msp430: turning on LED1 when S2 is pressed

code8.2.c

// Source: Programmable Microcontrollers with Applications, p.115
// Turning on the red LED when the push button is pressed
// MSP430 LaunchPad Rev.1.5

#include <msp430g2553.h>

#define LED BIT0                        // P1.0
#define BUTTON BIT3                     // P1.3

void main(void){
  WDTCTL = WDTPW|WDTHOLD;               // stop WDT
  
  P1DIR = LED;                          // P1.0 set as output
  P1REN = BUTTON;                       // internal resistor connected to S2 enabled, p.112
  P1OUT = BUTTON;                       // internal resistor set as pull-up
  
  /* check input from the push button */
  while(1){                       
    if((P1IN & BUTTON) == 0x00)         // active low input, p.113
      P1OUT |= LED;                     // turn on LED
    else
      P1OUT &= ~LED;                    // turn off LED
  }
}