Adding documentation on edge detection. Use rising edge for noise pin…

… (as it is low by default, high on noise)

src/RadiationWatch.cpp

@@ -61,7 +61,13 @@ RadiationWatch::RadiationWatch(byte signPin, byte noisePin):
 
 void RadiationWatch::setup()
 {
+  // Enable the ~20kΩ pull-up resistor for radiation and noise
+  // pins.
+  // For pullup resistors on Arduino, see https://www.arduino.cc/en/Tutorial/DigitalPins
   pinMode(_signPin, INPUT_PULLUP);
+  // The noise signal is low by default (no noise), so we could install a pull-down resistor.
+  // However this is not available on Arduino platform, so we goes with a pull-up resistor.
+  // As the PocketGeiger board pull down the noise signal to low if working, this is not an issue.
   pinMode(_noisePin, INPUT_PULLUP);
   // Initialize _countHistory[].
   for(int i = 0; i < HISTORY_LENGTH; i++)

src/RadiationWatch.h

@@ -22,6 +22,7 @@
 #endif
 // Duration of each history array cell (seconds).
 #define HISTORY_UNIT 6
+// Main loop processing period. In milliseconds.
 #define PROCESS_PERIOD 160
 #include "Arduino.h"
 
@@ -94,6 +95,15 @@ class RadiationWatch
     static void _onNoiseHandler();
 };
 
+/* Notes on the PocketGeiger pulse listening.
+ * Raw data of Radiation Pulse: Not-detected -> High, Detected -> Low.
+ * --> We listen on falling edges.
+ * Raw data of Noise Pulse: Not-detected -> Low, Detected -> High.
+ * --> We listen on rising edges.
+ * Source:
+ * https://cdn.sparkfun.com/assets/learn_tutorials/1/4/3/GeigerCounterType5_connect_with_microcomputer.pdf
+ */
+
 /* Secure that the setupInterrupt() is only defined and compiled once.
  * To be able to change the setupInterrupt() during compile time, the
  * function must be define in the header file. Every include from
@@ -108,7 +118,7 @@ class RadiationWatch
  * support. */
 void RadiationWatch::setupInterrupt() {
   enableInterrupt(_signPin, _onRadiationHandler, FALLING);
-  enableInterrupt(_noisePin, _onNoiseHandler, FALLING);
+  enableInterrupt(_noisePin, _onNoiseHandler, RISING);
 }
 #else
 /* Default: Use external interrupts utilizing the attachInterrupt()
@@ -117,7 +127,7 @@ void RadiationWatch::setupInterrupt() {
  * Arduino UNO. */
 void RadiationWatch::setupInterrupt() {
   attachInterrupt(digitalPinToInterrupt(_signPin), _onRadiationHandler, FALLING);
-  attachInterrupt(digitalPinToInterrupt(_noisePin), _onNoiseHandler, FALLING);
+  attachInterrupt(digitalPinToInterrupt(_noisePin), _onNoiseHandler, RISING);
 }
 #endif
 #endif // LIBCALL_RADIATIONWATCH