CDC ACM USB-to-Serial Device Example

This example demonstrates how to implement a CDC ACM (Communication Device Class - Abstract Control Model) device using the CherryUSB stack, which emulates a USB interface as a serial port device. The device can be recognized by the host as a virtual serial port (COM port) and supports bidirectional data transmission.

Features

CDC ACM Device Class Implementation: Complies with USB CDC ACM standard, supports virtual serial port communication
Data Transfer Functionality: Supports bidirectional data transfer between USB and UART
DTR (Data Terminal Ready) Control: Supports DTR signal detection and control
High-Speed Transfer: Supports high-speed USB transfer with maximum packet length up to 512 bytes (high-speed mode)
Automatic Data Transmission: Periodically sends test data to demonstrate data transfer functionality
Real-Time Operating System Integration: Based on RT-Thread RTOS implementation
CherryUSB Stack: Uses lightweight, high-performance CherryUSB protocol stack

Hardware Connection

Supported Development Boards

SF32LB52_LCD_N16R8: LCD development board based on SF32LB52 chip

USB Connection

Connect the development board’s USB interface to PC using a USB cable
Ensure USB interface power supply is normal
Supports USB 2.0 full-speed mode and high-speed mode (depending on configuration)

Pin Configuration

This example uses the internal USB controller, mainly involving the following configurations:

USB VID: 0x38f4 (SiFli Technologies)
USB PID: 0xFFFF
Endpoint Configuration:
- CDC data input endpoint: 0x85
- CDC data output endpoint: 0x02
- CDC interrupt endpoint: 0x86

Compilation and Usage

1 Compilation

Switch to the example project directory and run the scons command to execute compilation:

scons --board=sf32lb52-lcd_n16r8 -j8

Run build_sf32lb52-lcd_n16r8_hcpu\uart_download.bat and select the port as prompted to download:

build_sf32lb52-lcd_n16r8_hcpu\uart_download.bat

Uart Download

please input the serial port num:5

2 Usage

2.1 Running the Program

Download the compiled firmware to the development board
Reset or restart the development board
After program startup, it will print: “cherryusb device cdc acm demo!”

2.2 Device Recognition

Connect the development board to PC using a USB cable
PC will recognize a new serial port device (COM port)
Device information:
- Manufacturer: CherryUSB
- Product Name: CherryUSB CDC DEMO
- Serial Number: 2022123456

2.3 Data Transfer Test

Automatic Data Transmission:
- Program automatically sends 2048 bytes of test data every second (when DTR is enabled)
- Data content: First 10 bytes are “1234567890”, followed by letter ‘a’ padding
Serial Tool Connection:
- Use serial debugging tool to connect to the virtual COM port
- Any baud rate (CDC ACM does not depend on baud rate settings)
- 8 data bits, 1 stop bit, no parity
Data Reception:
- Can receive data sent from PC serial tool
- Received data length will be output through USB LOG

2.4 DTR Control

Device supports DTR (Data Terminal Ready) signal control
Device only automatically sends data when DTR signal is enabled
Most serial tools automatically enable DTR when connected

Technical Details

Code Structure

main.c: Main program entry, initializes CDC ACM device and starts data transmission loop
cdc_acm_template.c: CDC ACM device implementation, including USB descriptors, endpoint configuration, and data processing
usb_config.h: CherryUSB configuration file, defines various USB parameters and options

Configuration Parameters

Buffer Size: 2048 bytes (read/write buffers)
Maximum Packet Length: 64 bytes (full-speed) / 512 bytes (high-speed)
USB Configuration: Bus-powered, maximum power consumption 100mA
Endpoint Type: Bulk transfer endpoints + interrupt endpoint

CherryUSB Features

Supports advanced descriptor registration API
Supports USB 2.0 device qualifier descriptor
Supports Zero Length Packet (ZLP) handling
Supports endpoint status management and flow control

Troubleshooting

Common Issues

Device Cannot Be Recognized
- Check USB cable and connection status
- Verify VID/PID configuration (0x38f4/0xFFFF)
- Check Windows Device Manager for unknown devices
- Try reconnecting USB connection
Virtual Serial Port Cannot Be Used
- Confirm device is correctly recognized as CDC device
- Check if correct CDC ACM driver is installed
- Windows 10/11 usually installs driver automatically
Data Transfer Abnormal
- Check DTR signal status (view using serial tool)
- Confirm serial tool is correctly connected and configured
- Check USB log output to verify data transmission/reception
Compilation or Download Failure
- Confirm SiFli SDK environment is configured correctly
- Check toolchain path settings
- Verify target board selection is correct

Development Guide

Custom Development

For custom development based on this example, you can modify the following:

USB Descriptor Customization:
- Modify VID/PID (requires legitimate USB ID)
- Change device string descriptors
- Adjust endpoint configuration
Data Processing Logic:
- Modify usbd_cdc_acm_bulk_out() function to handle received data
- Modify cdc_acm_data_send_with_dtr_test() function to customize sent data
- Add data format conversion or protocol processing
Configuration Parameter Adjustment:
- Modify buffer size to fit application requirements
- Adjust transmission frequency and data volume
- Configure USB transfer mode (full-speed/high-speed)

Reference Documentation

CDC ACM Device Class: https://cherryusb.readthedocs.io/zh_CN/latest/quick_start/cdc_acm.html
CherryUSB Documentation: https://cherryusb.readthedocs.io/
CherryUSB Official Repository: https://github.com/cherry-embedded/CherryUSB
RT-Thread Official Website: https://www.rt-thread.org/document/site/#/rt-thread-version/rt-thread-standard/README

Update Log

Version	Date	Release Notes
1.0.0	11/2025	Enhanced feature description and usage instructions, added technical details
0.0.1	9/2025	Initial version