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PICList Thread
'[PIC] USB Ground Loop'
2005\12\21@115926 by Josh Koffman

face picon face
Hi all. If you've been following my recent emails, you'd have surmised
that I'm playing with the PIC18F4550 and USB. Now I've run across a
problem and while I do seem to have a workaround, I'm wondering if
it's the best one.

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.

The other thing is that my ideal setup would be:

Laptop - ICD - PIC - USB Hub - Desktop

That way I can have the laptop for development and the desktop to
actually connect to. Problem there is that I tried a variant of that
(before I discovered the grounding issue) and it didn't work. Now that
I'm wise to the ground problem, I have absolutely no idea how to make
that work.

So short of getting an isolated USB hub ($$$), what can I do?

Thanks!

Josh
--
A common mistake that people make when trying to design something
completely foolproof is to underestimate the ingenuity of complete
fools.
       -Douglas Adams

2005\12\21@122047 by Spehro Pefhany

picon face
At 11:59 AM 12/21/2005 -0500, you wrote:
{Quote hidden}

Where is your ground loop happening?

Laptop-- probably isolated (unless you're connected to something else)
ICD- isolated switching supply
USB hub - isolated supply
Desktop- grounded to earth
Bench power supply - ???? *should* be floating, but check

That's only *one* ground for sure. You can't get a loop from one connection.
So where is the problem coming from?

>Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
spam_OUTspeffTakeThisOuTspaminterlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com
->> Inexpensive test equipment & parts http://search.ebay.com/_W0QQsassZspeff


2005\12\21@130522 by Wouter van Ooijen

face picon face
> Where is your ground loop happening?

PIC - USB hub - Laptop - ICD - PIC

I have had similar problems with

PC parralel port - ARMchip - hub - hub - PC.

Programs that work flawlessly elsewhere fail randomly.

Wouter van Ooijen

-- -------------------------------------------
Van Ooijen Technische Informatica: http://www.voti.nl
consultancy, development, PICmicro products
docent Hogeschool van Utrecht: http://www.voti.nl/hvu


2005\12\21@130613 by Josh Koffman

face picon face
On 12/21/05, Spehro Pefhany <.....speffKILLspamspam@spam@interlog.com> wrote:
> Where is your ground loop happening?
>
> Laptop-- probably isolated (unless you're connected to something else)
> ICD- isolated switching supply
> USB hub - isolated supply
> Desktop- grounded to earth
> Bench power supply - ???? *should* be floating, but check
>
> That's only *one* ground for sure. You can't get a loop from one connection.
> So where is the problem coming from?

To be honest, I'm not sure. I believe the power supply is floating
because it also has a ground output, and on the (more often that I'd
care to admit) occasion that I attach my leads to +V and GND, I get no
output. I then sheepishly move to -V. In any case, my only thought is
that since most of the power supplies in question are switchers
(except the bench supply and the hub supply) that there may be
something in that. It definitely doesn't like having multiple ground
paths to my breadboard from the PC.

Josh
--
A common mistake that people make when trying to design something
completely foolproof is to underestimate the ingenuity of complete
fools.
       -Douglas Adams

2005\12\21@134043 by olin piclist

face picon face
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.


******************************************************************
Embed Inc, Littleton Massachusetts, (978) 742-9014.  #1 PIC
consultant in 2004 program year.  http://www.embedinc.com/products

2005\12\21@140715 by Mike Hord

picon face
> Where is your ground loop happening?
>
> Laptop-- probably isolated (unless you're connected to something else

Whoa, hold on a sec.  I'm discovering- much to my chagrin- that the
assumption of isolation in a laptop is no longer safe in the least.  Many
(most?) later model laptops are coming with a three-prong cord, with
the ground prong connected to the ground of the power supply, and
the ground of the laptop.  Thus, you end up with a situation where
the ground of the circuit under test connects to the ground of the ICD2,
which is (I assume) connected to the USB ground, which connects to
the laptop ground, which goes to earth ground in the house.

> That's only *one* ground for sure. You can't get a loop from one connection.
> So where is the problem coming from?

No, it's not one ground for sure.  The PC and laptop could very easily
be on different grounds.  Is your battery life good enough to pull the
power supply on the laptop?  Or can you find a "ground breaker",
sometimes called a cheater plug, which takes 3 prongs and reduces
to 2?

Mike H.

2005\12\21@144106 by Peter van Hoof

face picon face


Spehro Pefhany speffspamKILLspaminterlog.com wrote

{Quote hidden}

I don't believe you understand the concept of a groundloop
A groundloop actually has nothing to do with earth ground (though it can be part of it)
A groundloop exists when ground conductors of connecting wires form a closed loop
and thus a coil which picks up magnetic interference and couples this ground loop
current to the other conductors in the cables involved. In audio equipment this creates
a hum, in other signal cables other forms of interference.

Peter van Hoof

2005\12\22@033536 by Gerhard Fiedler

picon face
Spehro Pefhany wrote:

>>My development setup looks like this at the moment:
>>
>>PIC - USB hub - Laptop - ICD - back to PIC
>
> Where is your ground loop happening?

> That's only *one* ground for sure. You can't get a loop from one
> connection. So where is the problem coming from?

Not sure whether that's exactly the problem, but one loop here are the two
USB connections between the laptop and the PIC. That's a small loop, but it
is a loop.

If this is the problem, it should go away no matter where you pull the
ground in those two connections.

Gerhard

2005\12\22@050021 by Alan B. Pearce

face picon face
>Where is your ground loop happening?
>
>Laptop-- probably isolated (unless you're connected to something else)
>ICD- isolated switching supply
>USB hub - isolated supply
>Desktop- grounded to earth
>Bench power supply - ???? *should* be floating, but check
>
>That's only *one* ground for sure. You can't get a loop from one
connection.
>So where is the problem coming from?

If you use the 9V mains adapter that Microchip supply with the ICD, this
will have the DC output connected to mains ground. I have one that I wanted
to use for something that needed the output isolated, and I had to open the
supply and disconnect it from mains ground.

2005\12\22@170101 by Josh Koffman

face picon face
Hi all. Well, it's nice to know that other people are thinking of this
too! In my case, at the moment it seems that the problem wasn't as bad
as I thought. I believe I tracked it down to a faulty connector. I
believe it was giving an intermittent ground connection, and sometimes
it was somewhere in between connected and disconnected.

So, after replacing that, it seems to be working ok...for now. I look
forward to what others are able to find out in the future though. Rest
assured you'll be hearing from me if it starts acting up again!

Thanks,

Josh
--
A common mistake that people make when trying to design something
completely foolproof is to underestimate the ingenuity of complete
fools.
       -Douglas Adams

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