How to direct the AVR Libc stdout stream to send characters using a UART.

File(s):

arch/avr/tutorials/05_printf/printf.c

The UART is configured to 115200 BAUD, 8 Data Bits, No Parity and 1 Stop Bit. The following strings are sent upon start up:

printf - String in data memory (SRAM)
printf_P - String in program memory (FLASH)
PRINTF_P - String in program memory (FLASH)

Remember that the XMODEM-CRC bootloader will send a 'C' character upon reset and this will be shown in addition to the printf strings.

A custom handler "uart_putchar(...)" is linked to the stdout stream to send a character and inject a "\r" (Carriage Return) before every "\n" (Line Feed).

This example draws attention to the handling of the Harvard architecture by the GCC compiler and the AVR Libc library. The 8-bit AVRs have a small amount of SRAM (data memory) and therefore it is desirable to store constant strings in FLASH (program memory) and access it from there and not use a copy in SRAM.

The recommended solution is to use the macros in <avr/pgmspace.h> and string handling functions with the suffix "_P", e.g. "printf_P(...)" instead of "printf(...)". This handling has been encapsulated with the following variable argument macro:

#define PRINTF_P(format, ...) printf_P(PSTR(format), ## __VA_ARGS__)

All of this functionality has been encapsulated in the following modules:

See:

uart_printf : Microchip AVR printf to UART example

More info:

AVR-GCC and the PROGMEM Attribute by Dean Camera
avr-libc: Data in Program Space