MSP430FR5969: Mclk set

Hi,

I am trying to use MCLK at full frequency but my program does not respond well , i am using an external crystal oscillator of 12Mhz, if i set my MCLK prescalar to 1 it works perfectly which is (6 MHz) but for 12Mhz the program does not respond well

PJSEL0 |= BIT4 | BIT5 | BIT6 | BIT7; // For HFXT

/* clock configuration */

CSCTL0 = 0xA500;
CSCTL2 = 0x0255; /* Aclk is VLOclk, SMclk & Mclk is HFXT clock */
CSCTL3 = 0x0000; /* crystal frequency is not divided, Mclk not divided */
CSCTL4 = 0x0801;
do
{
CSCTL5 &= ~(LFXTOFFG | HFXTOFFG); /* Clear XT1 and XT2 fault flag */
SFRIFG1 &= ~OFIFG;
}
while (SFRIFG1&OFIFG); /* Test oscillator fault flag */
CSCTL0_H = 0;

Thanks and Regards,

Sasikumar E