Usage of UART for debug messages on AVR

If you need to print out a debug messages to UART port of AVR chip, you can use a simple way for it: switch stdout stream to UART. This is the code of the solution:

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

#ifndef NDEBUG

#  if !defined(F_DEBUG_UART) && !defined(F_CPU)
#    error F_DEBUG_UART frequency or F_CPU frequency should be defined!
#  endif

#  ifndef F_DEBUG_UART
#    define F_DEBUG_UART F_CPU
#  endif

#  ifndef DEBUG_NUART
#    error DEBUG_NUART should be defined!
#  endif

#  define __CONC3(X, Y, Z) X ## Y ## Z
#  define _CONC3(X, Y, Z) __CONC3(X, Y, Z)
// _CONC3(UDRE, DEBUG_NUART,) does not works
#  define _UDRE (_CONC3(UDR, E, DEBUG_NUART))
#  define _UDR (_CONC3(UDR, DEBUG_NUART,))
#  define _UBRRH (_CONC3(UBRR, DEBUG_NUART, H))
#  define _UBRRL (_CONC3(UBRR, DEBUG_NUART, L))
#  define _UCSRA (_CONC3(UCSR, DEBUG_NUART, A))
#  define _UCSRB (_CONC3(UCSR, DEBUG_NUART, B))
#  define _UCSRC (_CONC3(UCSR, DEBUG_NUART, C))
#  define _RXEN (_CONC3(RXEN, DEBUG_NUART,))
#  define _TXEN (_CONC3(TXEN, DEBUG_NUART,))
#  define _UCSZ0 (_CONC3(UCSZ, DEBUG_NUART, 0))
#  define _U2X (_CONC3(U2X, DEBUG_NUART,))
#  define DEBUG_UART_SUPPORT() \
static int debug_uart_putchar(char c, FILE *stream); \
static FILE mystdout = FDEV_SETUP_STREAM(debug_uart_putchar, NULL, _FDEV_SETUP_WRITE); \
static int debug_uart_putchar(char c, FILE *stream) \
{ \
        while (0 == (_UCSRA & (1<<_UDRE))) ; \
        _UDR = c; \
        return (0); \
}

/* 8 bits data, 1 stop bit, 9600 kbps, async */
#  define INIT_DEBUG_UART() do { \
        stdout = &mystdout; \
        _UCSRC = 0x06; \
        _UCSRA = 0x00; \
        _UCSRB = 0x18; /* receive, transmit, no interrupts */ \
        uint16_t ubrr = (F_DEBUG_UART/16/9600-1); \
        _UBRRL = (unsigned char)ubrr; \
        _UBRRH = (unsigned char)(ubrr>>8); \
} while (0)

#  define DEBUG_UART_PRN(...) printf(__VA_ARGS__)

#else
#  define DEBUG_UART_SUPPORT()
#  define INIT_DEBUG_UART()
#  define DEBUG_UART_PRN(...)
#endif

Save it into debug_uart.h, for example.
What you need to use it, it's only defining F_DEBUG_UART or F_CPU, DEBUG_NUART and calling 3 macros: DEBUG_UART_SUPPORT() on module level (it generates code, so call it only once!), INIT_DEBUG_UART() to init UART and DEBUG_UART_PRN(...) where you need message printing. The example is:

#define F_DEBUG_UART 7372800
#define DEBUG_NUART 0
#include "debug_uart.h"
DEBUG_UART_SUPPORT();
int
main(void)
{
        INIT_DEBUG_UART();
        DEBUG_UART_PRN("Hello world, %d\n", 99);
        ... so on ...

F_DEBUG_UART is the exact amount of the system oscillator frequency to calculate UART baud rate. Default value is equals to F_CPU, but it's possible, that you have value of fosc = 7.3728MHz (see AVR Datasheet about "Examples of UBRR settings") and you use in the Makefile F_CPU value as 8MHz - if you use 8MHz for UBRR calculation too, you will get errors in transmission! So use for F_DEBUG_UART exact values only!
DEBUG_NUART defined number of UART. If you define NDEBUG, all macros will be empty - you don't need to delete it's calls. For more information see help on WinAVR.