msp430: toggling LED2 when S2 is pressed

code8.4.c

// Source: Programmable Microcontrollers with Applications, p.117
// Toggling the green LED when the push button is pressed
// MSP430 LaunchPad Rev.1.5

#include <msp430g2553.h>

#define LED BIT6                        //P1.6 LED2
#define BUTTON BIT3                     //P1.3 S2

void delay_ms(int);                     // declare function delay_ms

void main(void){
  WDTCTL = WDTPW|WDTHOLD;               // stop WDT
  
  P1DIR = LED;                          // P1.0 set as output
  P1REN = BUTTON;                       // internal resistor connected to S2 enabled
  P1OUT = BUTTON;                       // internal resistor set as pull-up
  
  /* check input from the push button */
  while(1){
    if((P1IN & BUTTON) == 0x00){        // check if S2 (active low) is pressed
      delay_ms(5);                      // wait 5ms
      if((P1IN & BUTTON) == 0x00){      // if S2 still pressed ..
        P1OUT ^= LED;                   // .. toggle LED2
        while((P1IN & BUTTON) == 0x00); // wait for the release of S2
      }
    }
  }
}

void delay_ms(int a){                   // define function delay_ms
  while(a != 0){                        // while variable a is different from 0
    __delay_cycles(1000);               // 1000 cycles at 1MHz (SMCLK) gives 1ms
    a--;                                // decrement variable a
  }
}