Sequential Logic Circuits

Introduction to Sequential Logic

Introduction to Sequential Logic

  • Sequential logic circuits have outputs that depend on both:

    • Current inputs

    • Previous states (memory)

  • Key difference from combinational logic: feedback loops

  • Enable creation of memory elements and state machines

  • Essential components:

    • Latches & Flip-Flops (basic memory)

    • Registers (multi-bit storage)

    • Counters (sequential counting)

Latches and Flip-Flops

SR Latch (Set-Reset)

  • Basic memory element

  • Two stable states

  • Cross-coupled NOR gates

  • Asynchronous operation

SR latch
SR latch
S R Q Q’
0 0 Hold Hold
0 1 0 1
1 0 1 0
1 1 Invalid Invalid

Flip-Flops Overview

  • Edge-triggered memory elements

  • Synchronized with clock signal

  • More predictable than latches

  • Four main types:

Type Function
SR Flip-Flop Set-Reset with clock control
D Flip-Flop Data storage (most common)
JK Flip-Flop Improved SR (no invalid state)
T Flip-Flop Toggle on clock edge

D Flip-Flop

  • Data Flip-Flop

  • Stores input value on clock edge

  • Most widely used

  • No invalid states

  • Perfect for registers

D flip-flop
D flip-flop
D CLK Q
0 \(\uparrow\) 0
1 \(\uparrow\) 1
X 0 or 1 Hold

JK Flip-Flop

  • Jack-Kilby Flip-Flop

  • Resolves SR invalid state

  • J = Set, K = Reset

  • JK = 11 toggles output

  • Universal flip-flop

JK flip-flop
JK flip-flop
J K CLK Q
0 0 \(\uparrow\) Hold
0 1 \(\uparrow\) 0
1 0 \(\uparrow\) 1
1 1 \(\uparrow\) Toggle

Registers and Counters

Shift Register

  • Group of flip-flops for multi-bit storage

  • Data shifts left or right on each clock cycle

  • Applications: Serial-to-parallel conversion, delay lines

Shift register as group of flip-flops
Shift register as group of flip-flops

Ring Counter

  • Shift register with feedback

  • Only one bit is high at any time

  • Output rotates through positions

  • Used in timing and control circuits

Ring counter structure
Ring counter structure

Binary Counter

  • Counts in binary sequence

  • Each flip-flop divides frequency by 2

  • Cascaded T flip-flops

  • Applications: Frequency division, timing

Binary counter
Binary counter
CLK Q2 Q1 Q0
0 0 0 0
1 0 0 1
2 0 1 0
3 0 1 1
4 1 0 0
5 1 0 1
6 1 1 0
7 1 1 1

Applications of Sequential Logic

  • Memory Systems

    • RAM, ROM, Cache memory

    • Registers in processors

  • Control Systems

    • State machines

    • Sequence detectors

  • Data Processing

    • Serial-to-parallel conversion

    • Data synchronization

  • Timing and Counting

    • Frequency dividers

    • Digital clocks

    • Event counters

Summary

Key Takeaways

  • Sequential logic enables memory and state-dependent behavior

  • Latches provide basic asynchronous memory

  • Flip-flops offer synchronized, edge-triggered operation

  • Registers store multi-bit data efficiently

  • Counters provide counting and timing functionality

  • These components form the foundation of digital memory systems