concern is the current arrangement would allow the sensors to begin driving the pins before the SYS_RESET line goes high; however, I am a software guy just starting down the hardware path.  Thus far many in the class have been using the robot without issue.  Is my concern invalid?

Thank you for your time.

[Gerald Coley]

I put that in there for a reason. It may not happen all the time, but eventually you could blow up the processor.

Gerald

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[Eric Palmer]

If everything is driven form the same power source, even with voltage dividers or regulators, can we assume that turning on all devices at the same time, with one power switch, is safe or do we need to somehow put a delay into powering on everything but the BBB board?

Eric

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Eric Palmer

[Matthew Witherwax]

Thanks Gerald.  That is exactly the response I was expecting and exactly the feeling I had.  Never really like playing Russian roulette myself.  Do you see any issue with powering the sensors via a voltage regulator like http://www.pololu.com/product/2098 that can be turned on via a pin on the BBB once it is up and running?

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[Matthew Witherwax]

The way I read the manual and understand Gerald is that it isn't safe any more than it is safe to play Russian roulette because you don't always get shot.  As Gerald said, you just might get lucky and not blow anything... until you do.

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[Gerald Coley]

Yes you can do that  Use the 3.3V to enable the regulators.

Gerald

[Matthew Witherwax]

Thanks for all you do Gerald.

[Eric Palmer]

Awesome

Gerald we are lucky to have you on this mail list and be participating so much.

Thanks

Eric Palmer

[Gerald Coley]

I am not sure how to take that!Not sure if it is luck or something else.

Gerald

[Matthew Witherwax]

:) Attempts at humor on only one cup of coffee might not be so funny!

[Eric Palmer]

Yes I get that.  

[Gerald Coley]

That is my secret. I don't need coffee! The inside story is that I am always here. I have no life!

Gerald

[Matthew Witherwax]

Next time you are in Houston, coffee is on me!

[Gerald Coley]

Houston! Us folks in this part of the state try to stay away from Houston!

Actually I have spent a lot of time over the years in Houston. Been to a few Astros games as well. 

Gerald

[Matthew Witherwax]

I sure hope that was years ago when they actually won a game or two.  We have other attractions.  We even almost have NASA.

[Gerald Coley]

But now you have Nolan Ryan.

[Brad Hopper]

Maybe you can still go to the Astrodome - the eighth wonder of the world!

[ky...@cranehome.info]

On a related note: If my sensors are buffered and gated to sys_resetn am I safe if the BBB powers down a few ms before the rest? My circuit watches the 3.3V regulator output and when it falls away the power for the entire circuit is killed as well but it takes 5-9ms for the power controller to notice and react to the condition. My buffers are connected to SYS_RESETN via a schmidt-inverter driving the /OE gates for the pins.

[Gerald Coley]

You are fine. Reset is no the issue.  Power is. The fact that the power is removed makes everything safe. Reset is an easy way to make sure the board is powered up and provides a logic level indication of that fact..

Gerald

On Fri, Mar 7, 2014 at 12:47 AM, <ky...@cranehome.info> wrote:

On a related note:  If my sensors are buffered and gated to sys_resetn am I safe if the BBB powers down a few ms before the rest?  My circuit watches the 3.3V regulator output and when it falls away the power for the entire circuit is killed as well but it takes 5-9ms for the power controller to notice and react to the condition.  My buffers are connected to SYS_RESETN via a schmidt-inverter driving the /OE gates for the pins.

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