RX/TX Serial Communication

Question

I am trying to deal with the concept of serial communication with the ATmega328P. I'm quite new to this, but this is the code I've currently written. Basically we have enter in a character from serial communication into the RX of the ATmega chip and want the TX to transmit back same character that was sent from the serial terminal. I believe this code does receive the transmission, but I cannot figure out why it does not transmit the entered character.

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

There is no sense in your code.

1)

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

This has no sense: You're returning from main to ... where? Actually, when you exit main routine, the program is halted by entering the dead loop.

2)

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

you're transmitting content of the variable input. Which is declared above remains undefined. You never assign anything to the variable. I bet the compiler gave you a lot of warnings.

3)

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

what do you expect from 'printf'. Where it should to print the result, when running on a MCU?

In fact the code may be much simpler

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; 
    }

}