Split thermistor functions into separate logical units and add TODO

src/common/math.c

@@ -2,29 +2,67 @@
 
 #include <math.h>
 
-// https://bmcnoldy.earth.miami.edu/Humidity.html
+// https://en.wikipedia.org/wiki/Steinhart%E2%80%93Hart_equation
 // https://onlinelibrary.wiley.com/doi/abs/10.1002/andp.18280890511
 // https://onlinelibrary.wiley.com/doi/abs/10.1002/andp.18441370202
+// https://bmcnoldy.earth.miami.edu/Humidity.html
 
-// TODO: Make sure all results for conversion functions are correct.
+// TODO: Is resistance function semantically correct?
+// TODO: Check results for humidity conversion functions.
 
-// Conversion constants
-static const float C1 = 100.0f;
-static const float C2 = 243.04f;
-static const float C3 = 17.625f;
+// Thermistor coeficients
+static const float SC1 = 1.009249522e-3f;
+static const float SC2 = 2.378405444e-4f;
+static const float SC3 = 2.019202697e-7f;
+static const float R1  = 10000.0f;
+
+// Humidity conversion constants
+static const float TH1 = 100.0f;
+static const float TH2 = 243.04f;
+static const float TH3 = 17.625f;
+
+float Resistance(int adc_raw)
+{
+	float r2;
+
+	// TODO: Improve readability.
+	// TODO: Make sure output is correct.
+
+	// [GND] - [10K RESISTOR] - [ADC INPUT] - [THERMISTOR] - [VCC]
+
+	r2 = (float) adc_raw / 264.8f * 0.05f;
+
+	// XXX: Ohms Law solved for resistance?
+	//          R = V / I
+	return R1 / (5.0f / r2 - 1.0f);
+}
+
+// Resistance to temperature
+float SteinhartHart(float res)
+{
+	float logr, t;
+
+	//  25C = 10000 Ohms
+	// 100C =  6744 Ohms
+
+	logr = log(res);
+	t = 1.0f / (SC1 + SC2 * logr + SC3 * pow(logr, 3));
+
+	return t - 273.15f; // Kelvin to celsius
+}
 
 // Dewpoint TD
 float Dewp(float t, float rh)
 {
 	float a, b;
 
-	a = log(rh / C1);
-	b = C3 * t / (C2 + t);
-
 	// 243.04*(LN(RH/100)+((17.625*T)/(243.04+T)))/
 	// (17.625-LN(RH/100)-((17.625*T)/(243.04+T)))
 
-	return C2 * (a + b) / (C3 - a - b);
+	a = log(rh / TH1);
+	b = TH3 * t / (TH2 + t);
+
+	return TH2 * (a + b) / (TH3 - a - b);
 }
 
 // Temperature T
@@ -32,13 +70,13 @@ float Temp(float td, float rh)
 {
 	float a, b;
 
-	a = log(rh / C1);
-	b = C3 * td / (C2 + td);
-
 	// 243.04*(((17.625*TD)/(243.04+TD))-LN(RH/100))/
 	// (17.625+LN(RH/100)-((17.625*TD)/(243.04+TD)))
 
-	return C2 * (b - a) / (C3 + a - b);
+	a = log(rh / TH1);
+	b = TH3 * td / (TH2 + td);
+
+	return TH2 * (b - a) / (TH3 + a - b);
 }
 
 // Relative Humidity RH
@@ -46,11 +84,11 @@ float Rhum(float t, float td)
 {
 	float a, b;
 
-	a = C3 * td / (C2 + td);
-	b = C3 * t / (C2 + t);
-
 	// 100*(EXP((17.625*TD)/(243.04+TD))/
 	//      EXP((17.625*T)/(243.04+T)))
 
-	return C1 * (exp(a) / exp(b));
+	a = TH3 * td / (TH2 + td);
+	b = TH3 * t / (TH2 + t);
+
+	return TH1 * (exp(a) / exp(b));
 }