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Calculation with static double has different results

Thomas Johansen

Dear Forum,

I have experienced some strange behaviour using float/double values in a formula calculation.

Using the same static double values the result may differ each time I run the calculation?

I'm using a LPC824 (Cortex M0) with no FPU and KEIL-MDK.
No MicroLIB and default compiler version 5

I'm aware of the usage of double/float precision issue.
But this problem is when using the same values, the result is different each time I run a calculation.

I have a voltage (double) from a ADC, that is run through a formula, that uses 7 coefs (double).

The formula is this:


dResult = V0[paramIndex] + ( (dTemp-T0[paramIndex])*(P1[paramIndex]+(dTemp-T0[paramIndex])*(P2[paramIndex]+(dTemp-T0[paramIndex])*(P3[paramIndex]+P4[paramIndex]*(dTemp-T0[paramIndex]))))/( 1+(dTemp-T0[paramIndex])*(q1[paramIndex]+q2[paramIndex]*(dTemp-T0[paramIndex]))));

V0[], T0[], P1[], P2[], P3[], P4[], q1[] and q2[] contains static double values.
So they do neve change.

dTemp is the double value (voltage) passed to the function containing the formula.

If I also keep the dTemp stable (fixed static value of 27.8125) the dResult is different each time I run the formula.
If I use the value of 27.7812 it is the same value each time.

Why do I see this behavior? Why is the result not the same each time?

Thomas

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