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SIMULATION OF DIGITAL LOGIC CIRCUITS

TOPIC 3.0 - COURSE OUTLINE

 Understand the concept of digital logic circuit simulation
➢ Concept of the logic level simulation.
➢ Differentiate between combinational logic circuits and sequential logic circuits
➢ Methods to simulate digital electronic circuits and to obtain the digital/analogue outputs of circuits.

 Apply digital circuit simulation
➢ Show how to configure digital/logic sources
➢ Do digital/logic simulation setup
➢ Conduct simulation of the following combinational logic circuits suggested below;
✓ Logic gates
✓ Combinational logic circuits
✓ Half Adder, Full adder
✓ Encoder, Decoder
✓ Multiplexers
✓ Other combinatinational logic circuits

 Contrite the simulation of the following sequential logic circuits suggested below.

➢Flip-flop
➢Counter
➢Register
➢Other sequential logic circuits

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SIMULATION OF DIGITAL LOGIC CIRCUITS

TOPIC 3.0 - COURSE OUTLINE

➢ Show the input and output of the logic simulation.
 Analyze the logic level simulation results for the logic circuits

➢ Examine the simulation results in sction 3.2.3 and 3.2.4
➢ Compare the expected theoretical output of the circuit tot the simulated results
➢ Investigate the occurrence of ‘glitches’ in digital logic circuit if there ia any.

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SIMULATION OF DIGITAL LOGIC CIRCUITS

Concept of logic level circuit simulation

Digital circuit simulation involves the simulation of
pure digital circuit that is constructed of logic gates
and logic devices only. Examples of logic gates are
AND, OR, NAND, NOR and XOR and examples of
logic devices are multiplexer, demultiplexer,
encoder, decoder, adders, flip-flops, counters,
registers and analog-digital-analog converters.

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SIMULATION OF DIGITAL LOGIC CIRCUITS

Concept of Digital logic circuit simulation

During simulation, all inputs are simulated
using digital stimulus and digital signals.
Transient analysis is used as the analysis
setup.
Each of the logic gates and devices must
yield outputs that conform to the truth table
and all signals can be monitored using
Probe.

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SIMULATION OF DIGITAL LOGIC CIRCUITS

Concept of Digital logic circuit simulation

DIGITAL SOURCES
There are many types of digital sources and they are kept in the
different libraries. Among the sources are digital clock, digital
stimulus and preset digital signal such as HI and LO.

DIGITAL STIMULUS
Digital stimulus has the reference designator STIM1. How the
digital signal looks like can be set by the user. This is done by
setting the attributes of the digital signal in the COMMAND lines
section. The user can be guided to two types of timing for the
signal when writing the attributes. The two types are absolute
time and relative time.

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SIMULATION OF DIGITAL LOGIC CIRCUITS

Concept of Digital logic circuit simulation

ABSOLUTE TIME

Logic level Absolute time is time that refer to the origin (starting point)
1

0 4ms 6ms Time(ms)
0 1ms

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SIMULATION OF DIGITAL LOGIC CIRCUITS

Concept of Digital logic circuit simulation

RELATIVE TIME

Logic level 1

0

0 +1ms +3ms +2ms Time(ms)

Relative timing is similar to absolute time except that the time-
transition pairs specify the amount of time from the last transition.
Note the plus (+) signs before the time values.

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SIMULATION OF DIGITAL LOGIC CIRCUITS

How to setup digital/logic source
STIMULUS (STIM1)

Example : AND gate 2-input as figure 1

Input_A Input_B

Figure 1 00
01
10
11

Step to setup Stimulus source
1. Place AND part from part browser
2. Place part Digital Stimulus (STIM1) as input sources
3. Determine the logic level of inputs (refer to true table or equations given)
4. Setup Digital Stimulus (Double click) on input source.

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SIMULATION OF DIGITAL LOGIC CIRCUITS

How to setup digital/logic source

Place part – AND gate Place input source – STIM1

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SIMULATION OF DIGITAL LOGIC CIRCUITS

How to setup digital/logic source

Setup input_A Setup input_B

Explain for input_A? COMMAND1 indicates that at time 0, the logic
level is 0. COMMAND2 indicated that a 1ms
into the simulation the logic level changes to 1
and is set in the same manner as
COMMAND1. COMMAND3 forced the logic
back to 0 at 2ms. At 3ms the logic returns to 1
and at 4ms the logic returns to 0.

use abosulate time method 10
where each command indicates the change

to a specific logic state at a specific time.

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SIMULATION OF DIGITAL LOGIC CIRCUITS

How to setup digital/logic source

Setup input_A Setup input_B

Explain for input_A? COMMAND1 indicates that at time 0, the logic
level is 0. COMMAND2 indicated that a 1ms
into the simulation the logic level changes to 1
and is set in the same manner as
COMMAND1. COMMAND3 forced the logic
back to 0 at 2ms. At 3ms the logic returns to 1
and at 4ms the logic returns to 0.

use abosulate time method 11
where each command indicates the change

to a specific logic state at a specific time.

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SIMULATION OF DIGITAL LOGIC CIRCUITS

How to setup digital/logic source
DIGITAL CLOCK (DigClock)

Input_A Input_B
1
0 0
1 1
0
Figure 1 0
1

Step to setup Digital Clock source
1. Place NOT part from part browser
2. Place part Digclock as input sources
3. Double click on input source (digclock).

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SIMULATION OF DIGITAL LOGIC CIRCUITS

How to setup digital/logic source

Setup input_A

Note: to setup digital clock
consist of Frequency, F= 1Khz.
Off-time = 0.5ms
On-time = 0.5ms

0 1ms 2ms 3ms 4ms

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SIMULATION OF DIGITAL LOGIC CIRCUITS

How to setup digital/logic source

PRESET (HI or LO) HI LO OUTPUT
Figure 1 10 0

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SIMULATION OF DIGITAL LOGIC CIRCUITS

How to do digital/logic simulation setup
Same as the transient analysis setup

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SIMULATION OF DIGITAL LOGIC CIRCUITS

Conduct simulation of logic circuits

Example 1:
Do simulation for the logic gate figure below and compare output
Result with true table. It is same?

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SIMULATION OF DIGITAL LOGIC CIRCUITS

Conduct simulation of logic circuits

Solution: 1(a)

True table for this NAND gate

A BC
T1 =0 0 01
T2 =1ms 0 11
T3 =2ms 1 01
T4 =3ms 1 10

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SIMULATION OF DIGITAL LOGIC CIRCUITS

Glitches in digital logic circuits

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SIMULATION OF DIGITAL LOGIC CIRCUITS

Glitches in digital logic circuits

Glitches on output circuit 19

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SIMULATION OF DIGITAL LOGIC CIRCUITS

Glitches in digital logic circuits
Repair glitches in digital logic

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SIMULATION OF DIGITAL LOGIC CIRCUITS

Glitches in digital logic circuits

Repair glitches in digital logic

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