USB Request Block structure (USBH_URB)

USB Request Block (USBH_URB) structure represents communication structure between USB Host core and USB Host driver.

typedef volatile struct {               /* USB Request Block (URB)            */
  U8          *ptrDataBuffer;           /* Pointer to Data Buffer             */
  U8          *ptrCurrentDataBuffer;    /* Pointer to Current pos in Data Buff*/
  U32          DataLength;              /* Data Length                        */
  U32          DataTransferred;         /* Data Transferred                   */
  union {
    U32        Parameters;              /* Transfer Parameters                */
    struct {
      U32      PacketType        :  4;  /* Packet Type                        */
      U32      ToggleBit         :  1;  /* Toggle Bit Value                   */
      U32      ToggleForce       :  1;  /* Toggle Bit Forced (if cleared
                                           ToggleBit value is ignored)        */
    };
  };
  union {
    U32        Status;                  /* Status                             */
    struct {
      U32      Submitted         :  1;  /* URB Submit Status                  */
      U32      InProgress        :  1;  /* URB Processing In Progress Status  */
      U32      Cancelled         :  1;  /* URB Cancel Status                  */
      U32      Completed         :  1;  /* URB Completition Status            */
      U32      Timeout           :  1;  /* URB Timeout Status                 */
      U32      ResponsePacketType:  4;  /* Response Packet Type               */
      U32      Reserved          :  7;  /* Reserved bits to reposition other  */
      U32      Error             :  8;  /* URB Error        Status            */
    };
  };
  U32          NAKRetries;              /* Number of NAK retries              */
  U32          TimeoutCount;            /* Transaction Timeout Counter        */
  void       (*CompletedCallback)(void);/* URB Completition Callback Function */
} USBH_URB;

Explanation of USBH_URB structure elements:

• ptrDataBuffer represents a pointer to data for the transfer.
• ptrCurrentDataBuffer represents a pointer to current data for the transfer. It is used for restarting transmissions due to errors.
• DataLength represents number of bytes to transfer.
• DataTransferred represents number of bytes actually transferred.
• Parameters represents various transfer parameters.
  • PacketType represents packet type for the transfer (USBH_PACKET_xxxx).
  • ToggleBit represents starting toggle bit value for the first packet of the transfer.
  • ToggleForce determines if toggle bit value from ToggleBit is forced as a new toggle value for next transfer. For example this bit should be __FALSE for bulk transfers except first as toggle bit is alternated in packets during bulk transfers.
• Status represents various transfer status information.
  • Submitted represents submit condition. When application sets Submitted to __TRUE the transfer will be executed as soon as possible.
  • InProgress represents execution status of the transfer. If it is __TRUE means that transfer is in progress and currently being executed.
  • Cancelled represents cancelled condition. If it is __TRUE means that transfer was cancelled due to some error condition.
  • Completed represents completion condition. If it is __TRUE means that transfer has completed (it can also complete with errors, in that case transfer should be repeated).
  • Timeout represents timeout condition. If it is __TRUE means that transfer has not finished and did not complete due to the timeout condition.
  • ResponsePacketType represents responding packet type. For example if data was transmitted to a device response packet could be USBH_PACKET_ACK, USBH_PACKET_NAK or USBH_PACKET_STALL (for full/low speed transfer).
  • Reserved bits are used only for padding but can in future be used for some additional functionality.
  • Error represents error that happened during the transfer.
• NAKRetries represent number of NAKs that transfer will wait for before terminating.
• TimeoutCount represent number of milliseconds that can expire between request and response on the bus during this transfer before transfer will be terminated.
• CompletedCallback represent pointer to a function which will be called when transfer is completed (even when there were errors during the transfer).