Azure RTOS USBX CDC ACM: COM port enumerates successfully, but class is never activated (_ux_device_class_cdc_acm_activate() not called)

[ssp]

Hi everyone,

I'm working on Azure RTOS USBX CDC ACM on an STM32F429ZIT6. The USB device is detected successfully by Windows, and a COM port is created, but I'm unable to transmit or receive any data.

Environment

• MCU: STM32F429ZIT6
• STM32CubeIDE: 1.19
• STM32CubeF4: 1.28.3
• Azure RTOS USBX (X-CUBE-AZRTOS-F4)

Problem

The USB device enumerates successfully:

• Device Manager detects the USB CDC device.
• A COM port is created.
•  No data transfer occurs.
•  _ux_device_class_cdc_acm_activate() is never called.
• The CDC ACM class never becomes active.

While debugging, I found that execution reaches:

UINT _ux_device_stack_interface_start(UX_SLAVE_INTERFACE *interface)

where the following assignment is made:

class_command.ux_slave_class_command_class =
    interface->ux_slave_interface_descriptor.bInterfaceClass;

Later, USBX calls:

_ux_device_class_cdc_acm_entry(&class_command);

During the UX_SLAVE_CLASS_COMMAND_QUERY request, the following check is performed:

if (command->ux_slave_class_command_class == UX_SLAVE_CLASS_CDC_ACM_CLASS)
    return UX_SUCCESS;
else
    return UX_NO_CLASS_MATCH;

However, in my USBX version:

#define UX_SLAVE_CLASS_CDC_ACM_CLASS    10

which corresponds to 0x0A (CDC Data Interface).

According to the USB descriptor, the interface passed into _ux_device_stack_interface_start() has:

bInterfaceNumber = 0
bInterfaceClass  = 0x02

Therefore, the comparison becomes:

0x02 == 0x0A

which fails, returning UX_NO_CLASS_MATCH.

As a result:

• _ux_device_class_cdc_acm_activate() is never called.
• The interface is never associated with the CDC ACM class.
• Read/write APIs never work.