Corrects the raw ADC soil mesasure given the current input voltage

code/b-parasite/config/prst_config.h

@@ -12,14 +12,15 @@
 // Analog to digital converter (ADC).
 // Prints out ADC debug info, such as the values read for battery and soil
 // moisture.
-#define PRST_ADC_DEBUG 0
+#define PRST_ADC_BATT_DEBUG 0
+#define PRST_ADC_SOIL_DEBUG 0
 
 // BLE.
 // Prints out BLE debug info, such as the final encoded advertisement packet.
 #define PRST_BLE_DEBUG 0
 #define PRST_BLE_PROTOCOL_VERSION 1
 #define PRST_BLE_MAC_ADDR_LSB1 0x00
-#define PRST_BLE_MAC_ADDR_LSB0 0x01
+#define PRST_BLE_MAC_ADDR_LSB0 0x02
 #define PRST_BLE_ADV_NAME "prst"
 // Total time spend advertising.
 #define PRST_BLE_ADV_TIME_IN_MS 1000

code/b-parasite/src/main.c

@@ -59,7 +59,7 @@ static void rtc_callback() {
   prst_pwm_init();
   prst_pwm_start();
   prst_adc_batt_read_t batt_read = prst_adc_batt_read();
-  prst_adc_soil_moisture_t soil_read = prst_adc_soil_read();
+  prst_adc_soil_moisture_t soil_read = prst_adc_soil_read(batt_read.voltage);
   prst_pwm_stop();
   nrf_gpio_pin_clear(PRST_FAST_DISCH_PIN);
   prst_ble_update_adv_data(batt_read.millivolts, temp_humi.temp_millicelcius,

code/b-parasite/src/prst/adc.c

@@ -27,7 +27,7 @@ static nrf_saadc_value_t sample_adc_channel(uint8_t channel) {
 }
 
 // Caps the argument to the [0, 1] range.
-static inline double cap_value(double value) {
+static inline double cap_percentage(double value) {
   return value > 1.0 ? 1.0 : (value < 0.0 ? 0.0 : value);
 }
 
@@ -72,21 +72,48 @@ prst_adc_batt_read_t prst_adc_batt_read() {
   ret.raw = (uint16_t)result;
   ret.voltage = (3.6 * result) / (1 << PRST_ADC_RESOLUTION);
   ret.millivolts = ret.voltage * 1000;
-#if PRST_ADC_DEBUG
-  NRF_LOG_INFO("[adc] Read battery voltage: %d (raw); %d mV; ", ret.raw,
-               ret.millivolts, ret.voltage);
+#if PRST_ADC_BATT_DEBUG
+  NRF_LOG_INFO(
+      "[adc] Read battery voltage: %d (raw); %d mV; " NRF_LOG_FLOAT_MARKER " V",
+      ret.raw, ret.millivolts, NRF_LOG_FLOAT(ret.voltage));
 #endif
   return ret;
 }
 
-prst_adc_soil_moisture_t prst_adc_soil_read() {
-  nrf_saadc_value_t result = sample_adc_channel(PRST_ADC_SOIL_CHANNEL);
-  double percentage = cap_value(((double)result - PRST_SOIL_DRY) /
-                                (PRST_SOIL_WET - PRST_SOIL_DRY));
+// If you got this far and really want to see how the sausage is made,
+// this function estimates the soil moisture percent based on the raw
+// ADC value as returned from the saadc. It assumes 10 bits resolution.
+// Ideally, we're taking the ADC sample relative to the VDD voltage, so
+// this input value should be stable across the range of input voltages.
+// In practice, when varying the input voltage, this value is drifting
+// enough to be annoying. To account for this drift, I collected ADC readings
+// while varying the input voltage from 2V to 3V (CR2032 voltage range) and
+// fitted two second degree polynomials over them - one for the sensor
+// out in the air (representing a dry soil) and one while holding the
+// sensor in my hand (representing a wet soil).
+// This raw data is available at the data/ dir at the root of this repository.
+static inline double get_soil_moisture_percent(
+    double battery_voltage, nrf_saadc_value_t raw_adc_output) {
+  const double x = battery_voltage;
+  const double dry = -12.9 * x * x + 111 * x + 228;
+  const double wet = -5.71 * x * x + 60.2 * x + 126;
+#if PRST_ADC_SOIL_DEBUG
+  NRF_LOG_INFO("[adc] batt: " NRF_LOG_FLOAT_MARKER, NRF_LOG_FLOAT(x));
+  NRF_LOG_INFO("[adc] dry: " NRF_LOG_FLOAT_MARKER " wet: " NRF_LOG_FLOAT_MARKER,
+               NRF_LOG_FLOAT(dry), NRF_LOG_FLOAT(wet));
+#endif
+  return (raw_adc_output - dry) / (wet - dry);
+}
+
+prst_adc_soil_moisture_t prst_adc_soil_read(double battery_voltage) {
+  nrf_saadc_value_t raw_adc_output = sample_adc_channel(PRST_ADC_SOIL_CHANNEL);
+  const double percentage =
+      get_soil_moisture_percent(battery_voltage, raw_adc_output);
   prst_adc_soil_moisture_t ret;
-  ret.raw = result;
+  ret.raw = raw_adc_output;
+  ret.percentage = cap_percentage(percentage);
   ret.relative = percentage * (1 << 16);
-#if PRST_ADC_DEBUG
+#if PRST_ADC_SOIL_DEBUG
   NRF_LOG_INFO("[adc] Read soil moisture: %d (raw); " NRF_LOG_FLOAT_MARKER
                " %% (percentage); %u (relative)",
                ret.raw, NRF_LOG_FLOAT(percentage * 100), ret.relative);

code/b-parasite/src/prst/adc.h

@@ -18,14 +18,15 @@ typedef struct prst_adc_batt_val {
 
 typedef struct prst_adc_soil_moisture {
   int16_t raw;
-  // A value from 0 (cmopletely dry) to 2^10 (completely wet).
+  // A value from 0 (completely dry) to 2^10 (completely wet).
   uint16_t relative;
+  double percentage;
 } prst_adc_soil_moisture_t;
 
 void prst_adc_init();
 
 prst_adc_batt_read_t prst_adc_batt_read();
 
-prst_adc_soil_moisture_t prst_adc_soil_read();
+prst_adc_soil_moisture_t prst_adc_soil_read(double battery_voltage);
 
 #endif  // _PRST_ADC_H_