请先检查硬件，比如外部时钟是否起振
程序请参考：
 unsigned char i = 0;

/*--------------------------------------------------
 - Change on-chip oscillator clock to Main clock  -
 ---------------------------------------------------*/
    prc0 = 1;                               /* Protect off                                          */
    cm13 = 1;                               /* Xin Xout                                             */
    cm15 = 1;                               /* XIN-XOUT drive capacity select bit : HIGH            */
    cm05 = 0;                               /* Xin on                                               */
    cm16 = 0;                               /* Main clock = No division mode                        */
    cm17 = 0;
    cm06 = 0;                               /* CM16 and CM17 enable                                 */ 
    while (i <= 100)                                /* This setting is an example of waiting time for the   */
        i++;                         /* oscillation stabilization. Please evaluate the time  */
                                  /* for oscillation stabilization by a user.             */
    ocd2 = 0;                               /* Main clock change                                    */
    prc0 = 0;                               /* Protect on                                           */