Some battery saving improvements

Got it working on the nrf52840 breakout board!

code/parasite/lib/parasite/parasite/pwm.cpp

@@ -16,20 +16,24 @@ namespace {
 constexpr unsigned long kPWMFrequencyPrescale = PWM_PRESCALER_PRESCALER_DIV_1;
 }  // namespace
 
-void SetupSquareWave(double frequency, int pin_number) {
+SquareWaveGenerator::SquareWaveGenerator(double frequency, int pin_number)
-  // In conjunction with the PWM clock frequency, kPWMMaxCounter defines the
+    : frequency_(frequency), pin_number_(pin_number) {
+  // In conjunction with the PWM clock frequency, max_count defines the
   // frequency of the square wave.
   const int max_count = 16e6 / frequency;
 
   // Since we want a duty cycle of 0.5, we flip the PVM output when the counter
-  // reaches kPWMMaxCounter / 2;
+  // reaches max_count / 2;
   const int flip_at_count = max_count / 2;
 
   HwPWM0.addPin(pin_number);
   HwPWM0.setClockDiv(kPWMFrequencyPrescale);
   HwPWM0.setMaxValue(max_count);
   HwPWM0.writePin(pin_number, flip_at_count);
-  HwPWM0.begin();
 }
 
+void SquareWaveGenerator::Start() { HwPWM0.begin(); }
+
+void SquareWaveGenerator::Stop() { HwPWM0.stop(); }
+
 }  // namespace parasite

code/parasite/lib/parasite/parasite/pwm.h

@@ -10,5 +10,16 @@ namespace parasite {
 // and reusable.
 void SetupSquareWave(double frequency, int pin_number);
 
+class SquareWaveGenerator {
+ public:
+  SquareWaveGenerator(double frequency, int pin_number);
+  void Start();
+  void Stop();
+
+ private:
+  double frequency_;
+  int pin_number_;
+};
+
 }  // namespace parasite
 #endif  // _PARASITE_PWM_H_

code/parasite/src/main.cpp

@@ -7,16 +7,19 @@
 #include "parasite/ble_advertisement_data.h"
 #include "parasite/pwm.h"
 
-constexpr int kLED1Pin = 17;
+// variants/feather_nrf52840_express/variant.cpp
-constexpr int kLED2Pin = 18;
+constexpr int kLED1Pin = 17;             // P0.28
-constexpr int kPWMPin = 19;
+constexpr int kLED2Pin = 18;             // P0.02
-constexpr int kSoilAnalogPin = 4;  // AIN2
+constexpr int kPWMPin = 33;              // 0.09
-constexpr int kBattAnalogPin = 3;  // AIN3
+constexpr int kSoilAnalogPin = 21;       // P0.31, AIN7
-constexpr int kDischargeEnablePin = 16;
+constexpr int kBattAnalogPin = 15;       // P0.05, AIN3
+constexpr int kDischargeEnablePin = 16;  // P0.30;
 constexpr double kPWMFrequency = 500000;
 constexpr int kSoilMonitorAirVal = 680;
 constexpr int kSoilMonitorWaterVal = 60;
 
+// parasite::SquareWaveGenerator square_wave_generator(kPWMFrequency, kPWMPin);
+
 const parasite::MACAddr kMACAddr = {0x01, 0x02, 0x03, 0x04, 0x05, 0x06};
 parasite::BLEAdvertiser advertiser(kMACAddr);
 
@@ -49,21 +52,31 @@ void updateAdvertisingData(parasite::BLEAdvertiser* advertiser,
  * #define configTIMER_TASK_STACK_DEPTH                             (1024)
  */
 void timer_cb(TimerHandle_t timer_handle) {
-  Serial.println("timer!");
+  parasite::SquareWaveGenerator square_wave_generator(kPWMFrequency, kPWMPin);
+
   digitalToggle(kLED1Pin);
 
-  double battery_voltage = batt_monitor.Read();
+  square_wave_generator.Start();
-  Serial.printf("Batt voltage: %f\n", battery_voltage);
+  digitalWrite(kDischargeEnablePin, HIGH);
 
   parasite::soil_reading_t soil_reading = soil_monitor.Read();
-  Serial.printf("Moisture val: %d, %f%%\n", soil_reading.raw,
+  // Serial.printf("Moisture val: %d, %f%%\n", soil_reading.raw,
-                100 * soil_reading.parcent);
+  //               100 * soil_reading.parcent);
+
+  square_wave_generator.Stop();
+  digitalWrite(kDischargeEnablePin, LOW);
+
+  double battery_voltage = batt_monitor.Read();
+  // Serial.printf("Batt voltage: %f\n", battery_voltage);
 
   updateAdvertisingData(&advertiser, soil_reading, battery_voltage);
+
+  // Keep adversiting for 1 second.
   delay(1000);
+
   advertiser.Stop();
 
-  // TODO(rbaron): stop PWM; stop everything to save battery.
+  // TODO(rbaron): what else can we turn off to save battery?
 }
 
 void setup() {
@@ -71,12 +84,6 @@ void setup() {
   pinMode(kLED1Pin, OUTPUT);
   pinMode(kDischargeEnablePin, OUTPUT);
 
-  // Activate the PWM signal.
-  parasite::SetupSquareWave(kPWMFrequency, kPWMPin);
-
-  // Enable fast discharge cycle.
-  digitalWrite(kDischargeEnablePin, HIGH);
-
   timer.begin(2000, timer_cb, /*timerID=*/nullptr, /*repeating=*/true);
   timer.start();
 

resources.md

@@ -132,6 +132,18 @@ int sens_val = analogRead(kSensAnalogPin);
 ```
 I'm getting ~680 when in the air; ~65 while holding the sensor. The default resolution is 10 bits, so 1024 -> 3.3V. On the scope, I'm reading the sensor output value to be 2.16V. This matches the value I'm reading: 1024/3.3 * 2.15 = 667. Nice.
 
+The Vcc voltage seems to have an impact though (with nrf52840):
+Vcc = 3.5 => Moisture Air: 668; Hand: 65
+Vcc = 3.0 => Moisture Air: 640; Hand: 53
+Vcc = 2.5 => Moisture Air: 605; Hand: 32
+Vcc = 2.0 => Moisture Air: 547; Hand: 23
+
+Possible issues:
+* Fast discharging circuit is not fully open with lower voltage?
+
+Possible fixes:
+* Hack a software interpolation
+
 # Battery
 * [CR2032 datasheet](https://data.energizer.com/pdfs/cr2032.pdf)
   * 235 mAh (from 3.0V to 2.0V)