[参考译文] MSP-EXP430FR2355：UART TxD在字符后插入额外的NULL []

https://e2e.ti.com/support/microcontrollers/msp-low-power-microcontrollers-group/msp430/f/msp-low-power-microcontroller-forum/1090368/msp-exp430fr2355-uart-txd-inserts-extra-null-after-char

在UART TxD完成发送 字符串后发送了额外的NULL字符。

我将WFP 4.3 设置为我的UART TxD。  按下MSP-EXP430FR2355启动板上的S1按钮时，WFP 4.1 将触发中断，从而 启用UART IRQ。  逻辑 非常简单，我可以 在 串行终端中读取字符串OK (我正在发送一个简单的字符串"AB")。 但是当我检查示波器上的信号时，我注意到在字符串后面插入了两个NULL字符。  

为什么会发生这种情况，以及如何清除这些NULL？  

#include <msp430.h> 

char message[] = "AB";
unsigned int pos;

int main(void)
{
	WDTCTL = WDTPW | WDTHOLD;	// stop watchdog timer
	
	UCA1CTLW0 |= UCSWRST;       // put eUSCI_A1 into SW reset
	UCA1CTLW0 |= UCSSEL__SMCLK; // clock source: use SMCLK 1MHz
	UCA1BRW = 8;                // (from table) 115200 baud
	UCA1MCTLW |= 0xD600;        // (from table) 115200 baud

	P4DIR &= ~BIT1;             // P4.1 (SW1) clear bit to set as input
	P4REN |= BIT1;              // P4.1 (SW1) enable pull-up/down resistor
	P4OUT |= BIT1;              // P4.1 (SW1) pull-up
	P4IES |= BIT1;              // P4.1 (SW1) interrupt edge select, high-to-low transition (SW is pressed)

	P4SEL1 &= ~BIT3;            // P4.3 {UART} TxD
	P4SEL0 |= BIT3;             // P4.3 (UART) TxD

	PM5CTL0 &= ~LOCKLPM5;       // get out from LPM
	UCA1CTLW0 &= ~UCSWRST;      // take eUSCI_A1 out of SW reset (activate)

	P4IFG &= ~BIT1;             // P4.1 (SW1) clear interrupt flag
	P4IE |= BIT1;               // P4.1 (SW1) enable interrupt

	__enable_interrupt();

	while(1)
	{
	    // do nothing
	}

	return 0;
}

#pragma vector = PORT4_VECTOR        // triggered when SW1 is pressed
__interrupt void ISR_PORT4_S1(void)
{
    pos = 0;                        // when SW1 is pressed, initialize string index
    UCA1IE |= UCTXCPTIE;            // P4.3 (UART) enable IRQ
    UCA1IFG &= ~UCTXCPTIFG;         // P4.3 (UART) clear IFG
    UCA1TXBUF = message[pos];       // P4.3 (UART) put the first character in Tx buffer ... buffer starts shifting out

    P4IFG &= ~BIT1;                 // clear P4.1 (SW1) IFG
}

# pragma vector = EUSCI_A1_VECTOR   // triggered right after the first character is sent
__interrupt void ISR_EUSCI_A1(void)
{
    if(pos == sizeof(message))      // if all characters in a string is sent
    {
        UCA1IE &= ~UCTXCPTIE;       // P4.3 (UART) disable IRQ
    }
    else                            // more characters to be sent
    {
        pos++;
        UCA1TXBUF = message[pos];
    }
    UCA1IFG &= ~UCTXCPTIFG;         // P4.3 (UART) clear IFG
}