Written by Administrator, on 27-10-2007 13:30

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PIC 18F4550 and 18F2550 are powerful microcontrollers including a full-speed USB V2.0 compliant interface. With these MCU it's very easy for the hobbyist to design USB devices with very few components.

In these pages, I'm describing how to use the CDC firmware from Microchip. It permits to emulate a serial port with a PC running Windows or Linux. It's also very easy to build HID devices.

The 18F4550 experimentation board

The following schematic is very simple, only few components are needed. It's a self powered USB device.
Please note I'm using the Olimex tiny-ICSD programmer, if you want to use another PIC ICSP programmer, you should remove D1 and D2 and change the connector

(You can click on the picture to get an high resolution picture)

This device is compatible with the microchip USB bootloader

Please note C5 is 470nF (the datasheet recommend 220nF but more is better) and should be low ESR (for example a multi-layer ceramic). Don't forget the 100nF decoupling caps.

Here is a possible implementation

Click on the picture to get a 300dpi PCB

You can look at the wire's colour of your USB cable: Black=GND, Red=+5V, Green=Data+, White=Data-

4/1/2007 : IMPORTANT ERRATA - There's a little mistake in this PCB. R5 must be connected to +5V, not to the ground. Do not connect R5 to GND, but to +5V or it will not works at all. (thanks to Direk who pointed to this problem)

The software

You can download MCHPFSUSB_Setup  on the Microchip website. It's a windows .exe :(. The firmware we will use is written in C (for the C18 Microchip compiler). This code is (royaltyes) free, but it's not a free software, so it's not possible to redistribute modified source code

Some tips for your prototypes:

The source code is in the fw/cdc/MCHPUSB.mcw directory

You should comment the following lines in usbcfg.h for a self powered usb device:

#define USE_SELF_POWER_SENSE_IO
#define USE_USB_BUS_SENSE_IO

On my schematic, LED's are connected on port B. So you should modify io_cfg.h

#define mLED_1              LATBbits.LATB0
#define mLED_2              LATBbits.LATB1
#define mLED_3              LATBbits.LATB2
#define mLED_4              LATBbits.LATB3

You can put your own code on user.c. For example, look at Exercise_Example() in user.c.

Useful functions are getsUSBUSART() (get data) and putrsUSBUSART() (put data other the serial port)

Don't forget to check mUSBUSARTIsTxTrfReady() before to send data.

They are some examples in the exercise directory. For example, the following program (to put in user.c) toggle the LED number 4 when you press the key "1" in the terminal

They are some examples in the exercice directory. For example, the following program (to put in user.c) toggle the LED number 4 when you press the key "1" in the terminal

void Exercise_03(void){
if(getsUSBUSART(input_buffer,1))
{
if(input_buffer[0] == '1')
mLED_4_Toggle(); 
}

Under windows, you must install the provided driver and you can use the hyperterminal utility (19200bd with a 20MHz crystal). Under linux, check the cdc-acm module, a new device /dev/ttyACM0 should appear.

If you don't want to use the bootloader, you must remove the vector remapping section in main.c and change the linker script (look at important.readme.txt in the sources directory)

Next part -> Application: How to build a USB thermometer

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