Question

I Have this “simple” code.

union
{
    unsigned int res;
    char         bytes[2];
} ADC;

char ADC_num[5];
float temprature;

void vis_temp (void)        //  Show temp..
{ 
    signed int l, length;
    unsigned int rem;

    GO_nDONE=1;             // initiate conversion on the channel 0   

    while (GO_nDONE) continue;

    ADC.bytes[0]=ADRESL;
    ADC.bytes[1]=ADRESH;

    utoa(ADC_num, ADC.res, 10);

    temprature = (float) ADC.res * 478.1 / 1024;
    temprature = temprature - 50.0;

    l = (signed int) temprature;
    temprature -= (float) l;  
    rem = (unsigned int)(temprature* 1e1);

    sprintf(&ADC_num, "%i.%u", l, rem);

When reading ADC_res (voltage on pin, temperature sensor) that temperature is 0 degree or under then program writes “0.65500” instead of “-3.5” or similar.
I should have declared the right as signed and unsigned int.
Any hints to fix it, or have an other way of converting it.

Answer 1

temprature = (float) ADC.res * 478.1 / 1024;
temprature = temprature - 50.0;

Suppose now temprature has a negative value -x.yz.

l = (signed int) temprature;

Now l = -x, and

temprature -= (float) l;

temprature = -x.yz - (-x) = -0.yz.

rem = (unsigned int)(temprature* 1e1);

Multiply with 10, and convert to unsigned int. Usually, that results in undefined behaviour (6.3.1.4 (1)):

When a finite value of real floating type is converted to an integer type other than _Bool, the fractional part is discarded (i.e., the value is truncated toward zero). If the value of the integral part cannot be represented by the integer type, the behavior is undefined.⁶¹⁾

⁶¹⁾ The remaindering operation performed when a value of integer type is converted to unsigned type need not be performed when a value of real floating type is converted to unsigned type. Thus, the range of portable real floating values is (−1, Utype_MAX+1).

But converting the negative value to unsigned int would produce the wrong result anyway, even if the remaindering operation is done, what you want is the absolute value, so you should convert

rem = (unsigned int)fabsf(temprature * 1e1);

there.