USB-PCDC Usage Instructions

English | Chinese

Introduction

This example demonstrates how to use the RA8 Series MCU USB PCDC module (r_usb_pcdc) on the Titan Board to enumerate the device as a USB virtual serial port (CDC), allowing serial communication with the board on a PC as a “COM port/ttyACM”.

USB PCDC Instructions

1. Overview

USB PCDC (USB Peripheral Communications Device Class) is a specific implementation of the CDC (Communications Device Class) defined in the USB protocol.

It primarily allows embedded devices to appear as a virtual serial port (VCOM) over USB, enabling the host (PC or embedded host) to interact with the USB device just like a traditional UART serial port.

Common application scenarios:

  • MCU as a USB device, recognized by the PC as a virtual serial port (COMx).

  • Embedded devices communicating with a host computer for data transfer or debugging.

  • Replacing physical RS-232/RS-485 interfaces to reduce hardware cost.

2. PCDC Protocol Features

  1. CDC Class Protocol

    • USB-defined communication device class, uses standard class drivers (no extra driver installation needed; automatically recognized on Windows/Linux/macOS).

  2. Data Transfer Methods

    • Control Endpoint (Endpoint 0): Used for device enumeration and descriptor transfer.

    • Interrupt Endpoint: Used for status and control signals (e.g., DTR, RTS).

    • Bulk Endpoint: For high-efficiency, reliable serial data transfer.

  3. Virtual Serial Port Features

    • Appears as a standard serial port (COM port or ttyUSB) on the host.

    • Supports UART configurations such as baud rate, data bits, parity, and stop bits.

3. Typical Application Architecture

  • USB Device (MCU side): Implements the PCDC protocol stack, connecting USB data to the MCU’s internal UART/application layer.

  • USB Host (PC/embedded system): Maps a virtual serial port through the OS-provided CDC driver.

  • Application Layer: Host software (PuTTY, Tera Term, serial monitor tools) can communicate directly.

4. Advantages

  • No additional driver needed: Most OSs include built-in CDC drivers.

  • Replaces traditional serial ports: Enables debugging/communication even without a physical UART.

  • Higher data rates: Full-Speed (12 Mbps) or High-Speed (480 Mbps) USB exceeds typical UART bandwidth.

  • High compatibility: Works with various host applications (LabVIEW, Python pySerial, C# serial libraries).

RA8 Series USB PCDC Module (r_usb_pcdc) Overview

The RA8 Series MCU integrates a USB PCDC (Peripheral CDC) module, designed to enumerate the MCU as a USB virtual serial port (CDC device), enabling bidirectional data communication with a PC or other USB hosts. It works together with the RA8 basic USB module (r_usb_basic), and the r_usb_pcdc class driver provides full CDC functionality.

1. Module Functionality

  • Device Mode Support: MCU acts as a USB device connected to a host (Full-Speed / High-Speed, depending on MCU specification).

  • Virtual Serial Port (CDC) Features:

    • Handles class requests: SET_LINE_CODING, GET_LINE_CODING, SET_CONTROL_LINE_STATE, SEND_BREAK.

    • Supports standard Bulk IN/OUT endpoints and an Interrupt IN endpoint.

    • Generates serial port status notifications (e.g., DTR/RTS, line state).

  • Data Transfer Management: Supports high-efficiency transfer with DMA/DTC or FIFO buffers.

  • Event Callback Mechanism: For connection, disconnection, configuration completion, data transfer completion, class request handling, and error events.

  • Hot-plug and Low Power: Supports VBUS detection, suspend/resume, and remote wakeup.

  • Application Layer Interface: Provides API for read/write buffers, send-complete notifications, and event callbacks.

2. Supported Features

  • Standard USB Enumeration: Supports Device, Configuration, and Interface Descriptors.

  • Endpoint Management: Configures Bulk IN/OUT and Interrupt IN endpoints.

  • Class Request Handling: Parses line coding and control line state according to CDC protocol.

  • Reliable Transfers: Supports CRC, NAK, STALL, and timeout handling.

  • Multiple Device & Composite Device Support: Can dynamically manage multiple CDC devices via hubs.

  • DMA Collaboration: Optimizes data throughput and reduces CPU load.

3. Module Architecture

[ Application Layer Task/Thread ]
         ↑     Callback/Event notifications (connect, configure, transfer complete)
[ r_usb_pcdc Class Driver ]
         ↕     Class request parsing, data transfer management, endpoint configuration
[ r_usb_basic Basic Driver ]
         ↕     USB device enumeration, endpoint control, SOF interrupt, DMA support
[ USB Device Controller/PHY ]
         ↕     USB D+/D- signals (FS) or ULPI/HS PHY (HS)
[ USB Cable ]  <—> Host (PC)

Key Submodules:

  1. Class Request Handler: Parses CDC class requests and callbacks the application.

  2. Endpoint Control and Buffers: Manages Bulk/Interrupt endpoints and supports DMA/FIFO.

  3. Event Management: Handles connection/disconnection, configuration, suspend/resume, transfer complete, error events.

  4. Power and Clock Control: USB PHY, VBUS detection, suspend/wake management.

4. Workflow

  1. USB Initialization: Initialize USB controller, PHY, and class driver.

  2. Device Connection Detection: Detect VBUS/ID signals to trigger enumeration.

  3. Enumeration and Configuration: Upload descriptors and configure interface/endpoints.

  4. Endpoint Data Transfer: Application exchanges data with host via Bulk IN/OUT endpoints.

  5. Class Request Handling: Process CDC control requests from host.

  6. Event Callback Notification: Notify application of connection, configuration, and transfer completion.

  7. Disconnection Handling: Stop endpoint transfers, release buffers, and handle device removal events.

Hardware Description

image-20251015150816627

Titan Board provides a USB-Device peripheral, located on the development board as shown below:

image-20250905135855760

FSP Configuration Configuration

  • Create a r_usb_pcdc stack：

image-20250814174049728

  • Configure r_usb_basic：

image-20250902114335127

  • Configure USB FS pin：

image-20250814174855736

  • Configure USB HS pin：

image-20250814175041366

PS: Note that when switching to HS mode, the P413 pin needs to be pulled high, as shown in the figure below:

image-20250902114517445

RT-Thread Settings Configuration

Enable USB PCDC。

image-20250814175140744

Compilation & Download

  • RT-Thread Studio: Download the Titan Board resource package from the RT-Thread Studio package manager, then create a new project and compile.

After compiling, connect the USB-DBG interface of the development board to the PC and download the firmware to the development board.

Running Effect

Use USB cable to connect the USB-DEV interface of Titan Board with the computer, open the serial port terminal with the serial port assistant, input 1 into the serial port terminal, and see the output in the debugging terminal of the development board.

image-20250814175936476

Enter 2 into the USB-PCDC serial terminal, and you will see the following output in the USB-PCDC serial terminal:

image-20250814175955299