Josh Koffman wrote:
> My development setup looks like this at the moment:
>
> PIC - USB hub - Laptop - ICD - back to PIC
>
> The USB hub is self powered (via wall wart), the PIC is powered by my
> bench supply. I was having some very intermittent problems with my
> setup where for awhile my USB project wouldn't work at all. Then it
> would work fine for a few. Then not at all again. Very frustrating. It
> seems to be a ground loop issue as when I disconnect the ground on the
> USB plug to the PIC, it works great. However, if I then program the
> chip and disconnect the ICD, I get nothing again. So for prototyping
> the solution seems to be grounding the PIC through the ICD and letting
> the ground on the USB input float. I'm not sure if the ICD will work
> if I float that ground. I figure floating the USB is better as the USB
> signal is by definition a differential one. I get the feeling that
> isn't the best of solutions though.

I've been wondering about that too, but haven't had a chance to try anything
because I'm still waiting for boards to come back.  I thought about ground
loops when designing the circuit, but there wasn't much I could do since I'm
powering the circuit from the USB short of a switching power supply with
transformer coupling, which would be waaaay overkill for this board.

I ended up putting 100 ohms in series between the board ground and the
shield of the USB connector.  This should avoid ground loops thru the
shield, but I imagine they will still occur thru the actual ground line.
We'll see.  I don't have a good answer.


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