another ESD question

[circuitmangler]

First of all I'd like to thank everyone who responded to my earlier question about adding ESD protection - it all was very helpful information.

My new question is this: suppose we have a USB-powered microcontroller based device, and some of the uC pins are exposed as an external I/O port (i.e. DB-9, RJ-11, DIN, etc.) for connection to an external device. As far as ESD protection is concerned, is it better if the uC is powered before the external device is connected, or is it better to connect the external device while the uC is unpowered (that is, unconnected and with a floating ground.) How about unpowered but with its ground connected to the USB ground?

The external device will draw its power from the uC/USB device through the I/O port.

Thanks!

 

[zappedagain]

"Floating" is a tricky one for ESD. If someone is holding the device you can capacitively couple through them and have a capcitively-couple path to 'ground'; that is all the ESD pulse needs. I suggest you plug the USB device in AFTER power and ground are established. That way you have a known ESD path to work with.

If you chose to plug in the external device when not powered, then you need to dissipate the ESD at the IO connector and before it gets to the uC. That might work too with the right protection hardware.

 

[IRstuff]

I think that starting with grounding is always a good idea. If the new item is charged up, dumping its ground node to a real ground will likewise couple its other nodes to a lower potential, thereby reducing the risk of a large potential difference on input or output connections.

TTFN

FAQ731-376

 

[GonzaloEE]

Agreeing with guys above, I always avoid applying any external signal to an unpowered uC, no matter how rough the clamp diodes or any other protection the datasheet said.
In order to prevent latch-up or other tricky headaches, I’d suggest connecting things in the following order:
1-GND
2-uC VCC
3-Alien signals, hopefully after uC is completely awake, and output pins are no longer tristated.

Good Luck!