Chapter 2: Sequential Logic Circuits
Introduction
Sequential logic circuits are digital circuits whose outputs depend not only on current inputs, but also on the history of inputs. This memory of past events makes them essential for designing systems like counters, registers, and memory units.
In contrast to combinational logic, sequential logic uses storage elements to maintain state across clock cycles. This chapter introduces the fundamental components of sequential logic circuits and shows how they are used in digital systems.
2.1 Flip-Flops
Flip-flops are bistable elements—circuits that can store one bit of data. They are triggered by clock edges and come in several types:
- SR Flip-Flop: Set-Reset, simplest form, sensitive to invalid input (S=R=1).
- D Flip-Flop: Data or Delay, stores input D on clock edge.
- JK Flip-Flop: More versatile, toggles when both J and K are high.
- T Flip-Flop: Toggle flip-flop, changes state on each clock edge when T is high.
These are the core building blocks for registers, counters, and memory.
2.2 Shift Registers
A shift register is a series of flip-flops connected in a chain so that data can be moved left or right one bit at a time on each clock pulse. Types include:
- Serial-In Serial-Out (SISO)
- Serial-In Parallel-Out (SIPO)
- Parallel-In Serial-Out (PISO)
- Parallel-In Parallel-Out (PIPO)
Shift registers are used in data conversion, delay circuits, and serial communication.
2.3 Counters
Counters are sequential circuits that increment or decrement a binary number with each clock pulse. They can be classified as:
- Asynchronous (Ripple) Counters: Flip-flops triggered in sequence; simpler but slower due to propagation delay.
- Synchronous Counters: All flip-flops triggered simultaneously; faster and more reliable in modern designs.
Counters are found in timers, digital clocks, frequency dividers, and more.
2.4 Memory Elements and Storage Devices
Memory elements store binary data for later retrieval. Basic types include:
- Latch: Level-sensitive storage element
- Flip-Flop: Edge-triggered storage
- Registers: Collections of flip-flops for multi-bit storage
- RAM: Random Access Memory for temporary data storage
These elements enable the design of CPUs, buffers, and cache systems.
Summary
Sequential logic circuits introduce memory and state into digital design. In this chapter, you learned:
- The different types and behaviors of flip-flops
- How shift registers move and store data
- The distinction between asynchronous and synchronous counters
- The function of basic memory components like latches, flip-flops, and RAM
These concepts are essential for designing control units, pipelines, and memory systems in digital hardware.
🧪 MicroSim
âś… Quiz: Check Your Understanding
1. Which flip-flop stores the D input on the rising edge of the clock?
- A) SR Flip-Flop
- B) T Flip-Flop
- C) D Flip-Flop
- D) JK Flip-Flop
Show Answer
Correct answer: C) D Flip-Flop
Explanation: A D Flip-Flop captures and stores the input on the rising edge of the clock.
2. What is the primary difference between asynchronous and synchronous counters?
- A) Asynchronous counters use fewer flip-flops
- B) Synchronous counters use edge-triggered latches
- C) Asynchronous counters have more propagation delay
- D) Synchronous counters count in reverse
Show Answer
Correct answer: C) Asynchronous counters have more propagation delay
Explanation: In asynchronous counters, each flip-flop is triggered by the previous one, causing delays.
3. What kind of shift register loads data in parallel and outputs it serially?
- A) SISO
- B) SIPO
- C) PISO
- D) PIPO
Show Answer
Correct answer: C) PISO
Explanation: A PISO register accepts parallel inputs and shifts them out one bit at a time.
4. Which of the following is level-sensitive rather than edge-triggered?
- A) D Flip-Flop
- B) JK Flip-Flop
- C) T Flip-Flop
- D) Latch
Show Answer
Correct answer: D) Latch
Explanation: Latches respond to the level of the enable signal, not the edge of a clock.