Em 22/3/2010 15:21, thomas@tgohome.com escreveu:
> 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
> __________|     __|__
>                  ___
>                   _
>
> Now, USB V+ is not connected; it is power from the computer (or host) at 5
> volts. USB GND is connected to my chip's ground as well as the ground of
> the power supply for the chip. I wonder however, should I opto-isolate my
> D+ and D- connections (to avoid ground loops, voltage spikes and other
> nasties), and use the USB V+ and USB GND wires to power the opto-isolator?
>
> I am asking this because I've always seen RS-232 interfaces isolated, and
> my configuration is odd (normally a stand-alone unit with a stand-alone
> power supply, and an optional USB data download/control interface.) Is
> there any harm from isolating an interface which doesn't requite it?
>
> Any help appreciated. Thanks!
>   =



First, I think you should pull D+ to 3.3V, not 5V.

Second, D+ and D- are bi-directional signals, with speeds up-to 12Mbps,
how would you opto-isolate them?


Best regards,

Isaac

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