The Eccles-Jordan Flip-Flop

/*
  Flip-flop behaviour and a ripple counter
  Youblob blueprint: The Eccles-Jordan Flip-Flop

  PART 1 exercises a real SR latch you have built on the bench.
  PART 2 models a chain of toggle flip-flops in software so you can SEE why a
  ripple counter is a binary counter, and why its carry takes time to settle.

  Hardware for PART 1:
    D2 -> SET input of your latch
    D3 -> RESET input
    D4 <- Q output (through a divider if your latch runs above 5 V)
*/

const int PIN_SET = 2, PIN_RESET = 3, PIN_Q = 4;
const int NBITS = 4;
bool ff[NBITS];                 // modelled toggle flip-flops

void pulse(int pin) { digitalWrite(pin, HIGH); delay(5); digitalWrite(pin, LOW); }

void setup() {
  Serial.begin(115200);
  pinMode(PIN_SET, OUTPUT); pinMode(PIN_RESET, OUTPUT); pinMode(PIN_Q, INPUT);
  digitalWrite(PIN_SET, LOW); digitalWrite(PIN_RESET, LOW);

  Serial.println(F("--- PART 1: the real latch ---"));
  pulse(PIN_RESET); Serial.print(F("after RESET, Q = ")); Serial.println(digitalRead(PIN_Q));
  pulse(PIN_SET);   Serial.print(F("after SET,   Q = ")); Serial.println(digitalRead(PIN_Q));
  delay(500);
  // no input asserted at all -- if Q is unchanged, the circuit is REMEMBERING
  Serial.print(F("500 ms later, inputs idle, Q = ")); Serial.println(digitalRead(PIN_Q));
  Serial.println(F("Unchanged means it stored a bit with no power to the inputs.\n"));

  Serial.println(F("--- PART 2: ripple counter ---"));
  Serial.println(F("clk  Q3 Q2 Q1 Q0  value  toggles"));
}

void loop() {
  static int clk = 0;
  if (clk > 16) { delay(5000); return; }

  // A toggle flip-flop flips on each rising edge of ITS input; bit n is clocked
  // by bit n-1. That cascade is why it counts in binary -- and why the carry
  // RIPPLES rather than arriving everywhere at once.
  int toggles = 0;
  for (int i = 0; i < NBITS; i++) {
    ff[i] = !ff[i]; toggles++;
    if (ff[i]) break;           // stopped propagating: this stage stayed high
  }

  int value = 0;
  for (int i = 0; i < NBITS; i++) if (ff[i]) value |= (1 << i);

  Serial.print(clk); Serial.print(F("    "));
  for (int i = NBITS - 1; i >= 0; i--) { Serial.print(ff[i]); Serial.print(' '); }
  Serial.print(F(" ")); Serial.print(value);
  Serial.print(F("      ")); Serial.println(toggles);

  clk++;
  delay(400);
}