Hello all - I haven't done anything USB yet, and would like to have it=20
as a feature on a new product.

I'm looking at the PIC18LF2455, and it seems like there is a gotcha that=20
microchip doesn't actually spell out.

According to the datasheet (page 32 table 2-3), for full-speed USB 48Mhz=20
is required.  However, at 3.3V according to datasheet page 370 figure=20
28-2, the max frequency the LF part can do is a bit of 25Mhz, and in=20
fact for 48Mhz the minimum voltage is 4.2V.

I think I'm confusing clocks here, but I really need it sorted before=20
starting design.  What I want is full-speed USB when running @ 3.3V (or=20
even better @ 3V, which computes to a bit over 20Mhz).

Can this be done?
For 3V operation, following table 2-3, can I use a 24Mhz xtal with HSPLL=20
set to div by 6 to give 16Mhz for the microcontroller?  Will I be able=20
to use full-speed USB?

However, according to 2.2.4, it seems that if I use a 4Mhz crystal,=20
enable the PLL to get 96Mhz reference internally, I can then configure=20
the divisor that creates the USB clock to be div by 2 for 48Mhz for=20
full-speed USB, and then divisor for the microcontroller to be by 6 for=20
16Mhz (3V operation) or by 4 for 24Mhz (3.3V operation) and I'm set.

I'm pretty confused.  Can anyone say exactly what is needed (xtal and=20
osc config) to use full-speed USB @ 3V or 3.3V with this line of parts?

I would love to dispense with all this and use the new PIC18F13K50/14K50=20
parts, except that RAM is limited to 768 bytes (512 for the 13K), and=20
that will be a risk for this project.

Thanks for any help from out there.

J
--=20
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