這是msp430g2xx3_uscib0_i2c_09例程，i2c從機接收數據的程序，請高手指點一下，為什么讀取UCB0RXBUF是在發送中斷中？接收機是在什么時候進入發送中斷的？

#include

unsigned char *PRxData; // Pointer to RX data

unsigned char RXByteCtr;

volatile unsigned char RxBuffer[128]; // Allocate 128 byte of RAM

int main(void)

{

BCSCTL2|=DIVS_3;

WDTCTL = WDTPW + WDTHOLD; // Stop WDT

P1SEL |= BIT6 + BIT7; // Assign I2C pins to USCI_B0

P1SEL2|= BIT6 + BIT7; // Assign I2C pins to USCI_B0

UCB0CTL1 |= UCSWRST; // Enable SW reset

UCB0CTL0 = UCMODE_3 + UCSYNC; // I2C Slave, synchronous mode

UCB0I2COA = 0x48; // Own Address is 048h

UCB0CTL1 &= ~UCSWRST; // Clear SW reset, resume operation

UCB0I2CIE |= UCSTPIE + UCSTTIE; // Enable STT and STP interrupt

IE2 |= UCB0RXIE; // Enable RX interrupt

while (1)

{

PRxData = (unsigned char *)RxBuffer; // Start of RX buffer

RXByteCtr = 0; // Clear RX byte count

__bis_SR_register(CPUOFF + GIE); // Enter LPM0 w/ interrupts

// Remain in LPM0 until master

// finishes TX

__no_operation(); // Set breakpoint >>here<< and read

} // read out the RxData buffer

}

//------------------------------------------------------------------------------

// The USCI_B0 data ISR is used to move received data from the I2C master

// to the MSP430 memory.

//------------------------------------------------------------------------------

#pragma vector = USCIAB0TX_VECTOR

__interrupt void USCIAB0TX_ISR(void)

{

*PRxData++ = UCB0RXBUF; // Move RX data to address PRxData

RXByteCtr++; // Increment RX byte count

}

//------------------------------------------------------------------------------

// The USCI_B0 state ISR is used to wake up the CPU from LPM0 in order to

// process the received data in the main program. LPM0 is only exit in case

// of a (re-)start or stop condition when actual data was received.

//------------------------------------------------------------------------------

#pragma vector = USCIAB0RX_VECTOR

__interrupt void USCIAB0RX_ISR(void)

{

UCB0STAT &= ~(UCSTPIFG + UCSTTIFG); // Clear interrupt flags

if (RXByteCtr) // Check RX byte counter

__bic_SR_register_on_exit(CPUOFF); // Exit LPM0 if data was

} // received

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