MSP430 flip clock project located at http://tinkeringetc.blogspot.com/

main.c

//*************************************************************************************************
// MSP430 Flip Clock
//
// By: Dillon Nichols
// http://tinkeringetc.blogspot.com/
//
// Created in Code Composer Studio 5.4.0
//
// Description:
// A frequency is created by the external 32768Hz crystal which causes an interrupt every minute.
// During this interrupt, the servo arm moves to increase the minute count, then quickly moves
// back to the neutral position.
//
// Based on code from http://www.msp430launchpad.com/2012/03/using-aclk-and-32khz-crystal.html
// and http://gushh.net/blog/msp430-servo/
//
// This work is licensed under a Creative Commons Attribution 3.0 Unported License.
//*************************************************************************************************

#include <msp430g2553.h>

#define MCU_CLOCK  		1000000	// 1MHz
#define PWM_FREQUENCY		46		// 46Hz

void main (void){

	// Disable watchdog timer (password and hold)
	WDTCTL	= WDTPW + WDTHOLD;

	// Set up Timer1_A for tracking seconds
	BCSCTL1 |= DIVA_3;				// ACLK/8 = 4096Hz, ACLK is sourced from 32768Hz crystal
	BCSCTL3 |= XCAP_3;				// ~12.5pF cap, setting for 32768Hz crystal
	TA1CCTL0 = CCIE;				// Capture/compare (TA1CCR0) interrupt enable
									// Counts to TA1CCR0 and triggers an interrupt and restarts at 0
	TA1CCR0 = 30720;					// 512 -> 1 sec, x60 = 30720 -> 1 min
	// TASSEL_1 - Timer1_A clock source select, 32768Hz /8 = 4096Hz
	// ID_3 - The divider for the input clock -> /8, 4096Hz /8 = 512Hz
	// MC_1 - The timer counts up to the value of TA1CCR0
	TA1CTL = TASSEL_1 + ID_3 + MC_1;

	// Set up Timer0_A for PWM
	// OUTMOD_7 - Reset/Set
		//The output is reset when the timer counts to the TA0CCR1 value.
		//It is set when the timer counts to the TA0CCR0 value.
	TA0CCTL1 = OUTMOD_7;
	// TASSEL_2 - Timer0_A clock source select, SMCLK = 1MHz
	// MC_1 - The timer repeatedly counts from zero to the value of TA0CCR0
	TA0CTL	= TASSEL_2 + MC_1;
	TA0CCR0	= ((MCU_CLOCK/PWM_FREQUENCY)-1);	// = 21738 -> PWM Period
	TA0CCR1	= 0;					// Initial PWM Duty Cycle
	
	// Connect the servo signal wire to P1.2 through a 1K ohm resistor
	P1DIR	|= BIT2;				// Direction of P1.2 set as output
	P1SEL	|= BIT2;				// P1.2 = TA0.1 output (MSP430G2553 datasheet page 43)

	// Initialize unused ports 2 and 3 to eliminate waste current consumption
	P2SEL = 0xBE;
	P3SEL = 0xBE;

	// Bit Set Status Register
	_BIS_SR(LPM3_bits + GIE);		// Enter LPM3 and recognize maskable interrupts
	// Maskable interrupts are caused by peripherals with interrupt capability
	// Low-power mode 3 (LPM3)
		// CPU is disabled
		// MCLK and SMCLK are disabled
		// DCO's DC generator is disabled

}

// Timer1_A interrupt service routine
#pragma vector=TIMER1_A0_VECTOR		// TIMER1_A0_VECTOR is found in msp430g2553.h
__interrupt void Tick (void){		// Tick is an arbitrary name

	TA0CCR1 = 1410;	// tick
	__delay_cycles(150000);	// enough time to move
	TA0CCR1 = 1260;	// neutral
	__delay_cycles(150000);	// enough time to move back

}