On 20/12/2010 00:12, Olin Lathrop wrote:
> 6 - If I remember right, 20MHz crystal doesn't work with USB.  Did you re=
ad
> the datasheet before designing the circuit?  I didn't think so.  Go read =
it.
>
It does.
Internal 96MHz  oscillator and PLL locked to the 20MHz.

2.3Oscillator Settings for USB
When these devices are used for USB connectivity,
they must have either a 6MHz or 48MHz clock for
USB operation, depending on whether Low-Speed or
Full-Speed mode is being used. This may require some
forethought in selecting an oscillator frequency and
programming the device.
The full range of possible oscillator configurations
compatible with USB operation is shown in Table2-3.

Table 2.3 OSCILLATOR CONFIGURATION OPTIONS FOR USB OPERATION

Possible Crystals for  24MHz  CPU (6MHz USB low speed) or 48MHz CPU=20
(48MHz USB high speed) are
4, 8, 12, 16, 20, 24 and 48MHz

Consult also
FIGURE 2-1:PIC18F2455/2550/4455/4550 CLOCK DIAGRAM

(page 26)

Crystal is divided by 1, 2, 3, 4, 5, 6,  10 or 12 to get 4MHz
96Mhz osc PLL to the 4MHz
For High Speed you divide by 2 for USB clock
CPU clock is 96 MHz divide by 2, 3, 4 or 6  -OR- Crystal  divided by 1,=20
2 ,3 or 4

For Low speed USB you can use either CPU Primary clock divider (the via=20
96MHz or direct external), but must be 24MHz as it's divided by 4

CPU clock can also be secondary external  or Internal Osc (not the 96MHz).

It's very flexible

--=20
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