Question about the power needed for USB in 18f4550

[abdalla]

Hi,

in 18f4550 DataSheet in USB section

I`m so confused ....
see the following paragraph -> P167

The USB specification requires 3.3V operation for
communications; however, the rest of the chip may be
running at a higher voltage. Thus, the transceiver is
supplied power from a separate source, VUSB.


HOW USB 3.3v.....I THINK ITS 5V :?:

THX

 

[segmex]

18f4550 datasheet

I DO HAVE THIS QUESTION /

 

[M!k]

18f4550,datasheet

Yes, the supply voltage from the USB port is around 5V (4.4V ... 5.25V). But the tranceivers for the signalling lines need 3.3V (see also USB specs). In the PIC18F4550 this voltage can be regulated internally (VREGEN enabled, capacitor at VUSB pin) or fed externally (VREGEN disabled, 3.3V at VUSB pin). Read chapter 17.2.2.8 "Internal Regulator" of the PIC datasheet.

 

[abdalla]

18f4550 datasheet

thx so much,

But i also have question

in datasheet page 169 NOTE: VUSB must be <VDD in all time
how that can be if my VDD is 3.3v

 

[blueroomelectronics]

18f4550 vregen

On page 362 it shows although the 18F4550 can run at as low as 2V it only runs at 4MHz
VDD of less than 4.2V means you don't get the USB hardware which must run at 96MHz