Stm32 printf float variable

Question

I want to log out from stm32f405 via usart. In my syscall.c file i realize function to print via usart:

int _write(int file, char *ptr, int len)
{
    int todo;
    for (todo = 0; todo < len; todo++)
    {
    usart_send_char( *ptr++ );
    }
    return len;
}

Function usart_send_char( *ptr++ ); work as expected. But when i call:

printf("%s, %d, %3.2f\r\n", "asd", 777, 13.2 );

I get: asd, 777, 0.00 The float variable not printed correctly.

Makefile:

PROCESSOR = -mcpu=cortex-m4 -mthumb -mfloat-abi=softfp -mfpu=fpv4-sp-d16
CFLAGS += $(PROCESSOR) $(INCLUDES) $(STFLAGS) -Wall -fno-strict-aliasing $(C_PROFILE)
LDFLAGS = $(PROCESSOR) -Wl,-Map=$(PROG).map,--cref,--gc-sections

Used compilator:

Sourcery CodeBench Lite 2014.05-28

Where i am mistaken?

Answer 1

I haven't used Sourcery Codebench gcc for STM32F4, but with the GCC ARM Embedded toolchain, floating point support in printf isn't enabled by default. To enable, add -u _printf_float to your LDFLAGS.

Answer 2

1. I am not sure, but I think the mfloat-abi-flag must be set for the CFLAGS.
2. STM32F4 has a FPU, so why do you use mfloat-abi=soft? Can't you use mfloat-abi=hard?
3. If you don't need to calculate floatingpointvalue very often, you could do it in this way.

Code example:

 int i = 132;
 printf("Result is: %d.%d", i/10, i%10);

Answer 3

To enable the floats in sprintf, add -u _printf_float to your project as this picture indicate.