PIC C18: Converting double to string

Question

I am using PIC18F2550. Programming it with C18 language.
I need a function that converts double to string like below:

void dtoa(  char *szString,             // Output string
            double dbDouble,            // Input number
            unsigned char ucFPlaces)    // Number of digits in the resulting fractional part
{
    // ??????????????
}

To be called like this in the main program:

void main (void)
{
    // ...
    double dbNumber = 123.45678;
    char szText[9];
    dtoa(szText, dbNumber, 3); // szText becomes "123.456" or rounded to "123.457"
    // ...
}

sprintf() then find the '.' and null the right decimal character? — kenny, Feb 07 '12 at 21:44
@MartinBeckett I'm sure it's available if you have the space. Or roll your own dtoa() — kenny, Feb 07 '12 at 21:52
"Available" can be a nuanced question since standards like MISRA ban the use of variable arguments... — Throwback1986, Feb 07 '12 at 22:02
I use sprintf all the time on the PIC18, and doubles as well. — Adam Casey, Apr 05 '12 at 15:41

score 1 · Answer 1 · answered Feb 07 '12 at 21:47

1

So write one!

5mins, a bit of graph paper and a coffee is all it should take.
In fact it's a good interview question

answered Feb 07 '12 at 21:47

Martin Beckett

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score 1 · Answer 2 · answered Feb 07 '12 at 21:51

1

Tiny printf might work for you: http://www.sparetimelabs.com/tinyprintf/index.html

answered Feb 07 '12 at 21:51

Throwback1986

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As mentioned on that page: "Of course being tiny, it has limitations, no floating point support..." – Michael Burr Feb 08 '12 at 17:00
Ah yes, good catch. Now I dimly recall rolling in our own floating point. – Throwback1986 Feb 08 '12 at 17:15

score 1 · Answer 3 · answered Feb 07 '12 at 22:35

Generally, the Newlib C library (BSD license, from RedHat, part of Cygwin as well as used in many many "bare-metal" embedded-systems compilers) is a good place to start for usefuls sources for things that would be in the standard C library.

The Newlib dtoa.c sources are in the src/newlib/libc/stdlib subdirectory of the source tree:

Online source browser: http://sourceware.org/cgi-bin/cvsweb.cgi/src/newlib/libc/stdlib/?cvsroot=src#dirlist
Direct link to the current version of the dtoa.c file: http://sourceware.org/cgi-bin/cvsweb.cgi/~checkout~/src/newlib/libc/stdlib/dtoa.c?rev=1.5&content-type=text/plain&cvsroot=src

The file is going to be a little odd, in that Newlib uses some odd macros for the function declarations, but should be straightforward to adapt -- and, being BSD-licensed, you can pretty much do whatever you want with it if you keep the copyright notice on it.

PIC C18: Converting double to string

3 Answers3