[thermistor] Use vcc reference to obtain an accurate value for the potential divider reference

ADC reference source now selected in hw_config

firmware/inc/thermistor.h

@@ -25,6 +25,6 @@
 #ifndef THERMISTOR_H
 #define THERMISTOR_H
 
-float thermistor_voltage_to_temperature(float voltage);
+float thermistor_ratio_to_temperature(float ratio);
 
 #endif /* THERMISTOR_H */

firmware/src/analogue.c

@@ -27,7 +27,7 @@
 #include "system/interrupt.h"
 #include "hw_config.h"
 
-float battery_v = 0.0, thermistor_v = 0.0, solar_v = 0.0;
+float battery_v = 0.0, thermistor_ratio = 0.0, solar_v = 0.0;
 
 #define ADC_GAINF		ADC_GAIN_FACTOR_DIV2
 #define ADC_GAINF_VAL		0.5
@@ -51,13 +51,13 @@ enum adc_phase_t {
 
 void adc_complete_callback(void);
 
-void configure_adc(enum adc_positive_input input)
+void configure_adc(enum adc_positive_input input, enum adc_reference reference)
 {
   struct adc_config config_adc;
   adc_get_config_defaults(&config_adc);
 
   config_adc.clock_source = GCLK_GENERATOR_0;
-  config_adc.reference = ADC_REFERENCE_INT1V;
+  config_adc.reference = reference;
   config_adc.clock_prescaler = ADC_CLOCK_PRESCALER_DIV64;
   config_adc.resolution = ADC_RESOLUTION;
   config_adc.gain_factor = ADC_GAINF;
@@ -96,6 +96,24 @@ enum adc_positive_input adc_get_channel(enum adc_phase_t phase)
     default:	return SOLAR_ADC_CHANNEL;
   }
 }
+/**
+ * Gets the reference to use in the current phase
+ */
+enum adc_reference adc_get_reference(enum adc_phase_t phase)
+{
+  switch (phase) {
+#if BATTERY_ADC
+    case ADC_PHASE_CONVERT_BATTERY:	return BATTERY_ADC_REFERENCE;
+#endif
+#if THERMISTOR_ADC
+    case ADC_PHASE_CONVERT_THERMISTOR:	return THERMISTOR_ADC_REFERENCE;
+#endif
+#if SOLAR_ADC
+    case ADC_PHASE_CONVERT_SOLAR: 	return SOLAR_ADC_REFERENCE;
+#endif
+    default:	return SOLAR_ADC_CHANNEL;
+  }
+}
 /**
  * Assigns the value for the current phase
  */
@@ -109,7 +127,7 @@ void assign_adc_value(enum adc_phase_t phase, float pin_v)
 #endif
 #if THERMISTOR_ADC
     case ADC_PHASE_CONVERT_THERMISTOR:
-      thermistor_v = pin_v / THERMISTOR_ADC_CHANNEL_DIV;
+      thermistor_ratio = pin_v / THERMISTOR_ADC_CHANNEL_DIV;
       break;
 #endif
 #if SOLAR_ADC
@@ -147,7 +165,7 @@ void adc_complete_callback(void) {
   if (!is_adc_sequence_done()) { /* Another channel still to do.. */
 
     /* Start conversion on this channel */
-    configure_adc(adc_get_channel(adc_phase));
+    configure_adc(adc_get_channel(adc_phase), adc_get_reference(adc_phase));
     adc_start_conversion();
   }
 }
@@ -162,7 +180,7 @@ void start_adc_sequence(void)
   adc_phase++;
 
   /* Start conversion on this channel */
-  configure_adc(adc_get_channel(adc_phase));
+  configure_adc(adc_get_channel(adc_phase), adc_get_reference(adc_phase));
   adc_start_conversion();
 }
 
@@ -175,7 +193,7 @@ float get_battery(void)
 }
 float get_thermistor(void)
 {
-  return thermistor_v;
+  return thermistor_ratio;
 }
 float get_solar(void)
 {

firmware/src/data.c

@@ -96,7 +96,7 @@ struct tracker_datapoint* collect_data(void)
   datapoint.battery = get_battery(); /* Will return zero by default */
   datapoint.solar = get_solar();     /* Will return zero by default */
   datapoint.radio_die_temperature = telemetry_si_temperature();
-  datapoint.thermistor_temperature = thermistor_voltage_to_temperature(get_thermistor());
+  datapoint.thermistor_temperature = thermistor_ratio_to_temperature(get_thermistor());
 
   /**
    * ---- Barometer ----

firmware/src/thermistor.c

@@ -25,15 +25,14 @@
 #include <stdio.h>
 #include <math.h>
 
-#define SERIES_RESISTOR 100000
+#define SERIES_RESISTOR 100000  /* 100k */
-#define ADC_MAX_VALUE (1.85)
 
 /**
  * Calculate the temperature corresponding to the voltage read from the potential divider.
  * Note: Uses full Steinhart–Hart equation
- * Parameters: volatge = raw ADC value from 0 - ADC_MAX_VALUE
+ * Parameters: ratio - divider voltage over supply voltage, 0 - 1
  */
-float thermistor_voltage_to_temperature(float voltage)
+float thermistor_ratio_to_temperature(float ratio)
 {
   float resistance;
 
@@ -43,7 +42,7 @@ float thermistor_voltage_to_temperature(float voltage)
   float C = 0.00000008763737806081768;
 
   /* convert the value to resistance */
-  resistance = (SERIES_RESISTOR * ADC_MAX_VALUE) / voltage;
+  resistance = SERIES_RESISTOR / ratio;
   resistance -= SERIES_RESISTOR;
 
   float logR = log(resistance);

firmware/test/tc/analogue_read.h

@@ -39,6 +39,6 @@ __verification__ void analogue_read_tc(void) {
   while (!is_adc_sequence_done());
 
   analogue_read_tc_results.battery = get_battery();
-  analogue_read_tc_results.thermistor = thermistor_voltage_to_temperature(get_thermistor());
+  analogue_read_tc_results.thermistor = thermistor_ratio_to_temperature(get_thermistor());
   analogue_read_tc_results.solar = get_solar();
 }

firmware/test/tc/thermistor_equation.h

@@ -34,5 +34,5 @@ __verification__ void thermistor_equation_tc(void) {
    */
 
   thermistor_equation_tc_results.temperature =
-    thermistor_voltage_to_temperature(thermistor_equation_tc_params.value);
+    thermistor_ratio_to_temperature(thermistor_equation_tc_params.value);
 }