AVR half duplex receiver interrupt error

Question

I am attempting to implement half duplex UART with an ATTiny841. The idea of the code is that when I transmit anything but the character 'd' I receive nothing back and when I transmit the 'd' I get that letter back. After testing this out, with various characters, I always receive the same letter that I have transmitted. One of the major issues I am facing here is within the USART0_RX_vect. For some reason the if statement does not appear to work. Any suggestions on what I could do to solve this?

#include <avr/io.h>
#include <avr/interrupt.h>

class CommsController {
public:
    uint8_t data;
    bool bjsonComplete;

    CommsController(uint8_t ubrr) {
        UCSR0B |= (1<<RXEN0)|(1<<TXEN0)|(1<<RXCIE0); // Enable Rx and Rx Complete Interrupt.
        UBRR0H = (ubrr>>8);
        UBRR0L = ubrr;
        // Select 8-bit data frame, single stop bit and no parity using UCSR0C (Default values are what we want).

        this->bjsonComplete = false;
    }

    void transmit(unsigned char data) {
        UCSR0B &= ~(1<<RXEN0) & ~(1<<RXCIE0); // disable Rx and interrupt.
        /* Wait for empty transmit buffer */
        while ( !( UCSR0A & (1<<UDRE0)) )
        ;
        /* Put data into buffer, sends the data */
        UDR0 = data;
        while (!(UCSR0A & (1<<UDRE0) & (1<<TXC0))) {}; // Wait for empty transmit buffer.
        UCSR0B |= (1<<RXEN0)|(1<<RXCIE0); // Re-enable Rx and interrupt.
    }
};


volatile uint8_t data = 0;

CommsController* commsController;

ISR(USART0_RX_vect) {
    data = UDR0;
    if(data == 'd') {
        commsController->data = data;
        commsController->bjsonComplete = true;
    }
}

/* Replace with your library code */
int main(void)
{
    cli();
    CommsController c(51);
    commsController = &c;
    sei();

    while(1) {
        if(commsController->bjsonComplete) {
            commsController->transmit(commsController->data);
        }
    }

    return 0;
}

Are you sure this is a complete example to reproduce the behavior? commsController->data should really never contain something else than 'd' except initially. What i noticed is that you should probably set commsController->bjsonComplete back to false after you transmitted? — Rev, Oct 12 '17 at 06:27

danieljimeneznz · Accepted Answer · 2019-04-20T22:51:06.050

I later found out that the issue was actually because I did not declare the commsController volatile. I will post the full working code below for anyone who is facing similar issues:

#define F_CPU 8000000UL

#include <avr/io.h>
#include <avr/interrupt.h>
#include <avr/delay.h>

class CommsController {
public:
    volatile uint8_t data;
    volatile bool bjsonComplete;

    CommsController(uint8_t ubrr) {
        UCSR0B |= (1<<RXEN0)|(1<<RXCIE0); // Enable Rx and Rx Complete Interrupt.
        UBRR0H = (ubrr>>8);
        UBRR0L = ubrr;
        // Select 8-bit data frame, single stop bit and no parity using UCSR0C (Default values are what we want).

        this->bjsonComplete = false;
        this->data = 0;
    } 

    void transmit(unsigned char data) volatile {
        UCSR0B &= ~(1<<RXCIE0); // disable Rx interrupt.
        /* Wait for empty transmit buffer */
        while (!( UCSR0A & (1<<UDRE0)));

        /* Put data into buffer, sends the data */
        UDR0 = data;
        while (!(UCSR0A & (1<<TXC0))); // Wait for empty transmit buffer.
        UCSR0B |= (1<<RXCIE0); // Re-enable Rx interrupt.
    }
};

volatile CommsController c(51);

ISR(USART0_RX_vect) {
    c.data = UDR0;
}

/* Replace with your library code */
int main(void)
{
    cli();
    sei();
    DDRB |= (1<<PORTB1);
    PORTB &= ~(1<<PORTB1);

    while(1) {
        if(c.data == 'd') {
            c.transmit('f');
            c.data = 0;
        }
    }

    return 0;
}

AVR half duplex receiver interrupt error

1 Answers1