Beginner question on what constitutes a device USB interface

Thanks both for the information. It was very helpful. I am currently using STM USB library as a way to learn how USB works.

Based on STM libraries, this is how a set of C codes functions that needs to be implemented by either a DFU, HID ... See codes below. USBD_ClassTypeDef is part of USBD_HandleTypeDef.

Would USBD_HandleTypeDef be considered an interface or a configuration? I guess I am a bit confused if a device supports more than one interfaces, which in the codes that needed to be implemented?

typedef struct _Device_cb
{ uint8_t (*Init) (struct _USBD_HandleTypeDef *pdev , uint8_t cfgidx); uint8_t (*DeInit) (struct _USBD_HandleTypeDef *pdev , uint8_t cfgidx); /* Control Endpoints*/ uint8_t (*Setup) (struct _USBD_HandleTypeDef *pdev , USBD_SetupReqTypedef *req); uint8_t (*EP0_TxSent) (struct _USBD_HandleTypeDef *pdev ); uint8_t (*EP0_RxReady) (struct _USBD_HandleTypeDef *pdev ); /* Class Specific Endpoints*/ uint8_t (*DataIn) (struct _USBD_HandleTypeDef *pdev , uint8_t epnum); uint8_t (*DataOut) (struct _USBD_HandleTypeDef *pdev , uint8_t epnum); uint8_t (*SOF) (struct _USBD_HandleTypeDef *pdev); uint8_t (*IsoINIncomplete) (struct _USBD_HandleTypeDef *pdev , uint8_t epnum); uint8_t (*IsoOUTIncomplete) (struct _USBD_HandleTypeDef *pdev , uint8_t epnum);

uint8_t *(*GetHSConfigDescriptor)(uint16_t *length); uint8_t *(*GetFSConfigDescriptor)(uint16_t *length); uint8_t *(*GetOtherSpeedConfigDescriptor)(uint16_t *length); uint8_t *(*GetDeviceQualifierDescriptor)(uint16_t *length);
#if (USBD_SUPPORT_USER_STRING == 1) uint8_t *(*GetUsrStrDescriptor)(struct _USBD_HandleTypeDef *pdev ,uint8_t index, uint16_t *length);
#endif

} USBD_ClassTypeDef;

typedef struct _USBD_HandleTypeDef
{ uint8_t id; uint32_t dev_config; uint32_t dev_default_config; uint32_t dev_config_status; USBD_SpeedTypeDef dev_speed; USBD_EndpointTypeDef ep_in[15]; USBD_EndpointTypeDef ep_out[15]; uint32_t ep0_state; uint32_t ep0_data_len; uint8_t dev_state; uint8_t dev_old_state; uint8_t dev_address; uint8_t dev_connection_status; uint8_t dev_test_mode; uint32_t dev_remote_wakeup;

USBD_SetupReqTypedef request; USBD_DescriptorsTypeDef *pDesc; USBD_ClassTypeDef *pClass; void *pClassData; void *pUserData; void *pData;
} USBD_HandleTypeDef;

Hi John,

Thanks for the link. I actually was reading the same document and reading the codes but it's taking awhile to digest all the information.

I am trying to use the STM32F4 in device mode with USB to interface to the PC. The STM32F4 will then be controlled from the PC to talk to other devices using either I2C or general purpose I/O. I am thinking of using HID interface because it seems like the driver is already part of the OS so a little less work. Currently I am using the STM USART to talk to the PC but I am trying to switch to USB because it's a better solution.

I was able to get some of the USB example such as the DFU, HID mouse ... to work but some of the concepts are still a little bit muddy.

I don't know much about USB, but Milorad Cvjetkovic knows USB very well.
Back to your last question,
-> Would USBD_HandleTypeDef be considered an interface or a configuration?

I believe USBD_HandleTypeDef represents an USB device.

/**
* @brief  USBD_SetClassConfig
*        Configure device and start the interface
* @param  pdev: device instance
* @param  cfgidx: configuration index
* @retval status
*/

USBD_StatusTypeDef USBD_SetClassConfig(USBD_HandleTypeDef  *pdev, uint8_t cfgidx)
{
  USBD_StatusTypeDef   ret = USBD_FAIL;

  if(pdev->pClass != NULL)
  {
    /* Set configuration  and Start the Class*/
    if(pdev->pClass->Init(pdev, cfgidx) == 0)
    {
      ret = USBD_OK;
    }
  }
  return ret;
}

/**
* @brief  USBD_ClrClassConfig
*         Clear current configuration
* @param  pdev: device instance
* @param  cfgidx: configuration index
* @retval status: USBD_StatusTypeDef
*/
USBD_StatusTypeDef USBD_ClrClassConfig(USBD_HandleTypeDef  *pdev, uint8_t cfgidx)
{
  /* Clear configuration  and Deinitialize the Class process*/
  pdev->pClass->DeInit(pdev, cfgidx);
  return USBD_OK;
}