1 files changed, 68 insertions, 0 deletions
diff --git a/src/tempmodule/temperature.c b/src/tempmodule/temperature.c
new file mode 100644
index 0000000..7986585
--- /dev/null
+++ b/src/tempmodule/temperature.c
@@ -0,0 +1,68 @@
+#include <stddef.h>
+#include "temperature.h"
+
+/*
+ * formula by applying Ohms law:
+ *
+ * Rt:	Thermistor resistance
+ * Ra:	Voltage divider resistor
+ * Vt:	Tap voltage
+ * Vcc:	Input Voltage (3.31V)
+ *
+ * Rt = Ra * Vcc / Vt - Ra
+ *
+ * Using 64-bit arithmetic as 32-bit would overflow.
+ * On WeMOS D1 Mini the A0 reads 3.2V as 1023; our Vcc is 3.31V
+ * The correct value of the reference volume obtained by measuring
+ * resistances with a multimeter, taking the ADC reading and applying
+ * Ohm's law.
+ * We shift the range by one to avoid division by zero.
+ */
+
+struct temptuple {
+	short res;	/* Resistance in ohms. */
+	short temp;	/* Temperatures in 0.1°K. */
+};
+
+int get_resistance(int64_t vt, int64_t ra)
+{
+	++vt;
+	return ra * 1081 / vt - ra;
+}
+
+int get_temperature(int res)
+{
+	int ret;
+	size_t i;
+	static const struct temptuple lt[] = {
+		{1495, 2630}, {1630, 2730}, {1772, 2830}, {1922, 2930},
+		{2080, 3030}, {2245, 3130}, {2417, 3230}, {2597, 3330},
+		{2785, 3430}, {2980, 3530}, {3182, 3630}, {3392, 3730},
+		{3607, 3830}};
+
+	ret = -1;
+
+	for (i = 0; i < 12; ++i) {
+		/* If we have a matching resistance, nothing to calculate. */
+		if (res == lt[i].res) {
+			ret = lt[i].temp;
+			break;
+		} else if (res == lt[i + 1].res) {
+			ret = lt[i + 1].temp;
+			break;
+		/* If no matching resistance is found, calculate temp from subrange. */
+		} else if (res > lt[i].res && res < lt[i + 1].res) {
+			ret = lt[i].temp + get_temp_subrange(lt[i].res, lt[i + 1].res, res);
+			break;
+		}
+	}
+
+	return ret;
+
+}
+
+/* Returns the last 0-10 part of the temperature, in 0.1°K. */
+int get_temp_subrange(short a, short b, int res)
+{
+	return (res - a) * 100 / (b - a);
+}