//******************************************************************************
//   MSP430F21x2 Demo - USCI_A0, Ultra-Low Pwr UART 9600 Echo ISR, 32kHz ACLK
//
//   Description: Echo a received character, RX ISR used. Normal mode is LPM3,
//   USCI_A0 RX interrupt triggers TX Echo.
//   ACLK = BRCLK = LFXT1 = 32768Hz, MCLK = SMCLK = DCO ~1.2MHz
//   Baud rate divider with 32768Hz XTAL @9600 = 32768Hz/9600 = 3.41
//   //* An external watch crystal is required on XIN XOUT for ACLK *//
//
//                MSP430F21x2
//             -----------------
//         /|\|              XIN|-
//          | |                 | 32kHz
//          --|RST          XOUT|-
//            |                 |
//            |     P3.4/UCA0TXD|------------>
//            |                 | 9600 - 8N1
//            |     P3.5/UCA0RXD|<------------
//
//   A. Dannenberg
//   Texas Instruments Inc.
//   October 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 WDT
  P1DIR = 0xFF;                             // All P1.x outputs
  P1OUT = 0;                                // All P1.x reset
  P2DIR = 0xFF;                             // All P2.x outputs
  P2OUT = 0;                                // All P2.x reset
  P3SEL = 0x30;                             // P3.4,5 = USCI_A0 TXD/RXD
  P3DIR = 0xFF;                             // All P3.x outputs
  P3OUT = 0;                                // All P3.x reset
  UCA0CTL1 |= UCSSEL_1;                     // CLK = ACLK
  UCA0BR0 = 0x03;                           // 32kHz/9600 = 3.41
  UCA0BR1 = 0x00;                           //
  UCA0MCTL = UCBRS1 + UCBRS0;               // Modulation UCBRSx = 3
  UCA0CTL1 &= ~UCSWRST;                     // **Initialize USCI state machine**
  IE2 |= UCA0RXIE;                          // Enable USCI_A0 RX interrupt

  __bis_SR_register(LPM3_bits + GIE);       // Enter LPM3, interrupts enabled
}

// Echo back RXed character, confirm TX buffer is ready first
#pragma vector=USCIAB0RX_VECTOR
__interrupt void USCI0RX_ISR(void)
{
  while (!(IFG2&UCA0TXIFG));                 // USCI_A0 TX buffer ready?
  UCA0TXBUF = UCA0RXBUF;                     // TX -> RXed character
}