Class and Core Layers Interaction Through Callbacks

Upon reception of standard, class-specific and/or vendor requests, the Core layer can notify the Class layer about the event associated with the request via the use of class callbacks. Each Micrium class must define a class callbacks structure of type USBD_CLASS_DRV that contains function pointers. Each callback allows the class to perform a specific action if it is required. Listing - Class Callback Structure shows a generic example of class callback structure. In the listing, XXXX could be replaced with Audio, CDC, HID, MSC, PHDC or Vendor.

static USBD_CLASS_DRV USBD_XXXX_Drv = {
    USBD_XXXX_Conn,                                                           (1)
    USBD_XXXX_Disconn,                                                        (2)
    USBD_XXXX_AltSettingUpdate,                                               (3)
    USBD_XXXX_EP_StateUpdate,                                                 (4)
    USBD_XXXX_IF_Desc,                                                        (5)
    USBD_XXXX_IF_DescSizeGet,                                                 (6)
    USBD_XXXX_EP_Desc,                                                        (7)
    USBD_XXXX_EP_DescSizeGet,                                                 (8)
    USBD_XXXX_IF_Req,                                                         (9)
    USBD_XXXX_ClassReq,                                                      (10)
    USBD_XXXX_VendorReq,                                                     (11)
#if (USBD_CFG_MS_OS_DESC_EN == DEF_ENABLED)
    USBD_XXXX_MS_GetCompatID,                                                (12)
    USBD_XXXX_MS_GetExtPropertyTbl,                                          (13)
#endif
};

A class is not required to provide all the callbacks. If a class for instance does not define alternate interface settings and does not process any vendor requests, the corresponding function pointer will be a null-pointer. Listing - Class Callback Structure with Null Function Pointers presents the callback structure for that case.

static  USBD_CLASS_DRV  USBD_XXXX_Drv = {
    USBD_XXXX_Conn,
    USBD_XXXX_Disconn,
    0,
    USBD_XXXX_EP_StateUpdate,
    USBD_XXXX_IF_Desc,
    USBD_XXXX_IF_DescSizeGet,
    USBD_XXXX_EP_Desc,
    USBD_XXXX_EP_DescSizeGet,
    USBD_XXXX_IF_Req,
    USBD_XXXX_ClassReq,
    0,
#if (USBD_CFG_MS_OS_DESC_EN == DEF_ENABLED)
    0,
    0,
#endif
};

If a class is composed of one interface then one class callback structure is required. If a class is composed of several interfaces then the class may define several class callback structures. In that case, a callback structure may be linked to one or several interfaces. For instance, the Communication Device Class (CDC) is composed of one Communication Interface and one or more Data Interfaces. The Communication interface will be linked to a callback structure. The Data interfaces may be linked to another callback structure common to all Data interfaces.

The class callbacks are called by the core task when receiving a request from the host sent over control endpoints (refer to the Task Model page for more details on the core task). Table - Class Callbacks and Requests Mapping indicates which callbacks are mandatory and optional and upon reception of which request the core task calls a specific callback.