Silly timer implementation

I'm starting to explore ways to get into sleep. It seems like I need to:
1. Set/schedule an RTC interrupt
2. Get into the System ON sleep mode

code/parasite/src/main.cpp

@@ -24,6 +24,8 @@ parasite::BatteryMonitor batt_monitor(kBattAnalogPin);
 parasite::SoilMonitor soil_monitor(kSoilMonitorAirVal, kSoilMonitorWaterVal,
                                    kSoilAnalogPin);
 
+SoftwareTimer timer;
+
 void updateAdvertisingData(parasite::BLEAdvertiser* advertiser,
                            const parasite::soil_reading_t& soil_reading,
                            double battery_voltage) {
@@ -40,9 +42,10 @@ void updateAdvertisingData(parasite::BLEAdvertiser* advertiser,
   }
 }
 
+void timer_cb(TimerHandle_t timer_handle) { Serial.println("timer!"); }
+
 void setup() {
   Serial.begin(9600);
-
   pinMode(kLED1Pin, OUTPUT);
   pinMode(kDischargeEnablePin, OUTPUT);
 
@@ -51,6 +54,12 @@ void setup() {
 
   // Enable fast discharge cycle.
   digitalWrite(kDischargeEnablePin, HIGH);
+
+  timer.begin(5000, timer_cb, /*timerID=*/nullptr, /*repeating=*/true);
+  timer.start();
+
+  // waitForEvent();
+  // suspendLoop();
 }
 
 void loop() {
@@ -61,9 +70,5 @@ void loop() {
   Serial.printf("Moisture val: %d, %f%%\n", soil_reading.raw,
                 100 * soil_reading.parcent);
 
-  digitalToggle(kLED1Pin);
-
   updateAdvertisingData(&advertiser, soil_reading, battery_voltage);
-
-  delay(500);
 }

resources.md

@@ -212,4 +212,30 @@ Question: parasite will advertise a _lot_ of packets in short bursts. How is thi
 # OTA
 
 # Measuring current consumption
-* Good [issue](https://github.com/atc1441/ATC_MiThermometer/issues/134) on the xiaomi sensor tracker
+* Good [issue](https://github.com/atc1441/ATC_MiThermometer/issues/134) on the xiaomi sensor tracker
+
+# Data persistence
+* Adafruit has a internal filesystem implementation. See [example](https://github.com/adafruit/Adafruit_nRF52_Arduino/blob/master/libraries/InternalFileSytem/examples/Internal_ReadWrite/Internal_ReadWrite.ino). Here's the [library implementation](https://github.com/adafruit/Adafruit_nRF52_Arduino/tree/master/libraries/InternalFileSytem/src/flash)
+* Adafruit's [LittleFS](https://github.com/adafruit/Adafruit_nRF52_Arduino/tree/master/libraries/Adafruit_LittleFS)
+* Examples of using [fstorage](https://infocenter.nordicsemi.com/index.jsp?topic=%2Fcom.nordic.infocenter.sdk5.v15.0.0%2Flib_fstorage.html) from nordic: [link](https://github.com/NordicPlayground/nRF5-flash-storage-examples).
+* Example from the SDK: `flashwrite` in examples\peripheral\flashwrite
+* [sd_flash_write](https://infocenter.nordicsemi.com/index.jsp?topic=%2Fcom.nordic.infocenter.s132.api.v3.0.0%2Fgroup___n_r_f___s_o_c___f_u_n_c_t_i_o_n_s.html)
+
+Questions:
+* What addresses are safe to write to? We need to avoid:
+  * Bootloader
+  * SoftDevice
+  * Application
+  * More sections?
+
+# Deep sleep
+* [Adafruit_nRF52_Arduino](https://github.com/adafruit/Adafruit_nRF52_Arduino/blob/master/cores/nRF5/wiring.h#L34) has a waitForEvent function, which internally calls [sd_app_evt_wait](https://github.com/adafruit/Adafruit_nRF52_Arduino/blob/master/cores/nRF5/wiring.c#L101).
+* System ON/OFF. System ON mode is a power saving mode in which the RTCounters are active, so they can emit events for waking up the CPU. System off has no RTC active.
+* the RTC COMPARE event can be used to wake up. See the examples/peripheral/rtc.
+* [app_timer on nordicsemi](https://devzone.nordicsemi.com/f/nordic-q-a/46031/need-a-30-second-interrupt---using-rtc)
+* [Adafruit_nRF52_Arduino issue](https://github.com/adafruit/Adafruit_nRF52_Arduino/issues/165)
+  * `delay()` calls waitForEvent?
+  * [suspendLoop](https://github.com/adafruit/Adafruit_nRF52_Arduino/blob/master/libraries/Bluefruit52Lib/examples/Peripheral/beacon/beacon.ino#L60) can probably save energy when no deep sleep is used.
+
+# Arduino specifics
+* How `main` works by calling `setup` and `loop`. [Link for Adafruit](https://github.com/adafruit/Adafruit_nRF52_Arduino/blob/4d703b6f38262775863a16a603c12aa43d249f04/cores/nRF5/main.cpp#L74), [link for arduino-nrf5](https://github.com/sandeepmistry/arduino-nRF5/blob/master/cores/nRF5/main.cpp#L27). Or: why dropping the `setup` and `loop` doesn't work directly, as they do with ESP32.