tempmodule: add the logic to calculate voltages and temps.

1 files changed, 65 insertions, 0 deletions

diff --git a/src/tempmodule/main.ino b/src/tempmodule/main.ino
index f295d40..58f749a 100644
--- a/src/tempmodule/main.ino
+++ b/src/tempmodule/main.ino
@@ -23,6 +23,12 @@
 #include <Wire.h>
 #include <stddef.h>
 #include <string.h>
+#include <inttypes.h>
+
+struct temptuple {
+	short res;	/* Resistance in ohms. */
+	short temp;	/* Temperatures in 0.1°K. */
+};
 
 static const unsigned int iled = 2;
 static const unsigned int analog = A0;
@@ -41,6 +47,9 @@ static void mpu_wakeup(const int i2caddr);
 static void wifi_connect(const char * const ssid, const char * const password, const char doblink, const int ledpin);
 static void blink_led(const int pin, const int ontime, const int offtime);
 static void discover_client(void);
+static int get_resistance(int64_t vt, int64_t ra);
+static int get_temperature(int res);
+static int get_temp_subrange(short a, short b, int res);
 
 void setup(void)
 {
@@ -173,3 +182,59 @@ static void discover_client(void)
 		delay(95);
 	} while (!done);
 }
+
+/*
+ * formula by applying Ohms law:
+ *
+ * Rt:	Thermistor resistance
+ * Ra:	Voltage divider resistor
+ * Vt:	Tap voltage
+ * Vcc:	Input Voltage (3.3V)
+ *
+ * Rt = Ra * Vcc / Vt - Ra
+ *
+ * Using 64-bit arithmetic as 32-bit would overflow.
+ * On WeMOS D1 Mini the A0 reads 3.3V as 1023, that is our Vcc.
+ * We shift the range by one to avoid division by zero.
+ */
+static int get_resistance(int64_t vt, int64_t ra)
+{
+	++vt;
+	return ra * 1024 / vt - ra;
+}
+
+static int get_temperature(int res)
+{
+	int ret;
+	size_t i;
+	static const struct temptuple lt[] = {
+		{1452, 2630}, {1613, 2730}, {1754, 2830}, {1903, 2930},
+		{2059, 3030}, {2222, 3130}, {2393, 3230}, {2571, 3330},
+		{2757, 3430}, {2950, 3530}, {3150, 3630}, {3358, 3730},
+		{3571, 3830}};
+
+	for (i = 0; i < 9; ++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;
+	}
+
+	return -1;
+}
+
+/* Returns the last 0-10 part of the temperature, in 0.1°K. */
+static int get_temp_subrange(short a, short b, int res)
+{
+	return (res - a.res) * 100 / (b.res - a.res);
+}