USB module of PIC microcontroller

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Introduction: Configuring the USB Module of PIC18F4550 – Step-by-Step Guide

The PIC18F4550 microcontroller stands out from other 8-bit MCUs because it features a built-in USB module, allowing direct communication with a PC without external USB converters. From USB keyboards, data loggers, and custom HID devices to CDC virtual COM ports, this microcontroller gives you the power to design real USB-based embedded applications.

USB module of PIC microcontroller

In this post, we’ll explore how to configure the USB module in PIC18F4550, understand its key registers, and create a simple USB communication setup.

What Makes USB in PIC18F4550 Special?

The PIC18F4550 features a Full-Speed USB 2.0-compliant interface (up to 12 Mbps) and includes:

• Integrated USB transceiver
• HID (Human Interface Device) and CDC (Communication Device Class) support
• On-chip voltage regulator (VUSB) for internal 3.3V USB operation
• Plug-and-play connectivity with PCs (no extra FTDI chip required!)

USB Pins in PIC18F4550

Note: Connect a 470nF capacitor between VUSB (pin 18) and ground for stable USB voltage regulation.

Step 1: Hardware Setup

Connect the PIC18F4550 to your PC via a USB Type-B connector.

Typical circuit connections:

• D+ (RC5) → USB D+
• D– (RC4) → USB D–
• VBUS (RA1) → USB 5V detection
• VUSB (Pin 18) → 470nF capacitor to GND
• 20 MHz crystal → OSC1 & OSC2 (pins 13, 14)
• 5V regulated power supply

Tip: Always use a 20 MHz crystal for accurate USB timing since USB full-speed requires a precise 48 MHz clock internally.

Step 2: Configuring USB in Firmware

We’ll use MPLAB X IDE and XC8 with Microchip USB Framework or MCC (MPLAB Code Configurator).

Here’s a simplified USB initialization code snippet: USB module of PIC microcontroller (PIC18F4550)

#include <xc.h>
#include "usb.h"
#include "usb_device_hid.h"

void main(void)
{
    SYSTEM_Initialize(SYSTEM_STATE_USB_START);
    USBDeviceInit();        // Initialize USB module
    USBDeviceAttach();      // Enable USB connection

    while(1)
    {
        USBDeviceTasks();   // Manage USB tasks

        if(USBGetDeviceState() == CONFIGURED_STATE)
        {
            // Your USB communication logic here
        }
    }
}

#include <xc.h>
#include "usb.h"
#include "usb_device_hid.h"

void main(void)
{
    SYSTEM_Initialize(SYSTEM_STATE_USB_START);
    USBDeviceInit();        // Initialize USB module
    USBDeviceAttach();      // Enable USB connection

    while(1)
    {
        USBDeviceTasks();   // Manage USB tasks

        if(USBGetDeviceState() == CONFIGURED_STATE)
        {
            // Your USB communication logic here
        }
    }
}

Step 3: Using Microchip’s USB Stack

Microchip provides a USB Device Library supporting:

• HID (Mouse, Keyboard, Custom Data Transfer)
• CDC (Virtual COM Port)
• Mass Storage (Flash Drive Interface)

You can import sample firmware from MPLAB Harmony or Legacy USB Framework examples and modify them for your projects.

Step 4: Circuit Diagram

A circuit diagram typically includes:

• 5V USB VBUS input
• PIC18F4550 connected to USB Type-B connector
• 470nF capacitor at VUSB
• 20 MHz crystal

Step 5: Testing

Once you build and flash your code, connect the board to a PC.
If successful, Windows will detect a new USB device (like “Microchip Custom HID Device” or “Virtual COM Port”). You can then test communication using terminal software or custom PC applications.

Real-World Applications

• Custom HID/CDC Devices
• USB Data Logger
• PC Interface for Sensors
• USB Debug Interface

https://www.microchip.com/en-us/products/interface-and-connectivity/usb

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