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;
}
