1 files changed, 76 insertions, 0 deletions
diff --git a/movement/watch_faces/thermistor/thermistor_driver.c b/movement/watch_faces/thermistor/thermistor_driver.c
new file mode 100644
index 00000000..9e5d6fd7
--- /dev/null
+++ b/movement/watch_faces/thermistor/thermistor_driver.c
@@ -0,0 +1,76 @@
+#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);
+
+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);
+}
+
+void thermistor_driver_disable() {
+    // Enable the ADC peripheral, which we'll use to read the thermistor value.
+    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);
+}
+
+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];
+}
+
+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;
+}
+