PIC18F4550 problem with USB and Reset

Prefekt · Joined: 21 Oct 2010 Posts: 85

Hello,

I have a board with a pic18f4550, USB and RS232 Interface. The PIC is running perfect.
So I have develop a new board for this application with similar layout as the first board and a ground plane.
My problem is now, that the USB doesn't work (a window appears that the usb hardware was not correctly detected in windows XP). And when I tip with a metal item e.g. screwdriver on a GND pin or the ground plane the PIC do a reset...
In my first layout, the same pic works without these problems.

I have a 100nF between VDD and VSS ...

Anybody an Idea?

Best regards
Volker

Ttelmah · Joined: 11 Mar 2010 Posts: 19515

Seriously, designing a ground plane properly, involves quite a bit of knowledge. For instance, if you run it tightly round the oscillator pins, you introduce a lot of stray capacitance, and can take the crystal off frequency. Microchip have a good application note showing how to layout a ground plane round oscillator pins amongst other things to do with the oscillator. Other things that could affect the USB, are the total capacitance that is present around the USB wires. Maximum lengths, and allowable impedance limits are part of the USB spec, and need to be considered.
The resets suggest that though you are calling this a 'ground plane', it is not actually grounded, or connected to the point that local circuitry considers 'ground'...

Best Wishes