AVR-Chat

On Fri, Apr 22, 2005 at 03:05:51PM -0000, arhodes19044 wrote:

> I have not used the printf functions yet since I just started using
> these uC's.  But, don't I have to use printf and its variants with a
> certain output stream?

Not with 'sprintf()' which writes its output into a memory buffer.  A
simple way to convert a float to ascii is simply to:

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

       ...

       sprintf_P(my_buf, PSTR("%f"), my_float);

       ...

Where 'my_buf' is a char [] big enough to hold the result.

If you use 'printf()', then yes, you need to provide some type of
output function, but it is quite flexible.  Initialize like this:

int my_putc(char ch)
{
  ...

  return ch;
}

  ...

  /* initialize libc stdio so 'printf()' works */
  fdevopen(my_putc, NULL, 0);

  printf_P(PSTR("Hello world!\n"));

  ...


The 'my_putc()' function just needs to be able to handle output a
character at a time, but other than that, it can drive any output
device that you can dream up.  For simple polled serial, it's very
simple, something like:

int my_putc(char ch)
{
  loop_until_bit_is_set(UCSR0A, UDRE);
  UDR0 = ch;
  return ch;
}

Of course, you will need to initialize the UART also, before calling
'printf()'.

For interrupt driven serial, it's slightly more complicated -
basically put the character into a buffer and let interrupt driven I/O
continue the process.  That way, your 'printf()' can return
immediately if that is important.

> I have an LCD display setup as a 4-bit parallel device.  I do not have
> it set up for any interrupt driven output, but I might do that to
> increase speed in the calling program.  Right now I just send the
> stuff character bycharacter until it is done and then come back to the
> calling routine.  I will have to see if sprintf will work in such a
> way.

'sprintf()' will not work that way, but you can easily make 'printf()'
work as described above by simply providing your LCD equivalent of
'my_putc()' above.

For more information on this, see the avr-libc documentation here:

     http://www.nongnu.org/avr-libc/user-manual/

Specifically:

     http://www.nongnu.org/avr-libc/user-manual/group__avr__stdio.html

-Brian
-- 
Brian Dean
BDMICRO - ATmega128 Based MAVRIC Controllers
http://www.bdmicro.com/