Power lines from USB to PIC18F4550 ?

pic18f4550 filter capacitor values

Hi
In my bootloader circuit for 18F4550 (Microchip USB bootloader) I am looking for some clarifications.

My 18F4550board works omn 5V.

1. which is the best method for interfacing +5V and Ground lines from USB connector to PIC18F4550?

The Demo board of microchip used some special circuits there with inductors.
which is the simple method?

My 18F4550 has its own power supply,so can avoid USb power if complicated.
What is the actual purpose of that Vusb(3.3V)Pin of 18F4550?

Which is the ideal method to interface USB and Local power?

Thanks a lot

picstudent

pic18f4550 3.3

the USB module inside of the 18F4550 is working on 3.3V at a very high speed. At 5V, that speed will cause a hight temp and damage the whole chip.

pic18f4550 usb bootloader circuit

well, its true that the USB module works with 3.3v and you can configure, so it generates the 3.3v internally. or you can input the 3.3v externally.. (internal is just fine, but it needs a decoupling capacitor.. of 220nF (I think so)) (also, i don't think its useful to power other 3.3v chips near...)

the USB cable also have a +5v and GND to power your circuit... remeber that it can give 50mA, (but someday I charged up to 200mA with no change at all. It was an old card...) just feed it to your circuit... remember to filter with a capacitor(10uF) or/and an inductor (a small chocke coil maybe) 'cause the 5v comes from the PC power supply with a small 32KHz noise... maybe it doesn't matter. but it depends on you...

finally, If you can, always feed your circuit form an independent power supply... it's... better...

separate usb power local power

The usb port can supply 500mA.
When you first plug a device in, it must not draw more than 100mA from the usb power supply. During enumeraton the device can request up to 500mA, and once it is enabled, it can draw that current.

pic18f4550 usb 3.3v

Connect D+ and D- from the USB and don't worry about it. You can power the PIC with 5V from the PC or your own supply and it will make no difference to the communication. D+/D- is a differential voltage, so it is essentially floating. The PIC has an internal voltage regulator for it's USB module, so it doesn't care where the PIC supply comes from.

usb minimal

Hello

Thanks for all the responses.

Still one thing to clear.

Suppose I want to use both local and USB power, whichever availble,
can I short USB VCC and Local board VCC together? and Gnds together? (through a inductor or something)

or suppose I do not want to use PC power, can I keep +5 from PC not connected to board at all?

any diode arrangement reqired to block reverse flow to PC?

Any refernce schematics on how to do this ?

Thanks
picstudent

Difficult to say. Can you propose a better description of what you are trying to do with 2 separate sources driving the same board? You can generally tie one side, like the grounds, together without any trouble. It provides a common reference between the 2 supplies.

Hello

Thanks. So i can Short the ground lines together and keep the USB +5V line not connected to board if required . D+ and D- lines are differential and the +5 and GND lines need not be connected to board for establishing communications .Right?


thanks a lot

picstudent