< 1. 硬件上连接晶振
2. 软件上在systemctrl.c中，改成下面的
//---------------------------------------------------------------------------
// Example: XtalOscSel:
//---------------------------------------------------------------------------
// This function switches to External CRYSTAL oscillator and turns off all other clock
// sources to minimize power consumption. This option may not be available on all
// device packages

void XtalOscSel (void)  {
     EALLOW;
     SysCtrlRegs.CLKCTL.bit.XTALOSCOFF = 0;     // Turn on XTALOSC
     SysCtrlRegs.CLKCTL.bit.XCLKINOFF = 1;      // Turn off XCLKIN
     SysCtrlRegs.CLKCTL.bit.OSCCLKSRC2SEL = 0;  // Switch to external clock
     SysCtrlRegs.CLKCTL.bit.OSCCLKSRCSEL = 1;   // Switch from INTOSC1 to INTOSC2/ext clk
     SysCtrlRegs.CLKCTL.bit.WDCLKSRCSEL = 1;    // Switch Watchdog Clk Src to external clock
     SysCtrlRegs.CLKCTL.bit.INTOSC2OFF = 1;     // Turn off INTOSC2
     SysCtrlRegs.CLKCTL.bit.INTOSC1OFF = 1;     // Turn off INTOSC1
     EDIS;

}