TM34dz temp gauge with BS2

Comments

• Phil Pilgrim (PhiPi) Posts: 23,514

  2017-11-10 23:48

  How are you computing the temperature from the A/D data? My guess is that you're losing precision in the units place of your result during a divide operation.
  
  -Phil
• Tracy Allen Posts: 6,656

  2017-11-11 00:04 edited 2017-11-11 00:11

  The deal is, you have fill in the subroutine in the program that converts the raw reading of the ADC into physical units such as voltage or temperature. In the program, the subroutine is a stub that is developed at great length later on in the chapter:

  Calc_Volts:
  RETURN
  

  Temperature from the LM34 is 10 millivolts per °F, so finding °F and voltage are close.
  
  To keep it simple, each bit represents 19.53 millivolts. That is 5000 millivolts reference divided into 256 steps. So, one bit represents about 2°F and 40 bits (as you found) represents about 19.53*40 = 781 millivolts or 78.1°F. You'd be pretty close by multiplying the binary value times two to get °F.
  
  There more accurate ways to do the math. The text shows one, here is another:

  Calc_Volts:
     volts = adcBits */ 5000
  RETURN
  

  The */ operator implicitly divides by 256, so the result comes out the same as if it were (for example) degF = 40 * 5000 / 256 = 781. The Stamp can't compute 40 * 5000 directly because of the 65535 limit of a 16 bit word, but */ gets around that.
  
  
• Nomadic0 Posts: 21

  2017-11-11 01:09

  That makes a lot more sense! I was wondering why it seemed so proportionate.
  
  I was also looking around and have failed to find a way to display a decimal in a DEBUG statement
  
  Is this possible?
  
  I appreciate all the feedback
• Tracy Allen Posts: 6,656

  2017-11-11 02:25

  Page 58 of that book gets into displaying the value with a decimal point. You have to split the value into the integer and fractional parts.
  
  In the example where answer is 781 millivolts representing 78.1 °F, You could do as follows:
  
  DEBUG DEC milliVolts/10, ".", DEC1 milliVolts, "degF"
  
  The integer division throws out the ones digit, and the DEC1 calls it out.
  
• Nomadic0 Posts: 21

  2017-11-11 17:00

  You don't know how much you helped me out
  
  I really appreciate it!