typedef struct
{
  __IO uint32_t              MCR;                 /*!< CAN master control register,         Address offset: 0x00          */
  __IO uint32_t              MSR;                 /*!< CAN master status register,          Address offset: 0x04          */
  __IO uint32_t              TSR;                 /*!< CAN transmit status register,        Address offset: 0x08          */
  __IO uint32_t              RF0R;                /*!< CAN receive FIFO 0 register,         Address offset: 0x0C          */
  __IO uint32_t              RF1R;                /*!< CAN receive FIFO 1 register,         Address offset: 0x10          */
  __IO uint32_t              IER;                 /*!< CAN interrupt enable register,       Address offset: 0x14          */
  __IO uint32_t              ESR;                 /*!< CAN error status register,           Address offset: 0x18          */
  __IO uint32_t              BTR;                 /*!< CAN bit timing register,             Address offset: 0x1C          */
  uint32_t                   RESERVED0[88];       /*!< Reserved, 0x020 - 0x17F                                            */
  CAN_TxMailBox_TypeDef      sTxMailBox[3];       /*!< CAN Tx MailBox,                      Address offset: 0x180 - 0x1AC */
  CAN_FIFOMailBox_TypeDef    sFIFOMailBox[2];     /*!< CAN FIFO MailBox,                    Address offset: 0x1B0 - 0x1CC */
  uint32_t                   RESERVED1[12];       /*!< Reserved, 0x1D0 - 0x1FF                                            */
  __IO uint32_t              FMR;                 /*!< CAN filter master register,          Address offset: 0x200         */
  __IO uint32_t              FM1R;                /*!< CAN filter mode register,            Address offset: 0x204         */
  uint32_t                   RESERVED2;           /*!< Reserved, 0x208                                                    */
  __IO uint32_t              FS1R;                /*!< CAN filter scale register,           Address offset: 0x20C         */
  uint32_t                   RESERVED3;           /*!< Reserved, 0x210                                                    */
  __IO uint32_t              FFA1R;               /*!< CAN filter FIFO assignment register, Address offset: 0x214         */
  uint32_t                   RESERVED4;           /*!< Reserved, 0x218                                                    */
  __IO uint32_t              FA1R;                /*!< CAN filter activation register,      Address offset: 0x21C         */
  uint32_t                   RESERVED5[8];        /*!< Reserved, 0x220-0x23F                                              */
  CAN_FilterRegister_TypeDef sFilterRegister[28]; /*!< CAN Filter Register,                 Address offset: 0x240-0x31C   */
} CAN_TypeDef;

/*!<CAN control and status registers */
/*******************  Bit definition for CAN_MCR register  ********************/
#define  CAN_MCR_INRQ                        ((uint32_t)0x00000001)        /*!<Initialization Request */
#define  CAN_MCR_SLEEP                       ((uint32_t)0x00000002)        /*!<Sleep Mode Request */
#define  CAN_MCR_TXFP                        ((uint32_t)0x00000004)        /*!<Transmit FIFO Priority */
#define  CAN_MCR_RFLM                        ((uint32_t)0x00000008)        /*!<Receive FIFO Locked Mode */
#define  CAN_MCR_NART                        ((uint32_t)0x00000010)        /*!<No Automatic Retransmission */
#define  CAN_MCR_AWUM                        ((uint32_t)0x00000020)        /*!<Automatic Wakeup Mode */
#define  CAN_MCR_ABOM                        ((uint32_t)0x00000040)        /*!<Automatic Bus-Off Management */
#define  CAN_MCR_TTCM                        ((uint32_t)0x00000080)        /*!<Time Triggered Communication Mode */
#define  CAN_MCR_RESET                       ((uint32_t)0x00008000)        /*!<bxCAN software master reset */
#define  CAN_MCR_DBF                         ((uint32_t)0x00010000)        /*!<bxCAN Debug freeze */
/*******************  Bit definition for CAN_MSR register  ********************/
#define  CAN_MSR_INAK                        ((uint32_t)0x0001)            /*!<Initialization Acknowledge */
#define  CAN_MSR_SLAK                        ((uint32_t)0x0002)            /*!<Sleep Acknowledge */
#define  CAN_MSR_ERRI                        ((uint32_t)0x0004)            /*!<Error Interrupt */
#define  CAN_MSR_WKUI                        ((uint32_t)0x0008)            /*!<Wakeup Interrupt */
#define  CAN_MSR_SLAKI                       ((uint32_t)0x0010)            /*!<Sleep Acknowledge Interrupt */
#define  CAN_MSR_TXM                         ((uint32_t)0x0100)            /*!<Transmit Mode */
#define  CAN_MSR_RXM                         ((uint32_t)0x0200)            /*!<Receive Mode */
#define  CAN_MSR_SAMP                        ((uint32_t)0x0400)            /*!<Last Sample Point */
#define  CAN_MSR_RX                          ((uint32_t)0x0800)            /*!<CAN Rx Signal */

/* Clock configuration register (RCC_CFGR) */
typedef struct {
  __REG32  SW             : 2;
  __REG32  SWS            : 2;
  __REG32  HPRE           : 4;
  __REG32  PPRE1          : 3;
  __REG32  PPRE2          : 3;
  __REG32  ADC_PRE        : 2;
  __REG32  PLLSRC         : 1;
  __REG32  PLLXTPRE       : 1;
  __REG32  PLLMUL         : 4;
  __REG32  USBPRE         : 1;
  __REG32                 : 1;
  __REG32  MCO            : 3;
  __REG32                 : 5;
} __rcc_cfgr_bits;

....
__IO_REG32_BIT(RCC_CFGR,          0x40021004,__READ_WRITE ,__rcc_cfgr_bits);

  RCC_CFGR_bit.PLLMUL = 2;