How can I learn USB communication for STM32?

Learning USB communication on STM32 involves understanding USB protocols, STM32 hardware support, and software libraries. Here’s a structured approach to mastering it:

1. Understand USB Basics
Before diving into STM32, grasp core USB concepts:

USB Types: USB 2.0 (Full Speed/12 Mbps, High Speed/480 Mbps), USB 3.0, etc.

Communication Models:

• HOST (e.g., STM32 acts as a USB host to interface with keyboards, flash drives).
• DEVICE (e.g., STM32 emulates a mouse, CDC serial port, or mass storage).
• OTG (On-The-Go): Allows dual-role (host/device) functionality.

USB Classes: CDC (Serial), HID (Keyboard/Mouse), MSC (Mass Storage), DFU (Firmware Update), etc.

Resources:

• USB.org Specifications
• USB Made Simple

2. Choose the Right STM32 Hardware
Not all STM32 chips support USB! Look for:

• STM32F1/F2/F4/F7/H7 series (e.g., STM32F103C8T6 for Full Speed, STM32F429 for High Speed).
• Check the datasheet for USB FS (Full Speed) or HS (High Speed) support.
• USB PHY: Some STM32s (e.g., F4/F7) require an external PHY for USB HS.

3. Software Tools & Libraries
A. STM32CubeMX (Essential)

• Auto-generates USB initialization code with HAL/LL drivers.
• Supports USB Device, Host, and OTG modes.
• Configures USB middleware (CDC, HID, MSC, etc.).

B. Libraries/Frameworks

1. STM32 HAL/LL Libraries (Beginner-friendly):

Use USB_DEVICE or USB_HOST modules in STM32CubeIDE.

1. libopencm3 (Lightweight, no HAL overhead):

Example: USB CDC with libopencm3.

1. TinyUSB (Advanced, supports multiple MCUs):

TinyUSB for STM32.

4. Step-by-Step Implementation
Example 1: USB CDC (Virtual Serial Port)
1. Setup in STM32CubeMX:

• Select your STM32 chip.
• Enable USB (Device Mode) → CDC (Communication Device Class).
• Configure pins (DP/DM for USB FS).
• Generate code in STM32CubeIDE/Keil/IAR.

2. Code Snippet (Send/Receive Data):

c

#include "usbd_cdc.h"
USBD_HandleTypeDef hUsbDeviceFS;

void USB_CDC_Transmit(const char* data) {
  CDC_Transmit_FS((uint8_t*)data, strlen(data));
}

void CDC_Receive_Callback(uint8_t* buf, uint32_t len) {
  // Process received data (e.g., echo back)
  CDC_Transmit_FS(buf, len);
}

3. Test with a Terminal:

• Connect STM32 to a PC via USB.
• Open PuTTY/TeraTerm and select the virtual COM port.

Example 2: USB HID (Custom Device)
1. CubeMX Setup:

• Enable USB (Device Mode) → HID (Human Interface Device).
• Define a HID report descriptor (e.g., for a custom joystick).

2. Send HID Data:

c

uint8_t report[4] = {0x01, 0x02, 0x03, 0x04};
USBD_HID_SendReport(&hUsbDeviceFS, report, sizeof(report));

5. Debugging USB
Check Power: USB requires stable 3.3V (some STM32s need external 5V-to-3.3V regulator).

Pull-Up Resistor: USB FS needs a 1.5kΩ pull-up on DP (D+) pin (internal in most STM32s).

Software Issues:

• Ensure correct USB clock (e.g., 48 MHz for Full Speed).
• Verify interrupt priorities (USB interrupts should have high priority).

6. Advanced Topics

• USB OTG: Dual-role projects (e.g., STM32 as host for flash drives or as a device).
• USB Audio/MIDI: Implement real-time streaming.
• WinUSB/LibUSB: Custom PC drivers for proprietary protocols.

7. Recommended Projects

1. USB-to-UART Bridge: Replace FTDI chips with STM32 CDC.
2. USB Keyboard/Mouse: Emulate input devices.
3. Mass Storage: Read/write SD cards via USB.
4. DFU Bootloader: Update firmware over USB.

Resources
Books:

USB Complete by Jan Axelson.

Videos:

STM32 USB Tutorials by Digi-Key.

Example Code:

STM32 USB CDC Example.

Troubleshooting Tips

• PC Not Detecting Device: Check USB descriptors (use USBlyzer or Wireshark).
• Data Corruption: Verify buffer sizes and clock settings.
• High Power Consumption: Disable VBUS sensing if not needed (USB_NO_VBUS_SENSING in CubeMX).

By following this roadmap, you’ll progressively master USB on STM32—from basic serial communication to complex custom devices!