Split thermistor functions into separate logical units and add TODO

src/bus/i2c.c

@@ -14,11 +14,6 @@
 #define TW_MR_SLA_NACK 0x48  // SLA+R transmitted, NACK received
 #define TW_MR_DATA_ACK 0x50  // Data received, ACK returned
 
-// NTC conversion constants
-#define NTC01 1.009249522e-3
-#define NTC02 2.378405444e-4
-#define NTC03 2.019202697e-7
-
 // TODO: Error handling and recovery besides watchdog timer.
 // TODO: Add more documentation from the atmel data sheet.
 // TODO: ADS1115 continuous mode instead of single-shot?
@@ -477,7 +472,7 @@ static bool I2C_ADS1115_IsReady(void)
 	return reg & BIT(15);
 }
 
-word I2C_ADS1115_Read(int channel)
+word I2C_ADS1115_ReadRaw(int channel)
 {
 	word config;
 	word os, mode, rate, gain, mux;
@@ -563,21 +558,3 @@ word I2C_ADS1115_Read(int channel)
 
 	return I2C_ADS1115_ReadRegister(0x00);
 }
-
-float I2C_ADS1115_ReadThermistor(int channel)
-{
-	word raw;
-	float r1, t;
-	float logr2;
-
-	// TODO: Improve readability
-	// https://en.wikipedia.org/wiki/Steinhart%E2%80%93Hart_equation
-
-	raw = I2C_ADS1115_Read(channel);
-
-	r1 = (float) raw / 264.8f * 0.05f;
-	logr2 = log(10000.0f / (5.0f / r1 - 1.0f));
-	t = 1.0f / (NTC01 + NTC02 * logr2 + NTC03 * pow(logr2, 3));
-
-	return t - 273.15f; // Convert fahrenheit to celsius
-}

src/bus/i2c.h

@@ -28,7 +28,6 @@ int  I2C_AHT20_Init(void);
 int  I2C_AHT20_Read(float *temp, float *rhum);
 
 // ADS1115 analog to digital converter
-word   I2C_ADS1115_Read(int channel);
+word  I2C_ADS1115_ReadRaw(int channel);
-float  I2C_ADS1115_ReadThermistor(int channel);
 
 #endif // MAD_CORE_BUS_I2C_H

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
+// Thermistor coeficients
-static const float C1 = 100.0f;
+static const float SC1 = 1.009249522e-3f;
-static const float C2 = 243.04f;
+static const float SC2 = 2.378405444e-4f;
-static const float C3 = 17.625f;
+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));
 }

src/common/math.h

@@ -1,6 +1,9 @@
 #ifndef MAD_CORE_COMMON_MATH_H
 #define MAD_CORE_COMMON_MATH_H
 
+float Resistance(int adc_raw);
+float SteinhartHart(float res);
+
 float Dewp(float t, float rh);
 float Temp(float td, float rh);
 float Rhum(float t, float td);

src/main.c

@@ -110,8 +110,8 @@ static int Init(void)
 
 static void Update(void)
 {
-	float t[3], rh[3];
+	float t[6], rh[3];
-	float adct[3];
+	word raw;
 
 	Info("Reading sensor values...");
 
@@ -124,13 +124,18 @@ static void Update(void)
 	I2C_SetChannel(AHT03);
 	I2C_AHT20_Read(&t[2], &rh[2]);
 
-	adct[0] = I2C_ADS1115_ReadThermistor(ADS01);
+	raw = I2C_ADS1115_ReadRaw(ADS01);
-	adct[1] = I2C_ADS1115_ReadThermistor(ADS02);
+	t[3] = SteinhartHart(Resistance(raw));
-	adct[2] = I2C_ADS1115_ReadThermistor(ADS03);
 
-	Info("T1=%.2fC, RH1=%.2f%%, ADCT1=%.2fC", t[0], rh[0], adct[0]);
+	raw = I2C_ADS1115_ReadRaw(ADS02);
-	Info("T2=%.2fC, RH2=%.2f%%, ADCT2=%.2fC", t[1], rh[1], adct[1]);
+	t[4] = SteinhartHart(Resistance(raw));
-	Info("T3=%.2fC, RH3=%.2f%%, ADCT3=%.2fC", t[2], rh[2], adct[2]);
+
+	raw = I2C_ADS1115_ReadRaw(ADS03);
+	t[5] = SteinhartHart(Resistance(raw));
+
+	Info("T1=%.2fC, RH1=%.2f%%, NT1=%.2fC", t[0], rh[0], t[3]);
+	Info("T2=%.2fC, RH2=%.2f%%, NT2=%.2fC", t[1], rh[1], t[4]);
+	Info("T3=%.2fC, RH3=%.2f%%, NT3=%.2fC", t[2], rh[2], t[5]);
 
 	// TODO: Implement state machine
 	// TODO: Handle serial commands