MOD5213, atof convert"-0.0" to -2.0

[linhm]

Hello every friends here,

It's been a long time I have not posted questions. However lately I found some bugs in the firmware I developed for the MOD5213 module. Apparently the atof() function convert "-0.0" to -2.0 instead of 0.0.

Also the following code gave me a huge negative number instead of 0.0, where x,a,b are float, range is char, MAXADCCNTS is 4095.0:
x = (a - b) / (MAXADCCNTS - ((range == 1) ? 819.0 : 0.0));

if I modify the above code by using a temperory variable then I can get the correct result:

float xx;
if (range == 1) xx = MAXADCCNTS - 819.0 ;
else xx = MAXADCCNTS;// 4095.0
x = (a - b) / xx;

Does anyone have similar experience?
Is it caused the compiler?

Thanks.

[rnixon]

What is the value of MAXADCCNTS ?

Have you tried casting it to float in your equation?

[linhm]

#define MAXADCCNTS 4095.0

[linhm]

Casting does correct it, but don't know why? BTW, a=-1.0, b=-1.0;
x= -166194084 without casting.

x = (a - b) / (float)(MAXADCCNTS - ((range == 1) ? 819.0 : 0.0));

[tod]

I would suggest following rule #1 from Effective C++, prefer const to #define. If you replace your #define with

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const float MAXADCCNTS = 4095.0;

I suspect you will get the correct answer. Plus you will explicitly state the type as float instead of allowing the precompiler to make it a double (with the subsequent loss of precision when things get converted).

Your code as posted won't even compile for me when I use a #define, maybe I did something wrong but a short complete compilable example from you would have helped.

[linhm]

Thanks for all the replies.

I also tried casting to double as follows. This code gives the same error as without casting.
I guess it might be the defects in the compiler.
I understand that using "const float MAXADCCNTS = 4095.0;" is better practice.
But i don't think it will solve my problem.

#define MAXADCCNTS 4095.0
float a = -0.1;
float b= - 0.1;
char range = 0;

x = (a - b) / (double)(MAXADCCNTS - ((range == 1) ? 819.0 : 0.0));


As for the atof(), i made a workaround for strings like "-0.0".
I really want to know if anyone has this atof() problem before.

[tod]

So here's a compilable example

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    void TestAgain()
    {
#define MAXADCCNTS 4095.0
        float a = -0.1;
        float b = -0.1;
        char range = 0;

        float x = (a - b) / (double) (MAXADCCNTS - ((range == 1) ? 819.0 : 0.0));
        std::cout << "Testing define with cast:" << x << " \n";

        double test = atof("-0.0");
        std::cout << "Testing atof:" << test << "\n";
    }

When called on a MOD5272 it produces

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Testing define with cast:0
Testing atof:-0

If the cast is removed the results are the same.
If the #define is removed (and the cast) and const double used the results are the same.
I then ran it through a HelloWorld example for the PC and the results are the same.
I'm using NNDK Rel_2_6_0_020

[linhm]

Thanks.
Mine is rel20_rc_4.

[rnixon]

Hi linhm,

Your comments here seem confusing to me, but I would really like to know how this works. Please see below:

Thanks for all the replies.

I also tried casting to double as follows. This code gives the same error as without casting.
I guess it might be the defects in the compiler.
I understand that using "const float MAXADCCNTS = 4095.0;" is better practice.
But i don't think it will solve my problem.

The original question was for float, not double. If you use the const method like Tod said, doesn't that solve the problem if you use float?

If you really want to use a #define rather than a typed constant, you might be able to use:
#define MAXADCCNTS ( (float) 4095.0)

Just out of curiosity, what type of data are you processing that needs the resolution of a double instead of a float? Of course, if something is wrong with double in the compiler it should be fixed.

#define MAXADCCNTS 4095.0
float a = -0.1;
float b= - 0.1;
char range = 0;

x = (a - b) / (double)(MAXADCCNTS - ((range == 1) ? 819.0 : 0.0));


As for the atof(), i made a workaround for strings like "-0.0".
I really want to know if anyone has this atof() problem before.

[tod]

I would suspect the compiler tool chain has not changed from the 20_rc4 release (at least the directory names which have version numbers haven't changed.). I always find it valuable to have MinGW installed. I use its toolchain for standard C++ projects (like HelloWorld). This allows you to isolate any anomalies that are strictly part of the cross-compiler toolchain for the m68K family. As I've said I can't replicate either of your problems using either the MinGW toolchain or the m68K toolchain. I also don't have your processor but unless it uses different libraries for math that shouldn't matter. In the past I have seen compiler problems that are optimization dependent. This seems unlikely in your case but the default is -O2 and -falign-functions=4, if you're using something different that would be key information. You could try turning the optimization level to None(-O0) and see if the behavior changes.

In case it isn't obvious I would like to point out that a literal defintion like

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float x = 1.0;

results in a conversion from double to float.

You can avoid this by using the float literal suffix 'f'.

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float x = 1.0f;

The default precision for a floating point number is always double. So even

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#define SOME_SYM = 1.0
float x = SOME_SYM;

results in a loss of precision conversion, since the pre-processor turns this into the first example.