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Apparent USB Power Problems


#1

I have a Panda II board. I have been able to download code to the unit which should tell you that I have a functioning USB port that is able to provide power. But now when I plug in the Panda II board to the same USB port, it doesn’t come up.

The board isn’t dead. If I power the board from an external 5v supply, the board comes up just fine and runs the code I’ve downloaded to it. If I remove the external 5v supply while also plugged into USB, the board sometimes continues running (which means it is powered by USB) and somtimes the board goes dead (presumably the USB port isn’t providing power).

Powering the board from a powered hub doesn’t change anything.

When I leave the Panda II board plugged in while rebooting the computer, I briefly see the power applied to the board but then goes off. I presume this is because the USB port hardware on my computer is in an inconsistent state when rebooting, prior to being initialized.

What I think this says is that the USB port hardware-- both on the Panda II and my computer-- are functioning. Maybe something is marginal there, but as it has worked in the past (again, I was able to download code), we aren’t talking about a system that never worked.

My hope is that someone has seen this kind of thing before and has a suggestion. Alternatively, someone with more knowledge than I about USB power signaling could tell me what to look at on my Panda II board. I have an oscilloscope and can probe around, but don’t know what is normal or what would help point me to the problem.


#2

What else is attached to the Panda? Sensors, motor controller, etc?


#3

[quote]The board isn’t dead. If I power the board from an external 5v supply, the board comes up just fine and runs the code I’ve downloaded to it. If I remove the external 5v supply while also plugged into USB, the board sometimes continues running (which means it is powered by USB) and somtimes the board goes dead (presumably the USB port isn’t providing power).

Powering the board from a powered hub doesn’t change anything.[/quote]

Have you measured the 5V when using powered HUB?


#4

There is nothing connected to the Panda II.

There was something connected in the past. It was a circuit (that I described elsewhere) that drove a transistor so that I could switch a 5v rail from the 3.3v PWM. That circuit when connected to the PWM 6 output pulls the pin down low enough to affect the bootloader. But again, there is nothing currently currently connected to the Panda II board.


#5

Hello, Japanls.
I had this exact same problem too…however, it was not with a FEZ device. It was with a TI Launchpad board I got, which uses a similar USB connector. When I plugged it in, I used to get the same momentary flash, and when I left it plugged in and rebooted, I’d see the same flash again.
The solution to the problem on the launchpad turned out to be none other than the USB Cable itself. If you can, buy a new one, and then try it.
I hope it works out for you.