How to Use 74HC74: Examples, Pinouts, and Specs

The 74HC74 is a dual positive-edge triggered D-type flip-flop with individual data, set, reset, and clock inputs; and Q and Q-bar outputs. This integrated circuit (IC) is widely used in digital electronics for storing and processing binary data. It is a fundamental building block in sequential logic circuits, counters, shift registers, and memory storage devices.

• Digital counters and dividers
• Shift registers
• Data storage
• Synchronous circuits
• Memory registers
• Flip-flop based state machines

• Logic Family: HC
• Operating Voltage Range: 2V to 6V
• High-Level Input Voltage (V_IH): Minimum 2V
• Low-Level Input Voltage (V_IL): Maximum 0.8V
• High-Level Output Current (I_OH): -5.2 mA (max)
• Low-Level Output Current (I_OL): 5.2 mA (max)
• Propagation Delay Time: Approx. 13 ns (V_CC = 4.5V)
• Setup Time: Approx. 20 ns (V_CC = 4.5V)
• Hold Time: Approx. 5 ns (V_CC = 4.5V)
• Operating Temperature Range: -40°C to +125°C

1. Power Supply: Connect the V_CC pin to a positive supply voltage within the 2V to 6V range and the GND pin to the ground of the circuit.
2. Clock Signal: Apply a clock signal to the CLK input(s) of the flip-flop(s) you wish to use. The flip-flop(s) will trigger on the rising edge of this signal.
3. Data Input: Set the desired data bit on the D input(s) that will be stored on the next rising edge of the clock.
4. Set/Reset Inputs: Optionally, use the PR (preset) and CLR (clear) inputs to set or reset the flip-flop outputs directly. These are active-low inputs.
5. Output: The Q and Q̅ outputs will reflect the stored data bit and its complement, respectively.

• Ensure that the power supply voltage is within the specified range to prevent damage.
• Avoid floating inputs by connecting unused set/reset inputs to V_CC (inactive state).
• Minimize noise by keeping the clock signal path short and using proper decoupling capacitors near the V_CC pin.
• To prevent unintended state changes, ensure that the setup and hold times for the data inputs are met relative to the clock signal.

• Unstable Outputs: This can be caused by floating inputs or noise on the clock line. Ensure all inputs are properly connected and that the circuit is well-decoupled.
• Unexpected State Changes: If the flip-flop changes state unexpectedly, check that the setup and hold times for the data inputs are being met.

• Floating Inputs: Connect unused inputs to V_CC or GND as appropriate.
• Noise Issues: Use a low-pass filter or a Schmitt trigger buffer on the clock input to reduce noise sensitivity.
• Power Supply Decoupling: Place a 0.1 µF capacitor close to the V_CC pin to filter out noise on the power supply line.

Q: Can I use the 74HC74 at 5V? A: Yes, the 74HC74 operates within a range of 2V to 6V, so it is compatible with a 5V supply.

Q: What happens if I don't connect the set or reset pins? A: The set (PR) and reset (CLR) pins are active-low inputs. If left unconnected, they can potentially float to a low level and undesirably set or reset the flip-flop. It is best practice to tie these pins to V_CC if they are not used.

Q: How fast can the 74HC74 operate? A: The maximum operating frequency depends on the supply voltage, but it can typically operate at several megahertz. Check the propagation delay specifications in the datasheet for more precise timing information.

// Example code to toggle the state of the flip-flop on an Arduino UNO

const int clockPin = 2; // Connect to 1CLK or 2CLK
const int dataPin = 3;   // Connect to 1D or 2D
const int resetPin = 4;  // Connect to 1CLR or 2CLR (active low)

void setup() {
  pinMode(clockPin, OUTPUT);
  pinMode(dataPin, OUTPUT);
  pinMode(resetPin, OUTPUT);

  // Reset the flip-flop at the start
  digitalWrite(resetPin, LOW);
  delay(10);
  digitalWrite(resetPin, HIGH);
}

void loop() {
  // Set data to HIGH
  digitalWrite(dataPin, HIGH);
  // Toggle the clock to store the data
  digitalWrite(clockPin, HIGH);
  delay(10); // Wait for more than the setup time
  digitalWrite(clockPin, LOW);
  
  // Set data to LOW
  digitalWrite(dataPin, LOW);
  // Toggle the clock to store the data
  digitalWrite(clockPin, HIGH);
  delay(10); // Wait for more than the setup time
  digitalWrite(clockPin, LOW);
  
  delay(1000); // Wait for 1 second
}

This code demonstrates how to toggle the state of the flip-flop using an Arduino UNO. The flip-flop stores the value present on the data pin at the rising edge of the clock signal. The reset pin is used to clear the flip-flop at the beginning of the program. Ensure that the 74HC74's V_CC and GND pins are connected to the Arduino's 5V and GND, respectively.