MSP430FR6972: 时钟源选择内部的dco，mclk为什么不能设置为24mhz

#include "driverlib.h"

uint32_t clockValue;

void main(void)
{
    //Stop WDT
    WDT_A_hold(WDT_A_BASE);
    //Set DCO frequency to 16MHz
    CS_setDCOFreq(CS_DCORSEL_1, CS_DCOFSEL_6);

    CS_initClockSignal(CS_SMCLK,CS_DCOCLK_SELECT,CS_CLOCK_DIVIDER_1);
    CS_initClockSignal(CS_MCLK,CS_DCOCLK_SELECT,CS_CLOCK_DIVIDER_1);

    //Set wait state to 1
    FRAMCtl_configureWaitStateControl(FRAMCTL_ACCESS_TIME_CYCLES_1); 

    //Set P2.0 as Output Low (for setup as ACLK).
    /*

    * Select Port 2
    * Set Pin 0 to output Low.
    */
    GPIO_setOutputLowOnPin(
    	GPIO_PORT_P3,
    	GPIO_PIN4
    );

    //Set P2.0 as Ternary Module Function Output.
    /*

    * Select Port 2
    * Set Pin 0 to output Ternary Module Function, (ACLK).
    */
    GPIO_setAsPeripheralModuleFunctionOutputPin(
    	GPIO_PORT_P3,
    	GPIO_PIN4,
    	GPIO_TERNARY_MODULE_FUNCTION
    );

    /*
     * Disable the GPIO power-on default high-impedance mode to activate
     * previously configured port settings
     */
    PMM_unlockLPM5();

    while(1);

}