PIC18F4550 vs 18LF4550

[magestik]

Hello, i saw in the datasheet that the LF version stands for low voltage.

I need my PIC to operate @3V or 3.3V, i intend to use the LF version.

But my question is, since @3V the frequency decreases (~16Mhz), will the USB still work ?

I'm currently using a 18F4550 @5V, with these fuses :

#fuses HS,WDT,NOPUT,NOBROWNOUT,LVP,NOWRT,NOPROTECT
#use delay(clock=20000000,restart_wdt)

Will something change ?

Thank you.

[Ttelmah]

Actually quite a complex question.
Look first at the oscillator diagram. Note the dotted line 'round' the USB section. Then realise that the voltage versus frequency restrictions, apply to the CPU clock, not the USB stuff.
However then move to the USB peripheral diagram (17.1), and read up on the internal 3.3v regulator. This _requires_ 5v for use.
So, 'yes' you can use USB, _but_ you must select a clock mode, that runs the CPU itself below 16MHz. However you need an _external_ source of (preferably)3.3v, to run the USB pull up components. So if using 3v for the whole CPU, you would really need to generate 3.3v, with an external regulator from the USB bus, and have your own external pull ups to this (single one to D+, assuming you are running full speed).
You need a _minimum_ of 3v, to operate the USB PLL, and clock the USB 'full speed' (table 28.1), and a minimum of 3v, on VUSB (table 28.4).
If you can operate at 3.3v, rather than 3v, it is then 'easy', just to use this as Vusb. If however you must run at 3v, then you really need an external 3.3v source (low current) as well, or you will find yourself getting unreliable USB operation.

Best Wishes

[magestik]

Thank you for your very accurate answers, as always .

I'll try to use a 3.3v external regulator, from the 5v USB bus.

One more thing, since i was using a 20Mhz quartz @5V pic, do i need to change the quartz as well ? or just the fuses ?

[Ttelmah]

I'd keep Fosc, inside the clock figures specified. MicroChip, are a bit 'naughty' here on their data sheet. On most chips, Fosc, equals Fcpu. On these it doesn't, but they don't give a separate spec for Fosc, and Fcpu, so I treat their clock spec as applying to both.
It is only the USB PLL, that has a separate 'spec'.

Best Wishes