• State Not Answered
  • Locked Locked
  • Replies 26 replies
  • Subscribers 21 subscribers
  • Views 8231 views
  • Users 0 members are here
2025 survey

We are running a survey to help us improve the experience for all of our members. If you see the survey appear, please take the time to tell us about your experience if you can.

Options
Related
How was your experience today?
This discussion has been locked.
You can no longer post new replies to this discussion. If you have a question you can start a new discussion

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

Parents
  • 0 in reply to Thomas Johansen

    Hi again,

    I tried removing more and more code and it seems related to setting the CPU to low power mode.

    It is related to this:

    cmdData[0] = 12;
cmdData[1] = PWR_LOW_CURRENT;
cmdData[2] = 12;
__disable_irq();
LPC_ROM_API->pPWRD->set_power(cmdData, &response);
__enable_irq();

    Removing this does the trick. With both the changing value and the precision

    Any idea why? Because I need the low power mode.

    Thomas

Reply
  • 0 in reply to Thomas Johansen

    Hi again,

    I tried removing more and more code and it seems related to setting the CPU to low power mode.

    It is related to this:

    cmdData[0] = 12;
cmdData[1] = PWR_LOW_CURRENT;
cmdData[2] = 12;
__disable_irq();
LPC_ROM_API->pPWRD->set_power(cmdData, &response);
__enable_irq();

    Removing this does the trick. With both the changing value and the precision

    Any idea why? Because I need the low power mode.

    Thomas

Children