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PIC18F4550 USB Serial

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USB example for Microchip PIC18F4550.

Connect a USB cable directly to the pins of a pre-programmed PIC18F4550, add capacitor and crystal, and you have a virtual COMx serial port.

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Plug your PIC18F4550 into your PC using a USB cable. A virtual RS232 COM port is created, allowing your application to print RS232 messages into HyperTerminal. Allows porting of legacy code that used RS232 comms.

This project is a modified form of the CDC (Communications Class Driver) available from The forum has some discussion on this code.

Note: ensure that you read "readme (including install notes).txt" after download.

Schematic and PCB

This code runs on the PICDEM-FS board from Microchip.

This board is no longer available, but its easy to make your own circuit:

  • Place a preprogrammed PIC18F4550 on a breadboard.
  • Ensure that you are using a 20Mhz crystal.
  • Place a 470nF capacitor on port Vusb.
  • Hook the USB wires directly to the chip.
  • It should start to work straight away. Download the PICDEM-FS datasheet for the exact schematic to use with this code.


  • Start up any program which can monitor a standard serial port, such as HyperTerminal or putty.
  • Find the new COMx port that has been enumerated.
  • Set it to 115200,N,8,1.
  • The actual COM port speed doesn't matter, as its a virtual comm port.
  • You will see "[alive]" being continuously generated on virtual com port X over USB.
  • You can find the exact number of the virtual COM port by going into "Control Panel", then "System", then selecting the "Hardware" then "Device Manager", there will be a new COM port under "Ports (COM & LPT). You can change the number of the virtual comm port by going into properties, and selecting the new COM port.


11th Nov 2007 Tried it on Hi-Tech C v9.62, it worked. Had reports that it does not work on Hi-Tech C v9.51.

2nd Feb 2009 Customer got it working with Hi-Tech C v9.51, it worked after some tweaks. Added instructions to v1.10 of the download.

More C Sample Code

For more sample code, see

Do you have any enhancements to share with the community? We honour 98% of pull requests within a few days!


Connect a USB cable to a PIC18F4550, add capacitor and crystal, and you have a COMx serial port.






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