// Arduino Mega: read TWO 555 OUT signals on D2 and D3 (INT0/INT1)
// Counts edges on CHANGE for 1-second windows and prints frequency per channel
// Auto unit printing: Hz / kHz / MHz

const byte IN1_PIN = 2;   // 555 #1 OUT -> D2 (INT0)
const byte IN2_PIN = 3;   // 555 #2 OUT -> D3 (INT1)

volatile unsigned long edges1 = 0;
volatile unsigned long edges2 = 0;

// ---- ISRs: keep them short (increment only) ----
void ISR_edge1() { edges1++; }
void ISR_edge2() { edges2++; }

// ---- Helper: print frequency with automatic units ----
void printAutoHz(float hz) {
  if (hz >= 1.0e6f) {
    Serial.print(hz / 1.0e6f, 3); Serial.print(F(" MHz"));
  } else if (hz >= 1.0e3f) {
    Serial.print(hz / 1.0e3f, 3); Serial.print(F(" kHz"));
  } else {
    Serial.print(hz, 2);          Serial.print(F(" Hz"));
  }
}

void setup() {
  Serial.begin(115200);
  pinMode(IN1_PIN, INPUT_PULLUP);
  pinMode(IN2_PIN, INPUT_PULLUP);

  // Count both edges from the 555 square wave
  attachInterrupt(digitalPinToInterrupt(IN1_PIN), ISR_edge1, CHANGE);
  attachInterrupt(digitalPinToInterrupt(IN2_PIN), ISR_edge2, CHANGE);

  Serial.println(F("Dual 555 frequency monitor (D2 & D3, CHANGE mode)"));
  Serial.println(F("CH1=D2, CH2=D3"));
}

void loop() {
  static unsigned long tPrev = millis();
  const unsigned long now = millis();
  const unsigned long elapsed = now - tPrev;

  if (elapsed >= 1000UL) {
    tPrev += 1000UL;  // keeps schedule tight; reduces long-term drift

    // ---- Atomic snapshots ----
    noInterrupts();
    unsigned long e1 = edges1; edges1 = 0;
    unsigned long e2 = edges2; edges2 = 0;
    interrupts();

    // CHANGE counts two edges per cycle; compute using actual elapsed time
    const float seconds = (float)elapsed * 0.001f;
    const float freq1 = (seconds > 0) ? ( (e1 * 0.5f) / seconds ) : 0.0f;
    const float freq2 = (seconds > 0) ? ( (e2 * 0.5f) / seconds ) : 0.0f;

    // ---- Print nicely with auto units ----
    Serial.print(F("CH1: "));
    printAutoHz(freq1);
    Serial.print(F("    CH2: "));
    printAutoHz(freq2);
    Serial.println();
  }
}