Main USB Device Classes

Here are the big categories you’ll actually see in robotics / embedded:

1. CDC (Communication Device Class) — USB Serial

• Appears as: /dev/ttyUSB0, /dev/ttyACM0
• Emulates UART — behaves like a byte stream
• You read/write bytes like: write(fd, "hello", 5);
• Examples: Arduino, GPS modules, microcontrollers

This is stream-based communication.

2. HID (Human Interface Device) — Fixed Packet Reports

• Appears as: /dev/hidraw0
• No driver needed
• Uses fixed-size packets (called reports) — no concept of a byte stream
• Example: send exactly 64 bytes, receive exactly 64 bytes
• Examples: keyboard, mouse, gamepad, custom devices

This is packet-based communication.

3. Mass Storage (MSC)

• Appears as: /dev/sda
• Uses block storage protocol
• Examples: USB sticks, SD card readers, dev boards (drag-and-drop firmware)

4. UVC (USB Video Class)

• Appears as: /dev/video*
• Standardized video streaming
• Examples: webcams, depth cameras (sometimes)

5. USB Audio Class

• Appears in ALSA / PulseAudio
• Used for microphones and speakers

6. USB Networking (CDC ECM / RNDIS)

• Creates a network interface: usb0
• Examples: Raspberry Pi gadget mode, Jetson USB networking

7. Vendor-Specific (Custom Class)

• No standard driver
• Accessed via libusb or a custom kernel driver
• Examples: robotics sensors, firmware update tools, proprietary devices

8. DFU (Device Firmware Upgrade)

• Used for flashing microcontrollers
• Examples: STM32 DFU, some bootloaders

9. Smart Card / Security

• Used for authentication tokens
• Not common in robotics

10. Wireless / Bluetooth over USB

• USB Bluetooth adapters, Wi-Fi dongles

Composite Devices

A single USB device can expose multiple classes at once.

Example — a robot controller might appear as:

• HID (control commands)
• CDC (debug logs)
• Mass storage (firmware update)

Where libusb Fits

libusb is a low-level USB library that lets you talk to USB devices directly. It bypasses serial drivers, HID drivers, and kernel abstractions.

• Works with any class
• Especially useful for vendor-specific devices and bypassing existing drivers
• If a kernel driver already owns the device (e.g., HID, UVC), you may need to detach it first

“USB HID via libusb” = talking directly to a HID device at the USB level, not through /dev/ttyUSB*.

In Robotics: What You’ll Commonly See

Simple Mental Model

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