Теория управления / Л4-Миль-Мур-СА / pics / 6-sequential logic-31

Overview of chapter

•General memory system

•Building

–a memory element

•Latch; flip-flop (D, S-R and J-K)

–a finite state machine with memory elements

•State representations

–timing

–Tree, trellis, state diagram, state table

–State table minimization

• Registers and counters

Introduction: the concept of memory and sequential

•What do we mean by memory?

A memory should have at least three properties.

1.It should be able to hold a value, for instance 0 or 1.

2.You should be able to read the value that was saved.

3.You should be able to change the value that’s saved.

•What do we mean by sequential circuits?

The outputs of a sequential circuit depend on not only the inputs, but also the state, or the current contents of some memory

– Examples: combination lock, elevators, traffic lights, computers, etc.

Types of logic: combinational

logic where the output depends on current input only

Types of logic: sequential Mealy machine

Output depends on current input and memory content

For binary logic with n memory elements we have 2n states (different contents)

The memory contains all information about the past, necessary to account for the system‘s future behavior

What is next?

•We develope two basic elements with memory:

- Latch, ( Schnappriegel)

as a building block for the Flip-Flop - Flip-Flop ( a circuit with two stable states )

We define the clock signal

Example: Clock frequency = # of periods/second = 1 MHz (Herz)clock period 1 Sec

2 examples of circuits with memory

s

Q

Propagation delay in element

We use the freedom at the NAND input to change the output

Key idea: avoid 00 as input!

force SR = 11 if output must be fixed ( use C = 0 )

For C = 1, RS = D‘D. For C = 0, RS = 11.