automatically detect temperature sensor

watch-library/shared/driver/thermistor_driver.c

@@ -27,9 +27,53 @@
 #include "watch.h"
 #include "watch_utility.h"
 
+// assume we have no thermistor until thermistor_driver_init is called.
+static bool has_thermistor = false;
+
+bool thermistor_driver_init(void) {
+    // once called, assume we have a thermistor unless proven otherwise
+    has_thermistor = true;
+    uint16_t value;
+#if THERMISTOR_ENABLE_VALUE == false
+    const uint16_t threshold_value = 60000;
+#else
+    const uint16_t threshold_value = 5000;
+#endif
+
+    watch_enable_adc();
+    // Enable analog circuitry on the sense pin, which is tied to the thermistor resistor divider.
+    HAL_GPIO_TEMPSENSE_in();
+    HAL_GPIO_TEMPSENSE_pmuxen(HAL_GPIO_PMUX_ADC);
+
+    // Enable digital output on the enable pin, which is the power to the thermistor circuit.
+    HAL_GPIO_TS_ENABLE_out();
+    // and make sure it's DISABLED.
+    HAL_GPIO_TS_ENABLE_write(!THERMISTOR_ENABLE_VALUE);
+
+    // If the temperature sensor is connected, pulling the TS_ENABLE line to its disabled value
+    // connects both sides of the voltage divider to the same potential. If enable value is false,
+    // this will be high, if enable value is true it will be low, and TEMPSENSE should read the same.
+    value = watch_get_analog_pin_level(HAL_GPIO_TEMPSENSE_pin());
+
+    // If setting TS_ENABLE has no effect, there is no thermistor circuit connected to TEMPSENSE.
+    if (value < threshold_value) has_thermistor = false;
+
+    // now flip it to enable the temperature sensor
+    HAL_GPIO_TS_ENABLE_write(THERMISTOR_ENABLE_VALUE);
+
+    // If the temperature sensor is connected, pulling the TS_ENABLE line to its ENABLED value
+    // means we should get a reasonable temperature at this point.
+    value = watch_get_analog_pin_level(HAL_GPIO_TEMPSENSE_pin());
+
+    // value should be >15000 and <55000 and (between -4° and 76° C)
+    if (value < 15000 || value > 55000) has_thermistor = false;
+
+    return has_thermistor;
+}
 
 void thermistor_driver_enable(void) {
-#ifdef HAS_TEMPERATURE_SENSOR
+    if (!has_thermistor) return;
+
     // Enable the ADC peripheral, which we'll use to read the thermistor value.
     watch_enable_adc();
     // Enable analog circuitry on the sense pin, which is tied to the thermistor resistor divider.
@@ -39,11 +83,11 @@ void thermistor_driver_enable(void) {
     HAL_GPIO_TS_ENABLE_out();
     // and make sure it's off.
     HAL_GPIO_TS_ENABLE_write(!THERMISTOR_ENABLE_VALUE);
-#endif
 }
 
 void thermistor_driver_disable(void) {
-#ifdef HAS_TEMPERATURE_SENSOR
+    if (!has_thermistor) return;
+
     // Disable the ADC peripheral.
     watch_disable_adc();
     // Disable analog circuitry on the sense pin to save power.
@@ -51,11 +95,11 @@ void thermistor_driver_disable(void) {
     HAL_GPIO_TEMPSENSE_off();
     // Disable the enable pin's output circuitry.
     HAL_GPIO_TS_ENABLE_off();
-#endif
 }
 
 float thermistor_driver_get_temperature(void) {
-#ifdef HAS_TEMPERATURE_SENSOR
+    if (!has_thermistor) return (float) 0xFFFFFFFF;
+
     // set the enable pin to the level that powers the thermistor circuit.
     HAL_GPIO_TS_ENABLE_write(THERMISTOR_ENABLE_VALUE);
     // get the sense pin level
@@ -64,7 +108,4 @@ float thermistor_driver_get_temperature(void) {
     HAL_GPIO_TS_ENABLE_write(!THERMISTOR_ENABLE_VALUE);
 
     return watch_utility_thermistor_temperature(value, THERMISTOR_HIGH_SIDE, THERMISTOR_B_COEFFICIENT, THERMISTOR_NOMINAL_TEMPERATURE, THERMISTOR_NOMINAL_RESISTANCE, THERMISTOR_SERIES_RESISTANCE);
-#else
-    return (float) 0xFFFFFFFF;
-#endif
 }

watch-library/shared/driver/thermistor_driver.h

@@ -35,6 +35,7 @@
 #define THERMISTOR_NOMINAL_RESISTANCE (10000.0)
 #define THERMISTOR_SERIES_RESISTANCE (10000.0)
 
+bool thermistor_driver_init(void);
 void thermistor_driver_enable(void);
 void thermistor_driver_disable(void);
 float thermistor_driver_get_temperature(void);