.. zephyr:code-sample:: usb-cdc-acm
   :name: USB CDC-ACM
   :relevant-api: usbd_api _usb_device_core_api

   Use USB CDC-ACM driver to implement a serial port echo.

Overview

This sample app demonstrates use of a USB Communication Device Class (CDC) Abstract Control Model (ACM) driver provided by the Zephyr project. Received data from the serial port is echoed back to the same port provided by this driver. This sample can be found under :zephyr_file:`samples/subsys/usb/cdc_acm` in the Zephyr project tree.

Requirements

This project requires an USB device driver, which is available for multiple boards supported in Zephyr.

Building and Running

Reel Board

To see the console output of the app, open a serial port emulator and attach it to the USB to TTL Serial cable. Build and flash the project:

.. zephyr-app-commands::
   :zephyr-app: samples/subsys/usb/cdc_acm
   :board: reel_board
   :goals: flash
   :compact:

Running

Plug the board into a host device, for example, a PC running Linux. The board will be detected as shown by the Linux dmesg command:

usb 9-1: new full-speed USB device number 112 using uhci_hcd
usb 9-1: New USB device found, idVendor=8086, idProduct=f8a1
usb 9-1: New USB device strings: Mfr=1, Product=2, SerialNumber=3
usb 9-1: Product: CDC-ACM
usb 9-1: Manufacturer: Intel
usb 9-1: SerialNumber: 00.01
cdc_acm 9-1:1.0: ttyACM1: USB ACM device

The app prints on serial output (UART1), used for the console:

Wait for DTR

Open a serial port emulator, for example minicom and attach it to detected CDC ACM device:

minicom --device /dev/ttyACM1

The app should respond on serial output with:

DTR set, start test
Baudrate detected: 115200

And on ttyACM device, provided by zephyr USB device stack:

Send characters to the UART device
Characters read:

The characters entered in serial port emulator will be echoed back.

Troubleshooting

If the ModemManager runs on your operating system, it will try to access the CDC ACM device and maybe you can see several characters including "AT" on the terminal attached to the CDC ACM device. You can add or extend the udev rule for your board to inform ModemManager to skip the CDC ACM device. For this example, it would look like this:

ATTRS{idVendor}=="8086" ATTRS{idProduct}=="f8a1", ENV{ID_MM_DEVICE_IGNORE}="1"

You can use /lib/udev/rules.d/77-mm-usb-device-blacklist.rules as reference.