Everything You Need to Know about 74LS74 Dual D Flip-Flop

Flip-flops or latches are integrated circuit components with two stable states, making them essential for storing data in sequential logic systems. Among the various flip-flop ICs available are the basic set-reset latches, JK flip-flops, and D flip-flops, to name a few. The 74LS74 IC, the focal point of our discussion, falls under the D flip-flop category. Not only does it boast broad operational capabilities, but it also supports an extensive range of operating voltages. It's often termed a bistable multivibrator due to its characteristics.

The 74LS74 is a dual D flip-flop encompassing two standalone positive-edge-triggered D flip-flops, each with its complementary output. This article will explore its pin configuration, features, applications, equivalents, working principle, and associated circuit designs.

What is 74LS74?

The 74LS74 IC belongs to the dual D-type edge-triggered class of flip-flops, equipped with clear preset capabilities and complementary output terminals. This IC can retain data in binary form and possesses the flexibility to modify the stored data when necessary. It showcases a high operating voltage range of diverse functional prerequisites and facilitates seamless interfacing with CMOS, TTL, and NMOS systems. When a low level is present at the preset input terminal, the output is adjusted according to the given inputs' logic levels. This D-type flip-flop is also referred to as a data or delay flip-flop. It is designed to capture the D input value at specific points within a signal or clock cycle, such as the clock signal's falling edge. The data present at the D input is passed through the flip-flop at the pulse's positive edge.

Overview of 74LS74 Dual-D Flip-flop

Edge-triggered D Flip-Flop Structure:

When operating a negative edge-triggered master-slave flip-flop, the input signal must be applied before the positive edge. If interference signals appear at the input during the high level of CP, the state of the flip-flop might be corrupted. On the other hand, the edge-triggered flip-flop allows the input signal to be introduced just a moment before the triggering edge of CP. In this way, the time during which the input can be interfered with is greatly reduced, decreasing the possibility of interference. The edge-triggered D flip-flop is also called the hold-block edge-triggered D flip-flop.

Circuit Structure: This flip-flop comprises 6 NAND gates, of which G1 and G2 form the basic RS flip-flop.

Working Principle:

SD and RD are connected to the input terminals of the basic RS flip-flop, representing the preset and reset terminals, respectively, and are active low. When SD=0 and RD=1, regardless of the state of input D, it will set Q=1 and Q=0, meaning the flip-flop is set to 1. When SD=1 and RD=0, the flip-flop state is 0. SD and RD are often referred to as direct set and reset terminals. Assuming they both have a high level applied, this doesn't affect the circuit's operation. The working process is as follows:

1. When CP=0, the NAND gates G3 and G4 are blocked, producing outputs Q3=Q4=1, so the flip-flop state remains unchanged. Meanwhile, these two gates are activated due to the feedback signals from Q3 to G5 and Q4 to G6, allowing the input signal D to be received. Thus, Q5=D and Q6=Q5=D.

2. When CP transitions from 0 to 1, the flip-flop toggles. At this time, G3 and G4 are activated, and the outputs of G5 and G6 determine their inputs Q3 and Q4. We have Q3=Q5=D, and Q4=Q6=D. Based on the logical functionality of the basic RS flip-flop, it's inferred that Q=D.

3. After the flip-flop toggles, the input signal is blocked when CP=1. This is because when G3 and G4 are open, the outputs Q3 and Q4 are complementary, meaning one is 0. If Q3 is 0, the feedback from the output of G3 to the input of G5 blocks G5, thus blocking the path from D to the basic RS flip-flop. This feedback line maintains the flip-flop at 0 and prevents it from transitioning to 1, thus termed the '0-hold' and '1-block' lines. When Q4 is 0, both G3 and G6 are blocked, and the path from D to the basic RS flip-flop is also blocked. The feedback from Q4 output to G6 maintains the flip-flop at 1, termed the '1-hold' line. Feedback from Q4 output to G3 input blocks the flip-flop from resetting, termed the '0-block' line. Therefore, this flip-flop is often referred to as a hold-block flip-flop. 

In summary, this flip-flop accepts the input signal before the positive edge of CP, toggles at the positive edge, and blocks the input immediately after the positive edge. All these steps are completed after the positive edge, called an edge-triggered flip-flop. Compared to master-slave flip-flops, an edge-triggered flip-flop of the same process has stronger anti-interference capabilities and a higher operating speed.

74LS74 Pinout

Pin Configuration of 74LS74

Pin Number	Pin Symbol	Name	Description
5,9	1Q / 2Q	Output	Output Pin of the Flip Flop
6,8	1Q’(bar) / 2Q’(bar)	Complementary Output	Inverted output pin of Flip Flop
3, 11	1CLK / 2CLK	Clock Input Pin	These pins must be provided with clock pulse for the flip flop
1,13	1CLR (bar) / 2CLR (bar)	Clear Data	Resets the flip flop by clearing its memory
2,12	1D /2D	Data Input Pin	Input pin of the Flip Flop
4, 10	1PRE (bar) / 2PRE (bar)	PRE Input	Another Input pin for Flip Flop. Also referred to as a set pin
7	Vss	Ground	Connected to the ground of the system
14	Vdd/Vcc	Supply Voltage	Powers the IC typically with 5V

74LS74 Features

• Dual D-Type Flip Flop IC Package

• Supply Voltage Range: 2V to 15V

• Signal Transition Time: 40nS

• Threshold for High-Level Input: 2 V

• Upper Limit for Low-Level Input: 0.8V

• Temperature Range for Operation: 0 to 70°C

• Output Current at High Level: 8mA

• Packaging Options: 14-pin SO-14, SOT42

• Its input is bipolar, whereas the output employs a push-pull

74LS74 Application

• Shift Registers

• Memory or Control Registers

• Buffering Circuits

• Sampling Circuitry

• Latch Mechanisms

• It's employed across various latching device types

• Found in televisions

• Integrated into laptops and desktop computers

• Frequently utilized in digital electronics and networking tools

74LS74 Equivalent

The equivalents for 74LS74 include:

• 74LVC2G80

• HEF40312B

How to Use 74LS74

Operating a Flip-Flop is straightforward. Power the IC using the Vcc and GND pins. As mentioned earlier, each flip-flop functions independently. Connect the input signals to pins 2 and 3 to engage the first flip-flop, with the output reflected on pins 5 and 6. Pin 3 requires a clock source, typically a PWM signal from an MCU or a 555 timer. Activating the pin to a high state will clear the data and reset the flip-flop. For clarity on its operation, refer to the function table provided below. In this table, "X" denotes "irrelevant," and the up-arrow symbolizes the rising edge of a signal.

74ls74 Truth Table

INPUTS				OUTPUTS
PRE (bar)	CLR (bar)	CLK	D	Q	Q (bar)
L	H	X	X	H	L
H	L	X	X	L	H
L	L	X	X	H	H
H	H		H	H	L
H	H		L	L	H
H	H	L	X	Q0	Q0 (bar)

Note

Ensure pin3 is always supplied with a clock source, such as a PWM signal from a 555 timer or an MCU. Additionally, setting this pin high lets you reset the flip-flop and clear its data.

Simulation

Simulation is a useful method to verify the functionality of the flip-flop IC. Initially, employing logic bits and monitoring the logic status can help assess the standard operation of the IC. Nonetheless, validating with a real-world circuit can also be advantageous for confirming its specifications.

74LS74 Connection Diagram

Why is D flip-flop better?

The D flip-flop holds more prominence compared to other flip-flop variants. It ensures that the inputs S and R never possess the same value simultaneously. Gated flip-flops are employed in crafting delay flip-flops with an inverse linkage between their inputs.

74LS74 Package

74ls74 Datasheet

Download Datasheet PDF from Texas Instruments

Conclusion

To sum it up, the 74LS74 stands out as a rapid D-type flip-flop that triggers on the rising edge. Part of the 74XXYY IC series, it meshes well with microcontrollers and TTL devices due to its Schottky TTL architecture.

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