two’s - Tarleton State University...Moore vs. Mealy Moore and Mealy machines are equivalent, i.e....

transcript

QUIZ Ch. 2

two’s

QUIZ Ch. 2

Chapter 3

REVIEW of Boolean Algebra

and Digital Logic

Name the Boolean functions and draw the gates:

QUIZ: Boolean Algebra

Draw the truth table for the Boolean function:

Complete the Boolean identities:

Use Boolean identities to simplify:

• Sometimes it is more economical to build a

circuit using the complement of a function (and

complementing its result) than it is to implement

the function directly.

• DeMorgan’s law provides an easy way of finding

the complement of a Boolean function.

• Recall DeMorgan’s law states:

3.2 Boolean Algebra

• DeMorgan’s law can be extended to any number of

variables.

• Replace each variable by its complement and

change all ANDs to ORs and all ORs to ANDs.

• Thus, we find the the complement of:

3.2 Boolean Algebra

• Through our exercises in simplifying Boolean

expressions, we see that there are numerous

ways of stating the same Boolean expression.

– These “synonymous” forms are logically equivalent.

– Logically equivalent expressions have identical truth

tables.

• In order to eliminate as much confusion as

possible, designers express Boolean functions in

standardized or canonical form.

3.2 Boolean Algebra

• There are two canonical forms for Boolean

expressions: sum-of-products and product-of-sums.

– Recall the Boolean product is the AND operation and the

Boolean sum is the OR operation.

• In the sum-of-products form, ANDed variables are

ORed together.

– For example:

• In the product-of-sums form, ORed variables are

ANDed together:

– For example:

SOP and POS

Write the SOP (sum-of-products) form and

the gate diagram:

Write the SOP (sum-of-products) form and

the gate diagram:

• We have looked at Boolean functions in abstract

terms.

• In this section, we see that Boolean functions are

implemented in digital computer circuits called gates.

• A gate is an electronic device that produces a result

based on two or more input values.

– In reality, gates consist of one to six transistors, but digital

designers think of them as a single unit.

– Integrated circuits contain collections of gates suited to a

particular purpose.

3.3 Logic Gates

• The three simplest gates are the AND, OR, and NOT

gates.

• They correspond directly to their respective Boolean

operations, as you can see by their truth tables.

3.3 Logic Gates

• Another very useful gate is the exclusive OR

(XOR) gate.

• The output of the XOR operation is true only when

the values of the inputs differ.

Note the special symbol

for the XOR operation.

3.3 Logic Gates

Use DeMorgan to prove that the circuit below

implements the XOR gate:

Image source: Wikipedia →XOR gate

• NAND and NOR

are two very

important gates.

Their symbols and

truth tables are

shown at the right.

3.3 Logic Gates

• NAND and NOR

are known as

universal gates

because they are

inexpensive to

manufacture and

any Boolean

function can be

constructed using

only NAND or only

NOR gates.

3.3 Logic Gates

Gates can have multiple inputs and multiple

outputs.

• E.g., a second output can be provided for

the complement of the operation.

3.3 Logic Gates

• The main thing to remember is that combinations

of gates implement Boolean functions.

• The circuit below implements the Boolean

function:

We simplify our Boolean expressions so

that we can create simpler circuits.

3.3 Logic Gates

3.4 Integrated Circuits (IC)

Problem: Design a circuit using the SSI IC 7400

(Quad NAND) Small Scale Integrated

Integrated Circuits (IC)

Problem: Design a circuit using the SSI IC 7400

3.5 Combinational Circuits

• We have designed a circuit that implements the

Boolean function:

• This circuit is an example of a combinational logic

circuit.

• Combinational logic circuits produce a specified

output (almost) at the instant when input values

are applied.

– In a later section, we will explore circuits where this is

not the case.

• Combinational logic circuits

give us many useful devices.

• One of the simplest is the

half adder, which finds the

sum of two bits.

• We can gain some insight as

to the construction of a half

adder by looking at its truth

table, shown at the right.

Combinational Circuits

• As we see, the sum can be

found using the XOR

operation and the carry

using the AND operation.

• We can change our half

adder into to a full adder

by including gates for

processing the carry bit.

• The truth table for a full

adder is shown at the

right.

• How can we change the

half adder shown below

to make it a full adder?

• Here’s our completed full adder.

• Just as we combined half adders to make a full

adder, full adders can connected in series.

• The carry bit “ripples” from one adder to the next;

hence, this configuration is called a ripple-carry

adder.

Today’s systems employ more efficient adders.

Read and take notes: pp.137-158

(stop before Example 3.1.3 – Decoder)

End-of-chapter Exercises 1, 2, 3, 4, 22

Find the SOP expression and the truth table

for the following circuit:

Design the Full Adder directly, based on the truth table,

using SOP. Compare to the previous design.

QUIZ: Combinational Circuits

Minimize the FA using K-maps.

Use the following layout for each output:

QUIZ: Combinational Circuits

FA solution

Which implementation is best?

(Trick question!)

• Among other things, they are useful in selecting a

memory location according to a binary value placed

on the address lines of a memory bus.

• Address decoders with n inputs can select any of 2n

locations.

Decoders

2-to-4 decoder implementation. Each AND gate

generates a minterm!

If x = 0 and y = 1,

which output line

is enabled?

Decoders

Draw the gate diagram for a 3-to-8 decoder,

with inputs x, y, z.

QUIZ: Decoders

We’re designing a computer with 64 KB of RAM.

How many address bits are needed?

Typical decoder application:

memory addressing circuit

From cost analysis, it turns out that the cheapest

solution uses 8-KB RAM chips.

How many chips are needed?

How many address bits does each chip accept?

How many address bits are needed to distinguish

among the chips?

We implement it with 8 RAM chips, having 8KB each.

The 3 MSB are used to distinguish among the chips.

Write in hex the address range for each of the chips.

• A multiplexer is the opposite

of a decoder.

• It selects a single output

from several inputs.

• The particular input chosen

for output is determined by

the value of the multiplexer’s

control lines.

• To be able to select among n

inputs, log2n control lines are

needed.

Multiplexers

Implementation of a 4-to-1 multiplexer.

Each input is validated by a minterm!

If S0 = 1 and S1 = 0,

which input is

transferred to the

output?

This shifter

moves the bits

of a nibble one

position to the

left or right.

If S = 0, in which

direction do the

input bits shift?

Shifters

Draw the block

diagram of this

shifter.

QUIZ: Shifters

2-bit ALU

Which combination

f1f0 makes the ALU

Hint: Addition

needs Carry.

Individual work for next time

• Read pages 157-162 , and 199-208 (“Focus

on K-maps”)

• Solve in notebook:

– Ex. 48, 49 on p.195

– Ex. 1, 2 on p.209

3.6 Sequential Circuits

• Combinational logic circuits are perfect for

situations when we require the immediate

application of a Boolean function to a set of inputs.

• There are other times, however, when we need a

circuit to change its value with consideration to its

current state as well as its inputs.

– These circuits have to “remember” their current state; they

have memory

• Sequential logic circuits provide this functionality.

To retain their state values, sequential circuits rely

on feedback.

• Example:

– If Q is 0 it will always be 0, if it is 1, it will always be 1.

In asynchronous sequential circuits (a.k.a. latches),

state changes occur whenever the inputs change.

In synchronous sequential circuits (a.k.a. flip-flops),

state changes are controlled by clocks.

• A “clock” is a special circuit that sends electrical

pulses through a circuit.

• Clocks produce “square” electrical waveforms:

• Synchronously: on the rising edge/falling edge,

a.k.a. edge-triggered

• Asynchronously: when the clock pulse is at its

highest/lowest voltage, a.k.a. level-triggered

Sequential Circuits can change state:

You can see how feedback works by examining the

most basic sequential logic components, the SR

latch and flip-flop.

– The “SR” stands for set/reset.

The behavior of an SR latch is described by a

characteristic table:

• Q(t) means the value of the output at time t

• Q(t+1) is the value of Q after the next clock

pulse.

SR latch

QUIZ: Analyzing sequential circuits

What exactly does “undefined” mean here?

• To draw a truth table,

note that the SR flip-flop

actually has three inputs:

S, R, and its current

output, Q.

• Notice the two undefined

values. When both S and

R are 1, the SR flip-flop is

unstable (well, it’s a little

more complicated … see

Digital)

SR latch

QUIZ: Analyzing sequential circuits

Derive the characteristic table for this circuit

Clocked SR latch

Changes can

occur only on the

positive level of

Text calls it

flip-flop

SKIP JK flip-flop

The D flip-flop is the fundamental circuit of

computer memory

D flip-flop

QUIZ: Problem 55/196

• The behavior of sequential circuits can be

expressed using characteristic tables or finite state

machines (FSMs). – FSMs consist of a set of nodes that hold the states of the

machine and a set of arcs that connect the states.

• Moore and Mealy machines are two types of FSMs

that are equivalent. – They differ only in how they express the outputs of the

machine.

• Moore machines place outputs on each node, while

Mealy machines present their outputs on the

transitions.

Moore machine

Mealy machine

Moore vs. Mealy

Moore and Mealy machines are equivalent, i.e. they can be

used to implement circuits with the same behavior, but …

• Moore may need more states than Mealy

• Mealy may need more combinational logic than Moore

Draw the FSM diagram for a Moore machine

• Has one input and one output

• Turns its output to 1 if and only if at the

previous 3 clock “ticks” the input has been

QUIZ: Draw the characteristic table and the FSM

diagram for this circuit.

What function does it perform?

We can trade off flip-flops (memory) for (combinational)

logic: here is the same 4-state counter in the “walking ring”

implementation

QUIZ: If using ICs from the 7400 family, which

implementation requires fewer units?

7400 - quad inverter

7486 - quad XOR

7474 - double D w/positive and negative outputs Q

74174 - hex D w/positive outputs Q

74175 - quad D w/positive and negative outputs Q

• It is difficult to express the complexities of actual

implementations using only Moore and Mealy

machines. – For one thing, they do not address the intricacies of

timing very well.

– Secondly, it is often the case that an interaction of

numerous signals is required to advance a machine from

one state to the next.

• For these reasons, Christopher Clare invented the

algorithmic state machine (ASM).

The next slide illustrates the components of an ASM.

Algorithmic State Machines

ASM for a microwave oven (Moore or Mealy?).

Algorithmic State Machines

Individual work for next time

• Read pages 162-178

• Solve in notebook:

– Ex. 52 p.19

Draw the FSM diagram for a Moore machine

• Has two inputs (x, y) and one output (z).

• Turns its output to 1 if and only if at the

previous 2 clock “ticks” the inputs have

been identical.

Important Sequential

Circuits

4-bit register with D flip-flops

Real-life registers also have CLEAR

and ENABLE/WRITE inputs

They can also SHIFT or ROTATE their

bits (Left or Right)

4-bit binary counters (Use the D version, not the JK given in text!)

Source: http://teahlab.com/4-bit_counter_modulo-16_D_flipflop/

But note that the JK circuit needs less combinational logic!

QUIZ: Assume the counter is in state ‘7’ right

before the positive edge of CLK. Write the states of all

signals in the diagram.

Then write the states

of all signals after the

positive edge.

handout

QUIZ: Assume the counter is in state ‘7’ right

before the positive edge of CLK. Write the states of all

signals in the diagram.

Then write the states

of all signals after the

positive edge.

handout

Small Memory Circuit: four 3-bit words

QUIZ: Write the integer 6 in Word 1

handout

QUIZ: Write the integer 6 in Word 1

handout

Convolutional coding is performed on a string of bits

arriving in real-time (e.g. digital video, digital

telephony, satellite links, reading tape and disk)

The output bit is a function of the current and

previous input bits, therefore we need memory,

i.e. sequential circuits

Convolutional Coders and Decoders

Do you recognize

this combinational

circuit?

Do you recognize

this sequential

circuit?

Back to digital analysis: What is the output Q of this circuit?

Conclusion:

• The flip-flops are driven at half the frequency (i.e.

double the period) of the original CLOCK

• Two bits are output for every one bit input:

– A C from the top AND gate

– A B C from the bottom AND gate

Called (2,1)

convolutional

• It is a particular (2,1) convolutional code

• It is used to convert the (weak, noisy) analog

signal from the head of a magnetic disk or tape

drive into a digital signal.

• You can find more details on PRML at the end of

Chapter 2 (Section 2A.6 - p.134).

Partial Response Maximum Likelihood

Fig. 3.34 + explanations on p.180

→ Read and understand the operation of the PRML

encoder!

Initially all FFs

store 0s (RESET)

First bit Second bit Third bit

Fig. 3.34 – Why are there two zeros at the beginning?

This is the beginning

Continue the

analysis in Fig. 3.34

PRML encoder characteristic table

PRML encoder FSM

Moore or Mealy?

F(1101 0010)=11 01 01 00 10 11 11 10

Starting in state 0,

the encoder receives

the bits shown (from

left to right):

PRML encoding example

F(0100 1111) =

Use the FSM diagram

to encode:

0100 1111

(starting in state 0)

QUIZ: PRML encoding

Decoding is achieved by inverting the inputs and

outputs of the encoder Mealy machine:

PRML decoding

F(11 01 01 00 10 11 11 10) = 1101 0010

We can see from the

decoder FSM that, if no

errors have occurred,

the decoded sequence

matches the one

encoded in the previous

example:

PRML decoding

F(11 01 01 10 10 11 11 10) = 110? ????

What if there are

errors?

Example:

PRML decoding

What is the state of the decoder

when this pair of bits arrives?

F(11 01 01 00 10 11 11 10) = 1101 0010

F(1101 0010) = 11 01 01 00 10 11 11 10

The fact that there is a

limited set of possible

state transitions in the

encoding process is

crucial to the error

correcting capabilities of

We can see from the FSM

that, starting in state 0, we

PRML encoding example

F(11 01 01 10 10 11 11 10) = 110? ????

Conclusion: The error

is detected … but this is

not enough!

The decoder has to

be able to recover and

keep churning out the

PRML decoding

F(11 01 01 00 10 11 11 10) = 1101 0010

F(11 01 01 10 10 11 11 10) = 110? ????

Solution:

Minimize the total error of the

sequence of bits that starts w/the error!

PRML decoding

F(11 01 01 00 10 11 11 10) = 1101 0010

“PR”

“ML”

F(11 01 01 10 10 11 11 10) = 110? ????

Solution:

Minimize the total error of the

sequence of bits that starts w/the error!

PRML decoding

F(11 01 01 00 10 11 11 10) = 1101 0010

“PR”

“ML”

How long a sequence?

From an error-correction standpoint, the longer

the better …

… but longer sequences mean:

• longer decoding delay

• heavier computational load (CPU cycles and

memory)

In this example we use a sequence of 4 pairs

PRML decoding

F(00 10 11 11) = 1001

A more intuitive way of

understanding the

decoding process is

through a lattice,

famously known as

trellis diagram.

This is the trellis diagram

for the correct sequence

starting in state 1

Decoding PRML with Trellis diagrams

F(10 10 11 11) = ????

This is the diagram for

the incorrect sequence

starting in state 1

In the circles we have

the accumulated

(Hamming) errors

The path of least error

outputs 1001, thus 1001 is

the string of maximum

likelihood (ML).

F(00 10 11 11) = 1001

F(10 10 11 11) = 1001

Decoding PRML with Trellis diagrams

F(11 11 01 00 10 11 11 10) = 1??? ????

Draw the trellis

diagram and work the

ML sequence for the

error below.

(We still start from

state 0 and use a

sequence length of four

pairs)

QUIZ: Trellis decoding

Homework for Ch.3

End of chapter exercises

6, 23, 37, 43, 58, 65

Due next Tuesday at the beginning of class

two’s - Tarleton State University...Moore vs. Mealy Moore and Mealy machines are equivalent, i.e....

Documents