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

In article <bcf116ff-811c-4689-907d-0690213a4b7a@xxxxxxxxxxxxxxxxxxxxxxxxxxxx>, caagal@xxxxxxxxxxxxxx wrote:
I have built a basic circuit for the PIC18f4550 using a 20 MHz crystal
fromhttp://www.create.ucsb.edu/~dano/CUI/. =A0I have installed the HID
firmware fromhttp://www.lvr.com/usb.htm(USB complete). =A0The 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. =A0When 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.


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?

Could be that your 20MHz oscillator is barely oscillating. Check the solder
connections on both legs of the crystal, and make sure the connections between
the crystal and OSC1 / OSC2 are as short as possible and not accidentally
connected to ground or to the case of the crystal. Also make sure that you're
using the proper type crystal (series / parallel) and the proper load
capacitance; something around 18pF should be fine. Also make sure you have the
proper oscillator type selected in your config fuses (address 300001), I
believe it should be set to HSPLL (bits 3-0 set to 111x) for full speed USB.

--Tom.
.

Relevant Pages