//******************************************************************************
//   MSP430F21x2 Demo - USCI_A0, SPI 3-Wire Slave Data Echo
//
//   Description: SPI slave talks to SPI master using 3-wire mode. Data received
//   from master is echoed back.  USCI RX ISR is used to handle communication,
//   CPU normally in LPM4.  Prior to initial data exchange, master pulses
//   slaves RST for complete reset.
//   ACLK = n/a, MCLK = SMCLK = DCO ~1.2MHz
//
//   Use with SPI Master Incremented Data code example.  If the slave is in
//   debug mode, the reset signal from the master will conflict with slave's
//   JTAG; to work around, use IAR's "Release JTAG on Go" on slave device.  If
//   breakpoints are set in slave RX ISR, master must stopped also to avoid
//   overrunning slave RXBUF.
//
//                MSP430F21x2
//             -----------------
//         /|\|              XIN|-
//          | |                 |
//          | |             XOUT|-
// Master---+-|RST              |
//            |             P3.5|<- Data Out (UCA0SOMI)
//            |                 |
//            |             P3.4|-> Data In (UCA0SIMO)
//            |                 |
//            |             P3.0|<- Serial Clock In (UCA0CLK)
//
//   A. Dannenberg
//   Texas Instruments Inc.
//   April 2006
//   Built with CCE Version: 3.2.0 and IAR Embedded Workbench Version: 3.41A
//******************************************************************************
#include "msp430x21x2.h"

void main(void)
{
  WDTCTL = WDTPW + WDTHOLD;                 // Stop watchdog timer
  while (!(P3IN & 0x01));                   // If clock sig from mstr stays low,
                                            // it is not yet in SPI mode
  P3SEL |= 0x31;                            // P3.0,4,5 USCI_A0 option select
  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
}

// Echo character
#pragma vector=USCIAB0RX_VECTOR
__interrupt void USCI0RX_ISR (void)
{
  while (!(IFG2 & UCA0TXIFG));              // USCI_A0 TX buffer ready?
  UCA0TXBUF = UCA0RXBUF;
}