RX/TX串行通信

Question

我正在尝试使用ATmega328P处理串行通信的概念。我对此非常陌生，但这是我目前编写的代码。基本上，我们将串行通信中的字符输入到ATmega芯片的RX中，并希望TX传回从串行终端发送的相同字符。我相信这个代码确实接收到了传输，但我不知道为什么它不能传输输入的字符。

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);
}

Answer 1

你的代码中没有任何意义。

1)

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

这毫无意义:您将从main返回到...哪里?实际上，当您退出main例程时，程序会因为进入死循环而停止。

2)

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

您正在传输变量input的内容。上面声明的仍然是未定义的。您永远不会将任何内容赋给变量。我打赌编译器给了你很多警告。

3)

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

你对'printf‘有什么期望？当在MCU上运行时，它应该在哪里打印结果？

事实上，代码可能要简单得多

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; 
    }

}