Getting Started with MSP430: C Programming

Next is the declaration of functions and variables void Init_Sys(void), which is system initialization. System initialization is an overall concept, which generally includes the initialization of all peripheral modules. You can write the sub-functions of peripheral module initialization into Init_Sys(), or you can write the initialization of each module separately. However, for the simplicity of structure, it is best to initialize other modules (including some interrupt initialization) here after writing the system clock initialization.
void Init_Sys()
{
   unsigned int i;
   BCSCTL1&=~XT2OFF;                //Turn on XT2 oscillatordo
   {
   IFG1
   &= ~OFIFG;              //Clear oscillator failure flagfor
   (i = 0xFF; i > 0; i--);   // Delay, wait for XT2 to start oscillating
}
while ((IFG1 & OFIFG) != 0);    //Judge whether XT2 has started oscillatingBCSCTL2
=SELM_2+SELS;           //Select MCLK and SMCLK for XT2
 
//The following initializes various modules, interrupts, peripherals, etc
........................................
_EINT(); //Turn on global interrupt control
}
This involves the clock issue. Usually we choose XT2 as 8M crystal oscillator, that is, the system main clock MCLK is 8M, and the CPU executes commands based on this clock; but other peripheral modules can select other clocks in the corresponding control registers, ACLK; when you have low speed requirements and a large timing time interval, you can choose ACLK, such as setting it in the timer Timea initialization.
Main program:
void main( void )
{
   WDTCTL = WDTPW + WDTHOLD; //Turn off watchdog
  InitSys();              //Initialize
//Other function functions in your own task
 . . .
   . . . . . . . . . . . . . . . . . . . while(1);
}
After the main program, I will talk about the interrupt function. Interrupt is an indispensable part of your microcontroller task, and it can also be said to be the soul (exaggeration?).
 
Take an example of a timer interrupt:
//initialization
void Init_Timer_A(void)
{
    TACTL = TASSEL0 + TACLR;              // ACLK, clear TAR
        CCTL0 = CCIE;                         // CCR0 interrupt enable
        CCR0=32768;                          //timing 1s
        TACTL|=MC0;                          //increment count mode
   }
//    interrupt service
#pragma vector=TIMERA0_VECTOR
__interrupt void TimerA0()
{
//the task you request the interrupt to execute
}