thomas@tgohome.com wrote:
> Ok, I basically have a circuit like this:
> __________
>           |
>       Vdd |------+---- 5V
>           |      /
>           | 4.7k \
>           |      /
>        D+ |------+---- to USB D+
>           |
>        D- |----------- to USB D-
>           |
>    PIC    |
>  18F4550  |         -- to USB V+
>           |
>      Vusb |-------+
>           | 470n ---
>           |  cap ---
>       GND |-------+--- to USB GND
> __________|     __|__
>                  ___
>                   _


The resistor between D+ and Vdd doesn't belong there.  The appropriate
pullup resistors are already inside the PIC and will be enabled when
appropriate by the SIE.

> I wonder however, should I
> opto-isolate my D+ and D- connections

No, you shouldn't.  The USB lines are bi-directional, sometimes not strictly
digital, and at 12Mbit/sec too fast for most optos anyway.

> (to avoid ground loops, voltage
> spikes and other nasties), and use the USB V+ and USB GND wires to
> power the opto-isolator?

If your power supply is isolated, then there is no problem.  Getting a
isolated power supply would be much simpler than trying to isolate USB.


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