🛠 USB OTG FS sur le port micro-AB : se faire passer pour un périphérique classe CDC

Objective

The STM32F4DISCOVERY has two independent USB connectors: the one for the ST-LINK/V2-A debugger (which programs the board) and a real USB OTG FS micro-AB port wired to the STM32F407's USB 2.0 OTG FS controller (integrated PHY). This tutorial configures this second port as a CDC device — a USB serial port that always works, unlike the ST-LINK's VCP, which is not wired on stock boards.

Exact Pinout

• DM/DP OTG FS: PA11 / PA12
• VBUS detection: PA9, ID pin: PA10 (the micro-AB connector distinguishes device and host via the ID pin)
• LEDs LD7/LD8: OTG VBUS and overcurrent detection — they inform you about the port's power supply
• The F407 also embeds an HS OTG controller (high-speed PHY + ULPI): not exposed on this board, only the FS is usable here

Hardware

• 1 STM32F4DISCOVERY board + micro-USB (or micro-AB) cable to the PC, plugged into the OTG connector
• STM32CubeF4: the USB Device library (CDC class) is provided in the package, with complete examples for the board

Procedure

1. In the CubeF4 CDC example for STM32F4-Discovery, regenerate or write: 168 MHz clock, USB FS on PA11/PA12, pull-up on the DP line controlled by the USB stack.
2. The USB Device stack (STM32_USB_DEVICE_Library module) provides USBD_CDC: you implement reception in the class callback and send data with CDC_Transmit_FS.
3. Flash via the ST-LINK port (never via OTG on a blank board), then plug the cable into the OTG port: the PC enumerates a CDC serial device without drivers.

Application Skeleton

/* apres generation CDC sample CubeF4 — USB_DEVICE CDC */
USBD_HandleTypeDef husb;
USBD_Init(&husb, FS_PAD_VBUS, USB_OTG_FS_CORE_ID);
USBD_RegisterClass(&husb, USBD_CDC_CLASS);
USBD_CDC_RegisterInterface(&husb, &USBD_Interface_fops_FS);
USBD_Start(&husb);

/* Boucle principale : echo vers le terminal PC (port CDC, pas ST-Link) */
int main(void) {
  SystemClock_Config;              /* 48 MHz USB extraits de la PLL */
  /* ... USBD init ci-dessus ... */
  while (1) {
    if (rxReady) { CDC_Transmit_FS(rxData, rxLen); rxReady = 0; }
  }
}

Critical Points

• The USB clock requires exactly 48 MHz on the dedicated PLL output: with the board's 8 MHz HSE crystal, the standard 168 MHz configuration provides a correct 48 MHz — do not tweak the PLL without checking.
• The OTG port can also work as a host (host mass-storage demos exist in CubeF4): device/host detection comes from the ID pin PA10 and the type of micro-A/micro-B cable.
• MicroPython uses exactly this port (MICROPY_HW_USB_FS) for its REPL: when MicroPython is flashed, the ST-LINK port is only used for re-flashing.

Expected Verification

The PC sees a CDC serial device (named according to the firmware's USB descriptors); echo works at 115200 and higher. LED LD7 lights up when VBUS is present on the OTG port.

Common Errors

• Plugging the cable into the ST-LINK port while looking for the new CDC device: the two ports are electrically and logically separate.
• Forgetting the 48 MHz configuration for the USB PLL: enumeration fails randomly or not at all.
• Using a standard micro-USB cable on the micro-AB connector without checking ID detection: on some cables/positions, role detection takes time — try the other orientation of the connector.
• Relying on the ST-LINK's VCP as a second serial port: not wired on stock boards, only this OTG port provides a real USB serial.