prc0 = 1;                                  //打开保护
cm04 = 1;                                //选择XCIN/XCOUT
cm12 = 1;                                //连接内部电路
for( i = 0; i < 10000; i ++);      //等待时钟稳定
cm07 = 1;                               //使用附时钟
cm05 = 1;                                //关闭主时钟
prc0 = 0;                              //关闭保护