CCS/MSP430FR2355: CCS/MSP430FR2355

 In the given program the clock is taken from the external crystal where P2.6 and P2.7 assigned as XIN and XOUT pins of the crystal respectively.So according to me the frequency is decided by the crystal then how TBSSEL_x of the  TB0CTL register are controlling the frequency of the the clock.

#include <msp430.h>

int main(void)
{
    WDTCTL = WDTPW | WDTHOLD;                         // Stop WDT

    // Configure GPIO
    P1DIR |= BIT6 | BIT7;                             // P1.6 and P1.7 output
    P1SEL1 |= BIT6 | BIT7;                            // P1.6 and P1.7 options select
    P2SEL1 |= BIT6 | BIT7;                            // P2.6~P2.7: crystal pins
    do
    {
        CSCTL7 &= ~(XT1OFFG | DCOFFG);                // Clear XT1 and DCO fault flag
        SFRIFG1 &= ~OFIFG;
    }while (SFRIFG1 & OFIFG);                         // Test oscillator fault flag
    // Disable the GPIO power-on default high-impedance mode to activate
    // previously configured port settings
    PM5CTL0 &= ~LOCKLPM5;

    // Setup Timer0_B
    TB0CCR0 = 100-1;                                  // PWM Period
    TB0CCTL1 = OUTMOD_7;                              // CCR1 reset/set
    TB0CCR1 = 75;                                     // CCR1 PWM duty cycle
    TB0CCTL2 = OUTMOD_7;                              // CCR2 reset/set
    TB0CCR2 = 25;                                     // CCR2 PWM duty cycle
    TB0CTL = TBSSEL_1 | MC_1 | TBCLR;                 // ACLK, up mode, clear TBR

    __bis_SR_register(LPM3_bits);                     // Enter LPM3
    __no_operation();                                 // For debug
}