USB support features of PiCs...

[ScaleRobotics]

Thanks for your answers and care, i ve read all
lets assume that we'r using Vdd, Vpp, Gnd. D+ and D- from usb port. do we still need a bootloader? because as i know bootlader is like a manual for writing program to pic. thus we can do it with using our computer via usb.

Yes, we still need a bootloader to be loaded into some of the code space of the chip. Otherwise, there will not be anthing telling d+ and d- how to communicate, which registers to use, what memory to use, where to write on the chip, how to listen to the PC, etc, etc, etc. Even the PicKit is pre-programmed with code, so it can communicate and write to the chips.

It's kind of like this: Take your PC, then pull out the bios chip (can you even do that anymore?). Then try to start it up. Is it running? You still have a hard drive plugged in. All the hardware is there, but it doesn't work because nothing is telling them what to do.

Just what is it you are trying to do?