Electronic – need to wait a few moments before sending serial data on the AVR

avrserial

I have an AVR (ATMega644) connected to a Raspberry PI via serial connection. The AVR is powered with 5V and the AVR=>RPI Tx line is using a 1k8/3k3 voltage divider to get a 3v3 level.

If the 10ms wait (XXX in the code) is not present (according to the data sheet I do not have to wait at all) I receive this nonsense (python repr() of the received data):

 '\x00\xaa\x8a\x8a\xea\n'
 '\xe9\xf5%\xc5E\xd5\xa4\xfcBYE WORLD\r\n'

When the 10ms delay is there I receive this:

'\x00HELLO WORLD\r\n'
'BYE WORLD\r\n'

This is pretty much fine. I'd like to know why there's a \0 before the very first actual data byte ('H') though. I never send one for sure!

However, my main question is why the delay between initializing and sending data is necessary.

Notes: My F_CPU value is correct and so are the fuses. I also tried using a lower baudrate (4800) and a different chip.

This is the code I'm using: 

#include <avr/interrupt.h>
#include <util/delay.h>
#include <util/setbaud.h>


inline void uart_putc(char c)
{
    loop_until_bit_is_set(UCSR0A, UDRE0);
    UDR0 = c;
}


inline void uart_puts(const char *s)
{
    while (*s) {
        uart_putc(*s++);
    }
}


int main()
{
    // we don't need/use any interrupts
    cli();

    // -DF_CPU=18432000L -DBAUD=19200 used when compiling
    UBRR0H = UBRRH_VALUE;
    UBRR0L = UBRRL_VALUE;
    // 8 data bits
    UCSR0C |= (1 << UCSZ00) | (1 << UCSZ01);
    // enable transmitter
    UCSR0B |= (1 << TXEN0);

    _delay_ms(10);  // XXX
    uart_puts("HELLO WORLD\r\n");
    _delay_ms(250);
    uart_puts("BYE WORLD\r\n");

    // do nothing
    while (1)
        ;

    return 0;
}

Best Answer

Basically what you do in 

// enable transmitter
UCSR0B |= (1 << TXEN0);

_delay_ms(10);  // XXX

is inserting 10ms of stop bits (1). This helps your receiver to synchronize with the data stream (which conveniently starts with a start bit 0). Until then the AVR pin is high impedance and filling your input buffer with garbage to which your receiver is trying to synchronize.

Every time the receiver reads 10 bits that fit the frame (start-bit 0, 8 data bits, stop-bit 1) the data bits are pushed in the input buffer. Only when the frame doesn't match, bits get discarded until it does find a matching 10 bit string.

This also explains the importance for using start and stop bits in asynchronous transmission. This also explains why using 1.5 or 2 stop bits improves speed of synchronization; the probability of received data accidentally fitting the frame for a single byte decreased.

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