EEE2243Digital System Design

Chapter 5: Simple Design Case Studies

by Muhazam Mustapha, February 2011

Learning Outcome

• By the end of this chapter, students are expected to understand a few design case studies– Decoder– Multiplexer– Sequence Generator– Secure Car Key– Flight Attendant Call-Button

Chapter Content

• Decoder

• Multiplexer

• Sequence Generator

• Secure Car Key

• Flight Attendant Call-Button

Decoder

Decoder• A decoder is a combinational circuit that

activates its output according to the binary value of its input

• General block diagram of active high 3-bit decoder:

3-to-8 Decoder

I0

I1

I2

O0

O1

O2

O3

O4

O5

O6

O7

If I2I1I0 = 010, O2 will be set to HIGH, the rest will be LOW

Decoder• Most of the decoders available in the market are

inverted output (active low):

I0

I1

I2

O0

O1

O2

O3

O4

O5

O6

O7

If I2I1I0 = 010, O2 will be set to LOW, the rest will be HIGH

3-to-8 Decoder

Decoder• General truth table and circuit of 2-to-4 active

high decoder:

I1 I0 O0 O1 O2 O3

0 0 1 0 0 0

0 1 0 1 0 0

1 0 0 0 1 0

1 1 0 0 0 1

Decoder Verilog• From the

definition of decoder it might be obvious now that it easier to write its Verilog code in Boolean algebra rather than behavioral approach

module Decoder2to4(codein, codeout); input [1:0] codein; output [3:0] codeout; reg [3:0] codeout; parameter CO0 = 1; // code out 0 parameter CO1 = 2; parameter CO2 = 4; parameter CO3 = 8;

always@(codein) begin case (codein) 0: codeout = CO0; 1: codeout = CO1; 2: codeout = CO2; 3: codeout = CO3; endcase endendmodule

Multiplexer

Multiplexer• A multiplexer (mux) is a combinational circuit

that transfers its MULTI line inputs to a SINGLE line output according to the binary value of some selector lines

• General block diagram:

S0

I0

I1

I2

I3Output

I5

I6

I7

If S2S1S0 = 010, value at I2 will be sent to Output

8-to-1 Mux

I4

S1S2

Multiplexer• General truth table of 8-to-1 multiplexer:

S1 S0 Output

0 0 I0

0 1 I1

1 0 I2

1 1 I3

301201101001 ISSISSISSISSOutput

Multiplexer• Based on the previous truth table, multiplexer

can be built using decoder:S1 S0

I3

I2

I1

I0

Decoder

MultiplexerOutput

Multiplexer• The simplified circuit:

S1 S0

I3

I2

I1

I0

Output

Multiplexer Verilog• Multiplexer is

better be defined in behavioral approach

module Mux4to1(sel, lin, lout); input [3:0] lin; input [1:0] sel; output lout; reg [3:0] lout;

always@(sel) // latched, lin no effect begin case (sel) 0: lout = lin[0]; 1: lout = lin[1]; 2: lout = lin[2]; 3: lout = lin[3]; endcase endendmodule

Secure Car Key

Vahid Example 3.4 pg 127

Secure Car Key

• Many new car keys include tiny computer chip– When car starts, car’s

computer (under engine hood) requests identifier from key

– Key transmits identifier• If not, computer shuts off car

• FSM– Wait until computer requests

ID (a=1)– Transmit ID (in this case,

1101)

K1 K2 K3 K4

r=1 r=1 r=0 r=1

Wait

r=0

Inputs: a; Outputs: r

a’a

Vahid Example 3.4 pg 127

Secure Car Key

• Nice feature of FSM– Can evaluate output

behavior for different input sequence

– Timing diagrams show states and output values for different input waveforms

K1 K2 K3 K4

r=1 r=1 r=0 r=1

Waitr=0

Inputs: a;Outputs: r

a’a

Wait Wait K1 K2 K3 K4 Wait Wait

clk

Inputs

Outputs

State

a

r

clk

Inputsa

K1Wait Wait K1 K2 K3 K4 Wait

Output

State

r

Q: Determine states and r value for given input waveform:

a

Vahid Example 3.4 pg 127

Flight Attendant Call-Button

Vahid Example 3.1 pg 118

Flight-Attendant Call Button• D flip-flop will store bit• Inputs are Call, Cancel, and present

value of D flip-flop, Q• Truth table shown below

Preserve value: if Q=0, make D=0; if Q=1, make D=1

Cancel -- make D=0

Call -- make D=1

Let’s give priority to Call -- make D=1

Circuit derived from truth table, using Chapter 2 combinational

logic design process

Call

button

Cancel

button

Flightattendantcall-button

system

Bluelight

D Q’

QClk

Callbutton

Cancelbutton

Bluelight

Call

Cancel

Q

Vahid Example 3.1 pg 118

Verilog Code• Write the Verilog code yourself for the cases of

Secure Car Key and Flight Attendant Call-Button