move thermistor calculation to watch utilities

4 files changed, 67 insertions, 66 deletions

diff --git a/movement/watch_faces/thermistor/thermistor_driver.c b/movement/watch_faces/thermistor/thermistor_driver.c
index 9e5d6fd7..37b5ba3f 100644
--- a/movement/watch_faces/thermistor/thermistor_driver.c
+++ b/movement/watch_faces/thermistor/thermistor_driver.c
@@ -1,76 +1,34 @@
 #include "thermistor_driver.h"
 #include "watch.h"
-
-#define THERMISTOR_B_COEFFICIENT (3950.0)
-#define THERMISTOR_NOMINAL_TEMPERATURE (25.0)
-#define THERMISTOR_NOMINAL_RESISTANCE (10000.0)
-#define THERMISTOR_SERIES_RESISTANCE (10000.0)
-
-// TODO: we really need a math library.
-uint32_t msb(uint32_t v);
-double ln(double y);
+#include "watch_utility.h"
 
 void thermistor_driver_enable() {
     // Enable the ADC peripheral, which we'll use to read the thermistor value.
     watch_enable_adc();
-    // Enable analog circuitry on pin A1, which is tied to the thermistor resistor divider.
-    watch_enable_analog_input(A1);
-    // Enable digital output on A0, which is the power to the thermistor circuit.
-    watch_enable_digital_output(A0);
+    // Enable analog circuitry on the sense pin, which is tied to the thermistor resistor divider.
+    watch_enable_analog_input(THERMISTOR_SENSE_PIN);
+    // Enable digital output on the enable pin, which is the power to the thermistor circuit.
+    watch_enable_digital_output(THERMISTOR_ENABLE_PIN);
+    // and make sure it's off.
+    watch_set_pin_level(THERMISTOR_ENABLE_PIN, !THERMISTOR_ENABLE_VALUE);
 }
 
 void thermistor_driver_disable() {
-    // Enable the ADC peripheral, which we'll use to read the thermistor value.
+    // Disable the ADC peripheral.
     watch_disable_adc();
-    // Disable analog circuitry on pin A1 to save power.
-    watch_disable_analog_input(A1);
-    // Disable A0's output circuitry.
-    watch_disable_digital_output(A0);
+    // Disable analog circuitry on the sense pin to save power.
+    watch_disable_analog_input(THERMISTOR_SENSE_PIN);
+    // Disable the enable pin's output circuitry.
+    watch_disable_digital_output(THERMISTOR_ENABLE_PIN);
 }
 
 float thermistor_driver_get_temperature() {
-    // set A0 high to power the thermistor circuit.
-    watch_set_pin_level(A0, true);
-    // get the pin level
-    uint16_t val = watch_get_analog_pin_level(A1);
-    // and then set A0 low to power down the thermistor circuit.
-    watch_set_pin_level(A0, false);
-
-    double reading = (double)val;
-    reading = (1023.0 * THERMISTOR_SERIES_RESISTANCE) / (reading / 64.0);
-    reading -= THERMISTOR_SERIES_RESISTANCE;
-    reading = reading / THERMISTOR_NOMINAL_RESISTANCE;
-    reading = ln(reading);
-    reading /= THERMISTOR_B_COEFFICIENT;
-    reading += 1.0 / (THERMISTOR_NOMINAL_TEMPERATURE + 273.15);
-    reading = 1.0 / reading;
-    reading -= 273.15;
-
-    return reading;
-}
-
-uint32_t msb(uint32_t v) {
-  static const int pos[32] = {0, 1, 28, 2, 29, 14, 24, 3, 30, 22, 20, 15, 25, 17, 4, 8, 31, 27, 13, 23, 21, 19, 16, 7, 26, 12, 18, 6, 11, 5, 10, 9};
-  v |= v >> 1;
-  v |= v >> 2;
-  v |= v >> 4;
-  v |= v >> 8;
-  v |= v >> 16;
-  v = (v >> 1) + 1;
-  return pos[(v * 0x077CB531UL) >> 27];
+    // set the enable pin to the level that powers the thermistor circuit.
+    watch_set_pin_level(THERMISTOR_ENABLE_PIN, THERMISTOR_ENABLE_VALUE);
+    // get the sense pin level
+    uint16_t value = watch_get_analog_pin_level(THERMISTOR_SENSE_PIN);
+    // and then set the enable pin to the opposite value to power down the thermistor circuit.
+    watch_set_pin_level(THERMISTOR_ENABLE_PIN, !THERMISTOR_ENABLE_VALUE);
+
+    return watch_utility_thermistor_temperature(value, THERMISTOR_HIGH_SIDE, THERMISTOR_B_COEFFICIENT, THERMISTOR_NOMINAL_TEMPERATURE, THERMISTOR_NOMINAL_RESISTANCE, THERMISTOR_SERIES_RESISTANCE);
 }
-
-double ln(double y) {
-    int log2;
-    double divisor, x, result;
-
-    log2 = msb((int)y); // See: https://stackoverflow.com/a/4970859/6630230
-    divisor = (double)(1 << log2);
-    x = y / divisor;    // normalized value between [1.0, 2.0]
-
-    result = -1.7417939 + (2.8212026 + (-1.4699568 + (0.44717955 - 0.056570851 * x) * x) * x) * x;
-    result += ((double)log2) * 0.69314718; // ln(2) = 0.69314718
-
-    return result;
-}
-
diff --git a/movement/watch_faces/thermistor/thermistor_driver.h b/movement/watch_faces/thermistor/thermistor_driver.h
index 837eb15b..a0b197de 100644
--- a/movement/watch_faces/thermistor/thermistor_driver.h
+++ b/movement/watch_faces/thermistor/thermistor_driver.h
@@ -1,10 +1,16 @@
 #ifndef THERMISTOR_DRIVER_H_
 #define THERMISTOR_DRIVER_H_
 
-// NOTE: This implementation is specific to one prototype sensor board, OSO-MISC-21-009, but both
-// the sensor board design and this implementation are likely to change. Thermistor functionality
-// may even end up being baked into the Sensor Watch library. This is all by way of saying this
-// code is very temporary and the thermistor screens will likely get a rewrite in the future.
+// TODO: Do these belong in movement_config.h? In settings we can set on the watch? In an EEPROM configuration area?
+// Think on this. [joey 11/22]
+#define THERMISTOR_SENSE_PIN (A2)
+#define THERMISTOR_ENABLE_PIN (A0)
+#define THERMISTOR_ENABLE_VALUE (false)
+#define THERMISTOR_HIGH_SIDE (true)
+#define THERMISTOR_B_COEFFICIENT (3380.0)
+#define THERMISTOR_NOMINAL_TEMPERATURE (25.0)
+#define THERMISTOR_NOMINAL_RESISTANCE (10000.0)
+#define THERMISTOR_SERIES_RESISTANCE (10000.0)
 
 void thermistor_driver_enable();
 void thermistor_driver_disable();
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