MSP430 PWM duty cycle drift demo

pwm.c

#include "msp430.h"

/*
  Simple PWM generator using Timer A
  Output P1.0, PWM period about 2 ms
  Duty cycle changes gradually from 5% to 95%

  Compiled with msp430-gcc

  aarmot 2013-01-16
*/

#define PIN		BIT0
#define	PERIOD		2000
#define	MIN_DUTY	100
#define	MAX_DUTY	1900

unsigned int duty = MIN_DUTY;

void main(void)
{
	WDTCTL = WDTPW + WDTHOLD;	// Stop watchdog
	P1DIR |= PIN;			// Output direction
	P1OUT = PIN;			// Out = 1
	TA0CCTL0 = CCIE;		// CCR0 interrupt enabled
	TA0CCR0 = duty;
	TA0CTL = TASSEL_2 + MC_2;	// SMCLK, contmode
	__bis_SR_register(LPM0_bits + GIE);	// Enter LPM0 w/ interrupt
}

__attribute__((__interrupt__(TIMER0_A0_VECTOR)))
void Timer0_A(void)
{
	P1OUT ^= PIN;			// Toggle output
	if(P1OUT & PIN) {
		duty++;
		if(duty > MAX_DUTY) duty = MIN_DUTY;
		TA0CCR0 += duty;
	} else {
		TA0CCR0 += PERIOD - duty;
	}
}