move thermistor calculation to watch utilities

2 files changed, 37 insertions, 0 deletions

diff --git a/watch-library/watch/watch_utility.c b/watch-library/watch/watch_utility.c
index a0f361b4..bfa3073a 100644
--- a/watch-library/watch/watch_utility.c
+++ b/watch-library/watch/watch_utility.c
@@ -22,6 +22,7 @@
  * SOFTWARE.
  */
 
+#include <math.h>
 #include "watch_utility.h"
 
 const char * watch_utility_get_weekday(watch_date_time date_time) {
@@ -33,3 +34,23 @@ const char * watch_utility_get_weekday(watch_date_time date_time) {
     }
     return weekdays[(date_time.unit.day + 13 * (date_time.unit.month + 1) / 5 + date_time.unit.year + date_time.unit.year / 4 + 525) % 7];
 }
+
+float watch_utility_thermistor_temperature(uint16_t value, bool highside, float b_coefficient, float nominal_temperature, float nominal_resistance, float series_resistance) {
+    float reading = (float)value;
+
+    if (highside) {
+        reading = (1023.0 * series_resistance) / (reading / 64.0);
+        reading -= series_resistance;
+    } else {
+        reading = series_resistance / (65535.0 / value - 1.0);
+    }
+
+    reading = reading / nominal_resistance;
+    reading = log(reading);
+    reading /= b_coefficient;
+    reading += 1.0 / (nominal_temperature + 273.15);
+    reading = 1.0 / reading;
+    reading -= 273.15;
+
+    return reading;
+}
diff --git a/watch-library/watch/watch_utility.h b/watch-library/watch/watch_utility.h
index e8808923..aada783f 100644
--- a/watch-library/watch/watch_utility.h
+++ b/watch-library/watch/watch_utility.h
@@ -38,4 +38,20 @@
   */
 const char * watch_utility_get_weekday(watch_date_time date_time);
 
+/** @brief Returns a temperature in degrees Celsius for a given thermistor voltage divider circuit.
+  * @param value The raw analog reading from the thermistor pin (0-65535)
+  * @param highside True if the thermistor is connected to VCC and the series resistor is connected
+  *                 to GND; false if the thermistor is connected to GND and the series resistor is
+  *                 connected to VCC.
+  * @param b_coefficient From your thermistor's data sheet, the B25/85 coefficient. A typical value
+  *                      will be between 2000 and 5000.
+  * @param nominal_temperature From your thermistor's data sheet, the temperature (in Celsius) at
+  *                            which the thermistor's resistance is at its nominal value.
+  * @param nominal_resistance The thermistor's resistance at the nominal temperature.
+  * @param series_resistance The value of the other resistor in the voltage divider.
+  * @note Ported from Adafruit's MIT-licensed CircuitPython thermistor code, (c) 2017 Scott Shawcroft:
+  *       https://github.com/adafruit/Adafruit_CircuitPython_Thermistor/blob/main/adafruit_thermistor.py
+  */
+float watch_utility_thermistor_temperature(uint16_t value, bool highside, float b_coefficient, float nominal_temperature, float nominal_resistance, float series_resistance);
+
 #endif