* https://randomnerdtutorials.com/interrupts-timers-esp8266-arduino-ide-nodemcu/
  * https://circuits4you.com/2017/12/08/esp8266-external-interrupt-example/
  * https://www.switchdoc.com/2015/10/iot-esp8266-timer-tutorial-arduino-ide/
  * https://arduino-esp8266.readthedocs.io/en/latest/reference.html#timing-and-delays
  * https://www.linksprite.com/wiki/index.php?title=Audio_Jack_Modem_Arduino_Shield_for_iPhone_and_Android
  * https://www.st.com/resource/en/application_note/an4503-environmental-sensors-hardware-abstraction-layer-for-android-stmicroelectronics.pdf

===== Misc tips =====
  * software restart: ''void reset() { asm volatile ("jmp 0"); }''