Re: USB circuit disconnects when the circuit is put near another surface

On 29 Jan, 15:39, pa...@xxxxxxxxxxxxxxxxxxxxxxxxxxx (Paul Carpenter)
wrote:
On Monday, in article
     <479defe5$0$22821$e4fe5...@xxxxxxxxxxxxxxxxxxxxxxxx>
     m...@xxxxxxxxxxxxxxxxxxxxxxxxx "Meindert Sprang" wrote:





<caa...@xxxxxxxxxxxxxx> wrote in message
news:0a55d245-04c6-4a73-a053-b1bcc8bb2167@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
Hello,

I have built a basic circuit for the PIC18f4550 using a 20 MHz crystal
fromhttp://www.create.ucsb.edu/~dano/CUI/.  I have installed the HID
firmware fromhttp://www.lvr.com/usb.htm(USB complete).  The circuit
works fine and connects ok then installs the driver as long as it is
suspended in free space.

As soon as the circuit comes near ( 2-3 cm) another surface (paper or
the wooden table) the usb disconnects.  When I investigated I found
the voltage on D+ signal line went down from the required 3.1V(approx)
to 1.45V as it neared the surface, thus initiating the disconnection.

Connecting just the ground side of an oscilloscope to the ground on
the circuit also cured the problem.

Seems to me there is no ground connection between the PC and the CUI
board....

Yes but caused by an intermittent connection on the ground as the circuit
is moved nearer the surface something is flexing, causing an open circuit.

Whenever movement causes a problem I look for intermittent connections or
cable breaks.

--
Paul Carpenter          | p...@xxxxxxxxxxxxxxxxxxxxxxxxxxx
<http://www.pcserviceselectronics.co.uk/>    PC Services
<http://www.gnuh8.org.uk/>              GNU H8 & mailing list info
<http://www.badweb.org.uk/>             For those web sites you hate- Hide quoted text -

- Show quoted text -

Thanks for the advice. However the circuit definately disconnects
when something is brought to within approximately 1-2 cm of it. To
prove this I clamped the USB plug with the circuit board in free space
vertically. With no contact at all with the circuit board and it not
moving, a book brought slowly towards the circuit makes it disconnect
once it comes within 1-2 cm and reconnects when I take it away.

I have tried swapping the usb cable and measured the resistance of the
board ground to the computer end of the usb cable, they are all fine.

I have connected all the ground points shown in the circuit to the
ground of the usb supply (pin 4), should there need to be any
connection to the usb cable sheath earth? Or does it need to be in a
metal box connected to the sheath to stop the stray reflections?

Thanks

Mike.
.

Relevant Pages