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74LS74: An All-inclusive Guide of the Dual Flip-flop

Flip-flops/latches are integrated circuits with two stable states storing information in a sequential logic element. Types of flip-flop ICs include simple set-reset latches, JK flip-flops, and D flip-flops, among others.74LS74 IC, our interest of discussion today, is a subtype of the D flip-flop. Besides having a wide operating condition, it can also operate on a wide operating voltage range. As such, you can also refer to it as a bistable multivibrator. 

Our guide will cover its pinout, features, application, equivalents, working principle, and circuitry. 

An implementation of dual-edge-triggered D flip-flop

An implementation of dual-edge-triggered D flip-flop

Source; Wikipedia 

74LS74 Pinout Configuration

The table summarizes the pin configuration of 74LS74 IC

Pin No. Pin SymbolPin NamePin Function/Description
14Vcc/VddSupply voltage It provides power of approximately 5V to the IC. 
7Vss GND/groundIt has a connection to the system’s ground/GND. 
10, 42 PRE (bar)1 PRE (bar)PRE input/ Set pinIt’s an input pin for the IC. 
12, 22D/1DData input pin It’s also an input pin of the IC flip-flop.
13, 1 2 CLR (bar)1 CLR (bar)Clear dataIt clears the flip-flop’s memory hence resetting it. 
11, 3 2 CLK1CLKClock input pinYou must provide a clock pulse for the flip-flops when using the pins. 
8, 61Q’ (bar)2Q’ (bar)Complementary output pinIt’s the inverted output pin of the flip flop. 
9, 52Q/1QOutput pinIt’s the output pin of the flip-flop. 

74LS74 Trigger Table

INPUTSOUTPUTS
PRE (bar)CLR (bar)CLKDQQ (bar)
LHXXHL
HLXXLH
LLXXHH
HHHHL
HHLLH
HHLXQ0Q0 (bar)

X is a representation of ‘Don’t care.’

The up-arrow is an indication of the signal’s rising edge. 

The flip-flop table consists of individual set inputs and precise inputs. In addition, it also has complementary Q (bar) and Q outputs. 

74LS74 Flip-flop Features

74LS74 IC features and specifications are; 

  • First,It has an operating voltage ranging from 2V to 15V. 
  • Secondly, its operating temperature range is from 0°C to 70°C. 
  • Then,It has a bipolar input type while the output type is push-pull. 
  • It also has a high-level output current of 8mA and a propagation delay of 40nS. 
  • Its maximum low-level input voltage is 0.8V whereas, its minimum high-level input voltage is 2V. 
  • Lastly, you can find it in 14-pin SOT42 or SO-14 packages. 

74LS74 Flip-flop Equivalents 

You can also opt for the equivalents of 74LS74 IC like the examples listed here.

  • 74LVC2G80,
  • HEF40312B. 

74LS74 Dual-D Flip-flop Introduction

74LS74 IC falls under double D type edge-triggered flip flops with complementary output and clear preset terminals. D stands for delay or data. 

In other words, the flip flop module operates on features that help get D input value. Often, the value is at a point in the clock cycle or signal, e.g., in the clock signal’s falling edge. Eventually, you’ll get the D input data at the positive end of the pulse via the flip flop. 

It stores data as binary numbers, and one of its features allows for a change in stored data when needed. Its specifications include making direct interfaces with NMOS, TTL, and CMOS and high value of operating voltage range.

Finally, the low level at the input terminal of the preset resets the output value as per the inputs’ logic level. 

How Do We use 74LS74?

Operating a 74LS74 flip flop is simple since it’s straightforward, as we’ll show you in the steps below.

  • Start by powering the IC using GND and Vcc pins. 
  • Then, because each flip flop works autonomously, use the first flip flop to connect input signals 3 and 2. Consequently, you’ll receive an output at pin6 and pin5. 

Note; 

Always provide pin3 with a clock source like a PWM signal from 555 timers or Microcontroller Unit (MCU). Moreover, you can also use the pin to reset the flip flop by raising it and clearing its data. 

555 timer IC

Source; Wikipedia 

The function table below shows the complete operation of the flip-flop. 

INPUTSOUTPUTS
PRE (bar)CLR (bar)CLKDQQ (bar)
LHXXHL
HLXXLH
LLXXHH
HHHHL
HHLLH
HHLXQ0Q0 (bar)

Simulation 

The simulation will help determine if the flip-flop IC works. In the beginning, you can use logic bits and logic status to examine the IC’s standard operation. However, you can also use actual circuitry instead to confirm the requirements. 

You can incorporate the gif illustration below and combine its usage with the function table for a better outcome. 

Gif illustration of practical use of 74LS74 IC

Gif illustration of practical use of 74LS74 IC

Application 

You can find 74LS74 flip flop applications in the following components/devices;

Current buffer circuit 

Current buffer circuit 

Source; Wikipedia

  • Latching devices,
  • Network equipment, 
  • Shift registers,
  • Digital electronics, 
  • PCs, TVs, and notebooks, 
close-up view of a typical desktop computer baseboard

(close-up view of a typical desktop computer baseboard). 

  • Sampling circuits, and 
  • Control or memory registers. 

Conclusion

In summary, 74LS74 is a high-speed D-type flip-flop with positive edge triggering. It belongs to the 74XXYY IC series and can work with microcontrollers and TTL devices (using Schottky TTL circuitry). 

For further queries, kindly reach out to us. 

Hommer Zhao
Hi, I am Hommer, the founder of WellPCB. So far, we have more than 4,000 customers worldwide. If you have any questions, you can feel free to contact me. I really appreciate any help you can provide.

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