ELEC1303 - Lecture 16

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ELEC1303 - Lecture 15
Designing Sequential Circuits - Slides

Introduction

• Combinational circuits $-$ same inputs always produce same outputs
  • No internal memory
• Sequential circuits $-$ has memory elements and combinational logic
  • Outputs depend on inputs and the current state of memory

Two Types of Sequential Circuits

• Synchronous sequential circuit $-$ uses clock signal
  • Changes in memory are controlled by clock
    • Changes happen at discrete times
• Asynchronous sequential circuit $-$ no clock
  • Changes in memory can happen any time

We focus on synchronous circuits as they are easier to analyse and design

Sequence Recognisers

• A sequence recogniser is a special sequential circuit that looks for a special bit pattern in some input
  • One bit of input supplied on every clock cycle
  • Outputs “1” when pattern found

• Circuit needs to remember inputs from previous clock cycles

State Diagram

Sequential Circuit Design Procedure

• From our state table, we can assign flip-flop values
  e.g. we have four states ABCD, so we need two flip-flops Q${}_1$Q${}_0$

• Then assign flip-flop input values (e.g. D, JK, etc.)
  • An excitation table shows what flip-flop inputs are required in order to make a desired change
    
    More excitation tables in slides

Note: for some circuits, you will have unused states in your state table, you can simply fill in the table with don’t cares for the next states

OR, you could fill in the next states to reset to the first state to ensure that if the circuit enters an unused state, it will end up in a valid state

This is known as a self-starting counter

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ELEC1303 - Lecture 17