On Mon, 2009-12-28 at 16:05 -0500, Nathan House wrote:
I'm making a PIC18F4550 board so I can learn USB, is my schematic correct?
>
> http://www.roboticsguy.com/images/misc/PIC18F4550-board.png
>
> Appreciate any advice you might have!

The manuals for the Explorer 16 and the 28 pin starter board both include
schematics, both are available on the Microchip site, and both boards have
an 18Fx550 for USB.  When the board may be powered from the USB port,
Microchip sometimes likes to use an inductor in the line, I suspect to
eliminate any hash from the USB line, but studying someone else's
circuitry is always helpful for me.

Look at your MCLR circuitry.  Depending on the programmer, you may put +12
on the MCLR line during programming.  (Some programmers take care to
deliver something over +12.)  With only a 10K resistor to +5 you might
push +12 into your +5 supply, potentially damaging other parts.  10K is
fine if you aren't doing ICSP, but with the programmer connection, I'd be
a little nervous.

There are a few ways to deal with this.  Microchip suggests a diode (see
any of the ICSP documents online), but as long as you have SOME other
circuitry, changing this resistor to a very high value can also work. 
MCLR itself draws next to nothing, so as long as this resistor is
physically close to the pin (very little capacitance) the value can be
quite high.  If your other circuitry can be counted on to draw at least a
few mils during programming, then a large resistor can give you the 7 or 8
volts drop you need.  In a lot of cases, 10K might do it, but it feels a
little low for me, although I just noticed your power LED, it might just
be enough.

--McD


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