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D Flip-Flop (Set/Reset)

A full-featured digital memory element that captures data on a clock edge, with asynchronous Set and Reset pins.

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D Flip-Flop (SR)

This is a complete D Flip-Flop featuring both asynchronous Set (S) and Reset (R) pins. These pins allow you to force the flip-flop into a specific state independently of the clock signal.

Logic ComponentsSequential LogicMemory

Overview

The Set/Reset variant is commonly found in shift registers and state machines where the entire circuit must be initialized to a specific binary pattern (e.g., loading a starting value) before clocked operations begin.

Pin Reference

Pin	Type	Description
D	input	Data Input. The logic value to be stored on a clock edge.
CLK	input	Clock Input. Triggers data capture on the rising edge.
S	input	Asynchronous Set. When HIGH, forces Q to HIGH.
R	input	Asynchronous Reset. When HIGH, forces Q to LOW.
Q	digital	The current stored state.
Q'	digital	The inverted stored state (NOT Q).

Configurable Attributes

This component currently has no configurable attributes.

Working Principle

State Transition Table

S (Set)	R (Reset)	CLK (Edge)	D	Q (Next State)	Q' (Next State)
1	0	X	X	1	0
0	1	X	X	0	1
1	1	X	X	Invalid	Invalid
0	0	↑ (Rising)	0	0	1
0	0	↑ (Rising)	1	1	0
0	0	0, 1, ↓	X	No Change (Q)	No Change (Q')

• Asynchronous Overrides: S and R override the clock.
• Invalid State: Activating both Set and Reset simultaneously leads to an undefined or invalid state in physical hardware. The simulator will typically output LOW for both Q and Q' in this state, but it should be avoided.

Wiring Diagram

Example Arduino Code

const int pinD = 2;   // Data Input
const int pinCLK = 3; // Clock Input
const int pinS = 4;   // Set Input
const int pinR = 5;   // Reset Input
const int pinQ = 6;   // State Output

bool lastClkState = LOW;
bool stateQ = LOW;

void setup() {
  pinMode(pinD, INPUT);
  pinMode(pinCLK, INPUT);
  pinMode(pinS, INPUT);
  pinMode(pinR, INPUT);
  pinMode(pinQ, OUTPUT);
}

void loop() {
  bool currentClkState = digitalRead(pinCLK);
  bool setState = digitalRead(pinS);
  bool resetState = digitalRead(pinR);
  
  // Asynchronous Priority
  if (setState == HIGH && resetState == LOW) {
    stateQ = HIGH;
  } 
  else if (resetState == HIGH && setState == LOW) {
    stateQ = LOW;
  }
  // Detect Rising Edge
  else if (setState == LOW && resetState == LOW && currentClkState == HIGH && lastClkState == LOW) {
    stateQ = digitalRead(pinD);
  }
  
  // Drive Output
  digitalWrite(pinQ, stateQ);
  
  lastClkState = currentClkState;
}

Simulation Notes

• The Set and Reset pins act immediately.
• Floating pins cause unpredictable behavior. Connect unused Set/Reset pins to Ground.

Notes / Warnings

• Race Conditions: Ensure S and R are not toggled simultaneously to prevent metastability.

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