Gentle Backdrive on LPC17xx GPIO okay?

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Gentle Backdrive on LPC17xx GPIO okay?

NXP Employee
NXP Employee
Content originally posted in LPCWare by Adam.Garrison on Tue Feb 23 11:24:57 MST 2016
I am looking at the most recent rev of our board and have a question about the inputs on the processor.

Currently we are using a button to power the processer, effectively this pin drives the shutdown pin on our 3.3V regulator. Our charger circuit has a couple inputs/outputs that are fed into the processor for either status or control.

One of the controls the processor can provide is how much current our charger can draw off of USB charging.  I currently have a pull up on that pin to Vbus (5V).  This trace runs from the charger to a pin on the processor which is an output.  In the field I suspect that customers will use the USB Micro B port for charging to keep the battery topped off.  The problem I see is this….when they are plugged into USB, but the unit is not turned on, there is 5V on the pin to the processor (through a 10k resistor) which is effectively 500uA of current flowing into that pin.

I checked the power pins on the processor and none of them show any voltage at all (  I was worried about it backpowering the core through the GPIO pins).  The LPC1778 says that it is 5V tolerant when the core is powered.

If I limit the current like this, am I okay doing this with such a small amount of current flowing through it?
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