10/20/2009 Reading: Hambley Chapters 7.4-7 - EECS …ee40/fa09/lectures/Lec_1… · ·...

transcript

EE40 Lec 15

Logic Synthesis and

S CSequential Logic Circuits

P f N h ChProf. Nathan Cheung10/20/2009

Reading: Hambley Chapters 7.4-7.6Karnaugh Maps: Read following before reading textbook

EE40 Fall 2009 Prof. Cheung

Karnaugh Maps: Read following before reading textbookhttp://www.facstaff.bucknell.edu/mastascu/eLessonsHTML/Logic/Logic3.html

Synthesis of Logic CircuitsSuppose we are given a truth table for a logic function.

Is there a method to implement the logic function using basic logic gates?

Answer: There are lots of ways, but one way is the “ f d t ” (SOP) th d“sum of products” (SOP) method:

1) Write the sum of products expression based on the ) p ptruth table for the logic function

2) Implement this expression using standard logic gates.

• An alternative way is the “product of sums” (POS) method.

Logic Synthesis Example: AdderS S

A B C S1 S0

S1= carry, So=sum

0 0 0 0 00 0 1 0 1Truth Table of Adding

Three Inputs :0 1 0 0 10 1 1 1 0

Three Inputs : A, B, and C

0 1 1 1 01 0 0 0 11 0 1 1 01 1 0 1 0

1 1 1 1 1

Logic Synthesis Example: Adder

Sum-of-products method for S1

1) Find rows where S1 is 1A B C S1 S0

Input Output

1) Find rows where S1 is 1

2) Write down each product of inputs which create a 1 (invert

A B C S1 S0

0 0 0 0 00 0 1 0 1 inputs which create a 1 (invert

logic variables that are 0 in that row)

0 0 1 0 10 1 0 0 10 1 1 1 01 0 0 0 1 A B C A B C

A B C A B C

1) Sum all of the products1 0 1 1 01 1 0 1 0

A B C + A B C + A B C + A B C

2) Draw the logic circuit1 1 0 1 01 1 1 1 1

A B C + A B C + A B C + A B C

SOP Logic CircuitBC

SOP Logic Circuit

Creating a Better Circuit

What makes a digital circuit better?• Fewer number of gatesFewer number of gates• Fewer inputs on each gate

– multi-input gates are slower

• Let’s see how we can simplify the sum-of-products expression for S to make aproducts expression for S1, to make a better circuit…– Use the Boolean algebra relationsUse the Boolean algebra relations

A B )CC(ABCBABCAABCCABCBABCA+++=

ABCBABCA)(

SOP Simplification

Can we simplify this digital circuit further?

Add in two inversions (signal stays theA B (signal stays the same)

A B This becomes a NANDBC

Apply DeMorgan’s Theorem, i.e. “bubble

hi ”

pushing”

XYZZYX =++

NAND Gate Implementation

• De Morgan’s law tells us that

is the same as

• By definition,

is the same as

All sum-of-products expressions can be implemented with only NAND gates.

Product-of-sums method for S1

1) Find rows where S is 0A B C S1 S0

Input Output

1) Find rows where S1 is 0

2) Write down each sum of inputs which create a 0 (invert logic

0 0 0 0 0

0 0 1 0 1 which create a 0 (invert logic variables that are 1 in that row)

0 0 1 0 1

0 1 0 0 1

0 1 1 1 0 )CBA( ++ )CBA( ++0 1 1 1 0

1 0 0 0 1

)CBA( ++ )CBA( ++

)CBA( ++ )CBA( ++3) Product of the sums1 0 1 1 0

1 1 0 1 0 )CBA)(CBA)(CBA)(CBA( ++++++++

4) Draw the logic circuit1 1 0 1 0

1 1 1 1 1

SOP or POS ?

The Boolean Expression will appear shorter p pp• If the Truth table has less 1’s, SOP• If the Truth Table has less 0’s POS• If the Truth Table has less 0 s, POS

• After Minimization, both methods should give same results , unless there are “don’t care” rows in the Truth Table.

Notations of Hambley Textbook

Row # A B C DSum of Products (SOP)

Row # A B C D0 0 0 0 11 0 0 1 02 0 1 0 1

)7,6,2,0(mD Σ=2 0 1 0 1

3 0 1 1 04 1 0 0 0

Product of Sums (POS)5 1 0 1 06 1 1 0 17 1 1 1 1

)5,4,3,1(MD Π= )5,4,3,1(MD Π

Another Logic Synthesis Example: XOR

A B F0 0 00 1 1

Sum of Products (SOP)

)21(F ΣProduct of Sums (POS)

)30(MF Π0 1 11 0 11 1 0BABAF +=

)2,1(mF Σ=)BA)(BA(F ++=

)3,0(MF Π=

BABAF += )BA)(BA(F ++

Karnaugh Maps

2-variableKarnaugh Map

3-variableKarnaugh Map

4-variable Karnaugh Map

* Arrows show example locations of logic PRODUCTS

Comments on Karnaugh Maps• Required reading• http://www.facstaff.bucknell.edu/mastascu/eLessonsHTM

L/Logic/Logic3 htmlL/Logic/Logic3.html

• You may find more details there than the textbook.

• As the number of variables increases (say >4) it becomes more difficult to see patterns and computer methodsmore difficult to see patterns, and computer methods start to become more attractive.

EE40 ill f l 3 i bl d 4 i bl• EE40 will focus only on 3 variables and 4 variables Karnaugh Maps

Comments on Karnaugh MapsFor a 4-variables map1-cube: 1 square by itself(⇒ logic product of 4 variables)( g p )

2-cube: 2 squares that have a common edge

(⇒ logic product of 3 variables)(⇒ logic product of 3 variables)

4-cube: 4 squares with common edges (⇒ logic product of 2 variables)

8-cube: 8 squares with common edges (⇒ logic product of 1 variable)

Comments on Karnaugh Maps• In locating cubes on a Karnaugh map, the map should be considered

to fold around from top to bottom, and from left to right.

– Squares on the right-hand side are considered to be adjacent to those on the left-hand sidethose on the left hand side.

– Squares on the top of the map are considered to be adjacent to those CDon the bottom.

– Example: 1 100 01 11 10

00pThe four squares in the map corners form a 4-cube AB

4-Variables ExampleF T th T bl d S f P d t• From Truth Table and Sum of ProductsF=Σ m(1,3,4,5,7,10,12,13)

• Converting the row numbers to binary yields 0001 0011• Converting the row numbers to binary yields 0001,0011, 0100 etc..

• Place 1’s into the Karnaugh Mapg

CBDADCBAF ++=

I t O t t

3-Variables Example: Adder

A B C S1 S0

Input Output Simplification of expression for S1:B

0 0 0 0 00 0 1 0 1 00 01 11 10

0 1 0 0 10 1 1 1 0

0 0 0 1 01 0 1 1 1

A0 1 1 1 01 0 0 0 11 0 1 1 0

1 0 1 1 01 1 0 1 0

BC AC AB

S = AB + BC + AC

1 1 1 1 1S1 = AB + BC + AC

Miscellaneous Examples

3-Variable Exercise

4-Variable Exercise

Exercise with “Don’t Cares”

Sequential Logic Circuits• Sequential logic circuits that possess memory

because their present output value depends on i ll t i t lprevious as well as present input values.

Clock Signals• Often, the operation of a sequential circuit is

synchronized by a clock signal :vC(t)

positive-going edge(leading edge)

negative-going edge(trailing edge)

time0TC 2TC

• The clock signal regulates when the circuits respond to new inputs so that operations occur

TC 2TC

respond to new inputs, so that operations occur in proper sequence.

• Sequential circuits that are regulated by a clock

Sequential circuits that are regulated by a clock signal are said to be synchronous.

Flip-Flops• One of the basic building blocks for sequential

circuits is the flip-flop:– A simple flip-flop can be constructed using two

inverters:

0Q1QTwo possible states:

1Q,0Q0Q,1Q

Q,Q* Circuit can remain in either state indefinitely

The S-R (“Set”-“Reset”) Flip-Flop

S QS-R Flip-Flop Symbol:

• Rule 1:

– If S = 0 and R = 0, Q does not change.• Rule 2:

– If S = 0 and R = 1, then Q = 0• Rule 3:

If S = 1 and R = 0 then Q = 1– If S = 1 and R = 0, then Q = 1• Rule 4:

– S = 1 and R = 1 should never occur.

S 1 and R 1 should never occur.

Realization of the S-R Flip-Flop

R S Qn

0 0 Qn-1

0 1 11 0 0

1 1 (not allowed)

XOR and NAND Implementation

Exercise: Timing Diagram of SR flip-flop

R S Qn

0 0 Qn-1

0 1 11 0 01 1 (not allowed)( )

Clocked S-R Flip-Flop

Wh CK 0 di bl th i t R d S• When CK = 0, disables the inputs R and S• When CK = 1, enables inputs R and S

The D (“Delay”) Flip-Flop

D QD Flip-Flop Symbol:CK Q

• The output terminals Q and Q behave just as in the S-R flip-flop.Q h l h th l k i l CK• Q changes only when the clock signal CKmakes a positive transition. CK D Qn

0 × Qn-1

1 × Qn-1

↑ 0 0

↑ 1 1

D Flip-Flop Example (Timing Diagram)CKCK

Registers• A register is an array of flip-flops that is used to

store or manipulate the bits of a digital word.• Example: Serial In Parallel Out Shift Register using D• Example: Serial-In, Parallel-Out Shift Register using D

Flipflops

Q Q QParallel outputs

D0 Q0Data input D1 Q1 D2 Q2

Q0 Q1 Q2Parallel outputs

Cl k i t

Clock input

Shift Register Timing Diagram

J-K Flip Flop

Ripple Counter

Conclusion (Logic Circuits)

• Complex combinational logic functions can be achieved simply by interconnecting NAND gates (or NOR gates).

• Logic gates can be interconnected to form flip-flops.

• Interconnections of flip-flops form registers.• A complex digital system such as a computer

consists of many gates, flip-flops, and registers. Th l i t th b i b ildi bl kThus, logic gates are the basic building blocks for complex digital systems.

10/20/2009 Reading: Hambley Chapters 7.4-7 - EECS …ee40/fa09/lectures/Lec_1… · ·...

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