[求助] 搞不懂MSP430FR5739的一段IIC示范程序

青菜虫子 2013-7-24 00:14 楼主

程序如下，为IIC的master通讯程序：

#include
unsigned char TXData =0;                   // Pointer to TX data
unsigned char TXByteCtr;
int main(void)
{
WDTCTL = WDTPW + WDTHOLD;
// Init SMCLK = MCLk = ACLK = 1MHz
CSCTL0_H = 0xA5;
CSCTL1 |= DCOFSEL0 + DCOFSEL1;       // Set max. DCO setting = 8MHz
CSCTL2 = SELA_3 + SELS_3 + SELM_3;    // set ACLK = MCLK = DCO
CSCTL3 = DIVA_3 + DIVS_3 + DIVM_3;    // set all dividers to 1MHz
// Configure Pins for I2C
P1SEL1 |= BIT6 + BIT7;                // Pin init
UCB0CTLW0 |= UCSWRST;                // put eUSCI_B in reset state
UCB0CTLW0 |= UCMODE_3 + UCMST + UCSSEL_2;// I2C master mode, SMCLk
UCB0BRW = 0x8;                         // baudrate = SMCLK / 8
UCB0I2CSA = 0x48;                      // address slave is 48hex
UCB0CTLW0 &=~ UCSWRST;          //clear reset register
UCB0IE |= UCTXIE0 + UCNACKIE;          //transmit and NACK interrupt enable
while(1)
{
__delay_cycles(1000);                // Delay between transmissions
TXByteCtr = 1;                         // Load TX byte counter
while (UCB0CTLW0 & UCTXSTP);          // Ensure stop condition got sent
UCB0CTLW0 |= UCTR + UCTXSTT;          // I2C TX, start condition
    __bis_SR_register(CPUOFF + GIE);       // Enter LPM0 w/ interrupts
                                          // Remain in LPM0 until all data
                                          // is TX'd
TXData++;                            // Increment data byte
}

}
#pragma vector = USCI_B0_VECTOR
__interrupt void USCIB0_ISR(void)
{
  switch(__even_in_range(UCB0IV,0x1E))
  {
      case 0x00: break;                   // Vector 0: No interrupts break;
      case 0x02: break;
      case 0x04:
      UCB0CTLW0 |= UCTXSTT;          //resend start if NACK
      break;                         // Vector 4: NACKIFG break;
      case 0x18:
if (TXByteCtr)                   // Check TX byte counter
      {
        UCB0TXBUF = TXData;          // Load TX buffer
         TXByteCtr--;                   // Decrement TX byte counter
      }
      else
      {
         UCB0CTLW0 |= UCTXSTP;          // I2C stop condition
         UCB0IFG &= ~UCTXIFG;          // Clear USCI_B0 TX int flag
         __bic_SR_register_on_exit(CPUOFF);// Exit LPM0
      }
      break;                         // Vector 26: TXIFG0 break;
      default: break;
  }
}
这个程序用IAR单步执行和连续执行的结果是不一样的，所以搞不清其语句的执行次序。有疑惑的地方为高亮标出部分，主程序中执行UCB0CTLW0 |= UCTR + UCTXSTT; 执行后UCB0模块开始发送开始信号和从机地址，这部分动作可以脱离CPU执行，同时CPU继续执行下面的__bis_SR_register(CPUOFF + GIE);语句关闭CPU并打开全局中断。按照用户手册说，当UCB0模块开始发送开始信号时就置位了UCTXIFG0，等全局中断打开后就会立刻进入中断case 0x18:执行 UCB0TXBUF = TXData;          // Load TX buffer，现在的问题是这个语句是否需要CPU执行，但这时CPU还处于休眠状态如何执行？即使这句不需要CPU动作，那么后面的TXByteCtr--;  一定需要CPU执行，这时CPU不是进入休眠了吗？如何执行？