RX / TX串行通讯

问题描述

我正在尝试与ATmega328P进行串行通信的概念。我对此很陌生，但这是我当前编写的代码。基本上，我们已经将来自串行通信的字符输入到ATmega芯片的RX中，并希望TX回传从串行终端发送的相同字符。我相信这段代码确实可以接收传输，但是我无法弄清楚为什么它不传输输入的字符。

int main(void)
{
    /* For the microcontroller: set the baud rate to 19200 */
    UBRR0 = 25;
    /*
    ** Enable transmission and receiving via UART and also 
    ** enable the Receive Complete Interrupt.
    */
    // UCSR0A = (1<<RXC0)|(1<<UDRE0); 

    UCSR0B = (1<<RXCIE0)|(1<<UDRIE0)|(1<<RXEN0)|(1<<TXEN0);
    // Receiving data
    // wait for data
    while(!(UCSR0A & (1 << RXC0)));

    // return data
    return UDR0;

    //Transmitting data
    /* No need to set UCSR0C - we just want the default value */
    /* Enable interrupts */

    sei();
    /* Sit back and let it happen - this will loop forever */
    for (;;) {
    }
}
    
/*
 * Define the interrupt handler for UART Receive Complete - i.e. a new
 * character has arrived in the UART Data Register (UDR).
 */
/*UART Register*/
ISR(USART0_UDRE_vect)
{
    /* A character has been received - we will read it. If it is 
    ** lower case,we will convert it to upper case and send it
    ** back,otherwise we just send it back as is
    */
    char input;
    /* Extract character from UART Data register and place in input
    ** variable
    */
    UDR0 = input;
}
    
ISR(USART0_RX_vect) {
    char input = UDR0;
    printf_P(PSTR("%c"),input);
}

解决方法

您的代码没有意义。

1）

int main(void)
{
...
    return UDR0;

这毫无意义：您要从main返回到...哪里？实际上，当您退出主程序时，程序会通过进入死循环而暂停。

2）

ISR(USART0_UDRE_vect)
{
...
    char input;
...
    UDR0 = input;
}

您正在传输变量input的内容。上面声明的内容仍未定义。您永远不会为变量分配任何内容。我敢打赌，编译器给了您很多警告。

3）

ISR(USART0_RX_vect) {
    char input = UDR0;
    printf_P(PSTR("%c"),input);
}

您对'printf'有什么期望。在MCU上运行时，应在哪里打印结果？

实际上，代码可能更简单

int main(void)
{
    // Sets 19200 only when MCU is running at 8MHz
    UBRR0 = 25;
    // enable receiver and transmitter
    UCSR0B = (1<<RXEN0)|(1<<TXEN0);

    for(;;) { // infinite loop
      // Receiving data
      // wait for data
      while(!(UCSR0A & (1 << RXC0)));
      // read the data byte from the the receiver buffer
      unsigned char data = UDR0; 
      // Wait for the transmitting buffer to be empty
      while(!(UCSR0A & (1 << UDRE0)));
      // Send the data
      UDR0 = data; 
    }

}