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Topic: math with float not working (Read 784 times) previous topic - next topic


This code was working, but now all I get is 0.00 for temperature  :~ and I am confused about what happened

For these two routines, output is:
x2: 296.78, ReadTemperatureCal: 0.00°C
x3: 0.00  <<---- why is this zero? should be 23.63°C

Code: [Select]
float ReadTemperatureCal(void) {
  // reads calibration factor from EEPROM; float max. -10.0 - +10.0 degree offset range
  float Cal;
   Cal= EEPROMreadFloat(CAL_EEADDR); // read cal factor
   if ((Cal > 10.0) || (Cal < -10.0)) { // if garbage in fresh EEPROM
     Cal=0.0; //set to zero
     EEPROMwriteFloat(CAL_EEADDR, Cal); // save it;
     Serial.println("Reset Cal Factor to 0.0");
   Serial.print("ReadTemperatureCal: ");  Serial.print(Cal,2); Serial.print(0xB0,BYTE); Serial.println("C");
   return Cal;
double Thermistor(int RawADC) {

  double Temp;
  // The coefficients below for 10K @ 25C thermistor from Radio Shack 271-110   
  Temp = log(((30720000/RawADC) - 30000)); //30k pulldown
  // S-H coefficients
  Temp = 1 / (0.00090145879873 + (0.00024921185913 * Temp) + (0.00000020123928 * Temp * Temp * Temp));
    Serial.print("x2: ");   Serial.print(Temp,2); Serial.print(", ");
  Temp = Temp - 273.15 - ReadTemperatureCal();         // Convert Kelvin to Celcius
    Serial.print("x3: ");   Serial.println(Temp,2);
  return Temp;


I did a quick test, and the maths works as expected.

So "It suddenly stopped working" - the classic counter question is : What did you do ?

My best guess is that you are overflowing RAM, this usually results in weird behaviour. How large is the overall program? The size of code (~number of lines) isnt what I am asking about, but how many arrays are you using? Many strings used in print will break the ~1.5K you have. Including a library may have a large RAM-memory usage. Search for the "MemoryFree" routine and use that.


Thanks for verifying the code  :smiley-surprise:  I was running a Mega328 and tried it on a Mega2560, as a quick check if more RAM would help, but no change. It has to be a side-effect, stack mismatch. I found the EEPROM read routine seems to be the culprit, having a Union declared inside (?)
Code: [Select]
float EEPROMreadFloat(int address) {
  union floatStore {
    byte floatByte[4];
    float floatVal;
  } floatOut;
  for (int i = 0; i < 4; i++)
    floatOut.floatByte[i] = EEPROM.read(address + i);
  return floatOut.floatVal;

I moved the union declaration up to the top of the program it started working. I hope that's it. I checked all other routines for in/out variable passing and couldn't see anything off. I assume doubles and floats are the same (size).

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