add 3rd variant

wolke/fan3/fan3.ino

@@ -0,0 +1,57 @@
+// Frequency Measurement on 1 PIN using hardware interrupts
+// and measuring actual duratations (period lengths)
+
+// http://www.atmel.com/Images/Atmel-2549-8-bit-AVR-Microcontroller-ATmega640-1280-1281-2560-2561_datasheet.pdf
+// https://www.arduino.cc/en/Hacking/PinMapping2560
+
+/*
+82  PK7 ( ADC15/PCINT23 )   Analog pin 15
+83  PK6 ( ADC14/PCINT22 )   Analog pin 14
+84  PK5 ( ADC13/PCINT21 )   Analog pin 13
+85  PK4 ( ADC12/PCINT20 )   Analog pin 12
+86  PK3 ( ADC11/PCINT19 )   Analog pin 11
+87  PK2 ( ADC10/PCINT18 )   Analog pin 10
+88  PK1 ( ADC9/PCINT17 )    Analog pin 9
+89  PK0 ( ADC8/PCINT16 )    Analog pin 8
+
+Vector No.  Program Address Source  Interrupt Definition
+12          $0016           PCINT2  Pin Change Interrupt Request 2
+*/
+
+#define PIN_COUNT 8
+static const int PINS[PIN_COUNT] = {A8, A9, A10, A11, A12, A13, A14, A15};
+
+void enable_pci () {
+    cli();        // switch interrupts off while messing with their settings  
+    PCICR = 0x04; // Enable Pin-Change Interrupts on Block 3
+    PCMSK2 = 0b11111111;
+    sei();        // turn interrupts back on    
+
+    for (int i = 0; i < PIN_COUNT; ++i) {
+        pinMode(PINS[i], INPUT_PULLUP);
+    }
+}
+
+void setup () {
+    Serial.begin(115200);
+
+    // enable pin change mode interrupts on A8-A15
+    enable_pci();
+}
+
+int counter = 0;
+
+void loop () {
+    Serial.print(counter);
+    Serial.println();
+    counter += 1;
+
+    delay(500);
+}
+
+ISR(PCINT2_vect) {
+    // Interrupt service routine. Every single PCINT16..23 (= A8..A15) change
+    // will generate an interrupt: but this will always be the same interrupt routine
+    Serial.print("hello");
+    Serial.println();
+}