Why does the Atmega328 USART get trapped in a loop?

Question

I have an Atmega328 micrprocessor.

I connected its RX/TX pins to an SIM808 module and uploaded this code to the processor to communicate with it and send commands:

#include <util/delay.h>
#define F_CPU 12000000UL
#include<avr/io.h>

#define USART_BAUDRATE 1200
#define BAUD_PRESCALE (((F_CPU/(USART_BAUDRATE*16UL)))-1)

int main(void){

    // default settings
    char recieved_byte;

    UCSR0B |= (1<<RXEN0)  | (1<<TXEN0);
    UCSR0C |= (1<<UCSZ00) | (1<<UCSZ01);
    UBRR0H  = (BAUD_PRESCALE >> 8);
    UBRR0L  = BAUD_PRESCALE;

    for(;;){
    // wait until a byte is ready to read
    while( ( UCSR0A & ( 1 << RXC0 ) ) == 0 ){}

    // grab the byte from the serial port
    recieved_byte = UDR0;

    // wait until the port is ready to be written to
    while( ( UCSR0A & ( 1 << UDRE0 ) ) == 0 ){}

    // write the byte to the serial port
    UDR0 = recieved_byte;

    _delay_ms(100);
    }
    return 0;   /* never reached */
}

It gets trapped in a loop, for example after typing at in the serial terminal it prints this back and it continuous until restarted:

OK

atOK

OK

atOKatOKatOKatOKatOK

I've a feeling that the code must be changed, but I'm new to this and I couldn't find a simple example.

PS:

I test these:

1. I've used an Arduino instead of a bare processor. It works fine sending and receiving commands, so the SIM module is fine.

2. I've disconnected the SIM module and the echo program works well.

3. I've removed the echo part (I think) UDR0 = recieved_byte; but no luck.

Update 1:

I realized that the SIM808 works fine with an Arduino and/or other USB to serial if one connects RX to RX and TX to TX, it must be crossed inside the module. It doesn't help if I use this code and a bareare AVR MCU and it only echos back commands which the MCU gets, not the response from SIM module.

Answer 1

You cannot have the same UART wired to both the PC via USB serial and also the mobile modem, without causing such mixups.

You need a second UART, either hardware or software as typical Arduino examples for your platform in this role use.

You also do not want to use delays in serial code.

Basically, start over from a working example and only attempt to reimplement by hand once you thoroughly understand that example.

Ideally you would also switch to a better processor with at least two hardware UARTs. Likely few but the Arduino folks would try to force this one to do that job when there are better, cheaper choices for an application needing two UARTs.