slave.c

#include <msp430.h>
int main(void)
{
 WDTCTL = WDTPW + WDTHOLD; // Stop watchdog timer
 // If clock sig from mstr stays low, it is not yet in SPI mode
 while (!(P1IN & BIT4));
 P1SEL = BIT1 + BIT2 + BIT4;
 P1SEL2 = BIT1 + BIT2 + BIT4;
 UCA0CTL1 = UCSWRST; // **Put state machine in reset**
 UCA0CTL0 |= UCCKPL + UCMSB + UCSYNC; // 3-pin, 8-bit SPI master
 UCA0CTL1 &= ~UCSWRST; // **Initialize USCI state machine**
 IE2 |= UCA0RXIE; // Enable USCI0 RX interrupt
 __bis_SR_register(LPM4_bits + GIE); // Enter LPM4, enable interrupts
}
// Slave RX ISR
#pragma vector=USCIAB0RX_VECTOR
__interrupt void USCI0RX_ISR (void)
{
 while (!(IFG2 & UCA0TXIFG)); // USCI_A0 TX buffer ready?
}