msp430g2231 - use external square wave from ATTiny @ 12 MHZ as LFXT1 clock

runfast.c

//***********************************************************
// runfast.c - Using 12MHZ external square wave clock as LFXTCLK
//             source MCLK/SMCLK from 12MHZ wave and blink led
//             at 60Hz
//
// Use an external clock ( I'm using an attiny running at 12.00 MHZ
// with the CLKOUT fuse enabled ) as the MCLK/SMCLK for the MSP430.
// I've been told you could also use an external oscillator like an
// ECS-100A-160. I ordered some of these to verify.
//
// LFXT1 oscillator logic level square wave input frequency, LF mode
//
// XIN - external clock in from attiny CLKOUT pin ( set avr fuse to get clock )
// XOUT - NC
// P1.0 - led on until OSCFault cleared
// P1.4 - SMCLK frequency, measuer with multimeter/oscope
// P1.6 - 60Hz led flash, measure with multimeter/oscope
//
//***********************************************************

#include <msp430g2231.h>

void main(void) {
    WDTCTL = WDTPW + WDTHOLD;	// Stop watchdog timer

    P1DIR = BIT0 | BIT6 | BIT4;	// P1.0 and P1.6 output, P1.4 SMCLK frequency
    P1SEL = BIT4;               // P1.4 output SMCLK on P1.4, measure with multi meter
    P1OUT = BIT0;              	// red LED on until OSCFault is cleared

    DCOCTL = 0x00;

    BCSCTL2 = SELM_3 + DIVM_0 + SELS + DIVS_0;	// MCLK Source Select 3: LFXTCLK
    						// MCLK/1 SMCLK/1 SMCLK LFXTCLK
    BCSCTL1 |= XTS;				// XTS
    BCSCTL3 = LFXT1S_3 | XCAP_0;		// LFXT1 = 12MHZ crystal, No Capacitor

    // not sure if this is needed
    while(IFG1 & OFIFG) {
        IFG1 &= ~OFIFG;               // Clear OSCFault flag
        _delay_cycles(10000);  	      // delay for flag and visibility
    }

    P1OUT = 0;			      // red LED off

    __bis_SR_register(SCG0);          // Stop DCO, we just use the LFXT1 which is
    				      // actually running at 12MHZ
	
    while(1) {
        P1OUT ^= BIT6; 		      	   // green LED on, measure this, it should be 60Hz
        _delay_cycles((12000000/60/2)-35); // 12MHZ, 60/2, 35 overhead clocks
    }
}