Calculate temperature from thermistor resistance

src/bus/i2c.c

@@ -2,7 +2,9 @@
 #include "bus/i2c.h"
 
 #include <avr/io.h>
+#include <math.h>
 
+// I2C status flags
 #define TW_START       0x08  // Start condition transmitted
 #define TW_REP_START   0x10  // Repeated start condition transmitted
 #define TW_MT_SLA_ACK  0x18  // SLA+W transmitted, ACK received
@@ -12,6 +14,11 @@
 #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: Implement TWI_vect ISR? This may not actually be
@@ -528,7 +535,25 @@ word I2C_ADS1115_Read(int channel)
 
 	// Single-shot conversion, active low, continous mode
 	config = os | rate | gain | mux;
-
 	I2C_ADS1115_WriteRegister(0x01, config);
+
 	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

@@ -28,6 +28,7 @@ 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_Read(int channel);
+float  I2C_ADS1115_ReadThermistor(int channel);
 
 #endif // MAD_CORE_BUS_I2C_H