PSPICE HANDOUT BASIC DIGITAL LOGIC GATES
OBJECTIVE
At the conclusion of this Laboratory Exercise, the student will be able to draw and simulate digital
circuits using basic logic gates with digital voltage sources.
INTRODUCTION
Computers were developed to store and mathematically manipulate quantitative information. The
earliest systems were analog computers, usually either electrical or mechanical. Unique circuits
and mechanisms were built to represent fixed mathematical problems with results appearing in the
form of a final voltage or some motion. The electrical and mechanical equivalents of addition,
multiplication, integration, and differentiation were incorporated into these computers. Complex
problems could be solved.
Later the digital computer emerged. Here the electrical and mechanical analogies for mathematical
operations were replaced by the manipulation of bits, defined as a sequence of 1's and 0's instead
of specific voltage levels. With a single bit, you can represent any two distinct items. Examples
include zero or one, true or false, on or off, male or female, and right or wrong. However, you are
not limited to representing binary data types (that is, those objects which have only two distinct
values).
For instance, different bits can represent different things. For example, one bit might be used to
represent the values zero and one, while an adjacent bit might be used to represent the values true
and false. How can you tell by looking at the bits? The answer, of course, is that you can't. But
this illustrates the whole idea behind computer data structures: data are what you define it to be.
One purpose of this exercise is to learn about the names and behavior of basic logic gates, AND,
OR, NOT, and XOR.
Gate Name
Behavior
NOT (Inverter)
Output level is opposite of input level
AND Gate
Output is 1 when all inputs are 1
NAND (NOT AND)
Output is 1 when 1 or more inputs is 0
OR Gate
Output is 1 when any 1 or more inputs are 1
NOR (NOT OR)
Output is 1 when all inputs are 0
XOR (EXOR, Exclusive OR)
Output is 1 when both inputs are at different logic states
The presence of a bubble or circle on any input or output indicates that input or output is inverted.
LOGIC STATES
When a digital circuit is in operation, digital nodes take on values or output states shown in the
following Table.
Logic State
0
1
R
F
X
Z
Means
Low, false, no, off
High, true, yes, on
Rising (changes from 0 to 1 sometime during the R interval)
Falling (changes from 1 to 0 sometime during the F interval)
Unknown: may be high, low, intermediate, or unstable
High impedance: may be high, low, intermediate, or unstable
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PSPICE HANDOUT BASIC DIGITAL LOGIC GATES
V1
V2
TD
PW
Minimum Voltage
Maximum Voltage
Time Delay from 0 to rise
Pulse Width time at V2
TR Rise Time from 10% V1 to 90% V2
TF Fall Time from 90% V2 to 10% V1
PER Period time for 1 cycle
Duty Cycle PW/PER * 100%
DIGITAL VOLTAGE SOURCES
As mentioned earlier, digital systems are more concerned with the logic state of an output that
defined as logic 1 or 0 rather than a specific voltage. While there are different logic families of
Integrated Circuits and they have different voltage and current specifications, these considerations
are beyond the scope of this course, and are not considered when running PSpice simulations.
Digital or mixed Analog/Digital simulations accept inputs and provide outputs as logic states.
Digital input signals (source voltages) are available from a number of different defined parts. We
will use STIM1 in these simulations
STIMn part properties
Property
Description
WIDTH
Number of output signals (nodes).
FORMAT
Sequence of digits defining the number of signals corresponding to a digit in
any <value> term appearing in a COMMANDn property definition. Each digit
must be either 1, 3, or 4 (binary, octal, hexadecimal, respectively); the number
of all digits in FORMAT must equal WIDTH.
IO_MODEL
I/O model describing the stimulus’ driving characteristics.
IO_LEVEL
Interface subcircuit selection from one of the four analog/digital subcircuits
provided with the part’s I/O model.
DIG_PWR
Digital power pin used by the interface subcircuit.
DIG_GND
Digital ground pin used by the interface subcircuit.
TIMESTEP
Number of seconds per clock cycle or step.
COMMAND1 - Stimulus transition specification statements including time/value pairs, labels,
COMMAND16 and conditional constructs. The Time/Value pairs entered are space delimited.
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PSPICE HANDOUT BASIC DIGITAL LOGIC GATES
Using STIM1, STIM4, STIM8 and STIM16 parts
The STIMn parts have a single pin for connection. STIM1 is used for driving a single bit. STIM4,
STIM8 and STIM16 drive buses that are 4, 8 and 16 bits wide, respectively. The properties for all
of these parts are the same as those shown in the Table below.
When placed, you must connect each part to the wire or bus of the corresponding radix (number of
bits). Typically, each COMMANDn property contains only one command line. It is possible to
enter more than one command line per property by placing \n+ between command lines in a given
definition. (The n must be lower case and no spaces between characters; spaces may precede or
follow the entire key sequence.)
Typically, we will use the STIM1 Part Reference for digital Stimulus, and only modify the
Commandn values in the Property Editor. In the Property Editor, we will define the time in
seconds from t = 0 and the logic state at that time (either 1 or 0) for each COMMANDn entry.
When entering the commandn values, they are entered as time, a space, and the logic level entered
as a binary value (0 or 1).
We will use up to three different STIM1 sources. PSpice will sequentially number them from
DSTIM1 through DSTIM3.
Do not omit the m from the time value as it contains the order of magnitude. Also, if you
inadvertently enter the next COMMAND time with a value less than the previous entry, you will
receive an error message when you attempt to run a simulation.
The COMMANDn times and logic levels are listed in the table below.
DSTM1
DSTM2
DSTM3
Command1
0s 0
0s 0
0s 0
Command10
9m 1
18m 1
4.5m 1
Command11
10m 0
20m 0
5m 1
Command12
11m 1
22m 1
5.5m 0
Command13
12m 0
24m 0
6m 1
Command14
13m 1
26m 1
6.5m 0
Command15
14m 0
28m 0
7m 1
Command16
15m 1
30m 1
7.5m 0
Command2
1m 1
2m 1
0.5m 1
Command3
2m 0
4m 0
1m 0
Command4
3m 1
6m 1
1.5m 1
Command5
4m 0
8m 0
2m 0
Command6
5m 1
10m 1
2.5m 1
Command7
6m 0
12m 0
3m 0
Command8
7m 1
14m 1
3.5m 1
Command9
8m 0
16m 0
4m 0
Note: Grounds are not required when using digital stimuli and digital integrated circuits. They are
defined and connected as part of the part. We will not modify these grounds. However, it is
possible that when we connect analog devices to digital circuits, we may have to connect power or
ground to these devices.
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PSPICE HANDOUT BASIC DIGITAL LOGIC GATES
PROCEDURE
Place your PSpice work disk in the drive and run Capture.
The component part references (Gates and STIM1) and Properties are:
OR 7432
NOR 7402
AND 7411
NAND 7410 NOT 7404
DSTM1
DSTM2
DSTM3
1.
Create a New Blank Project named lab1a. Draw a digital circuit as follows:
ƒ Enter an OR gate, part reference 7432 with the output node (the single pin) facing to the
right.
ƒ Enter DSTM1 right 1 inch to the left and ½ inch above input pin 1 of the 7432. DCL on
DSTM1 and enter the Commandn values from the table above.
ƒ Enter DSTM2 right 1 inch to the left and ½ inch below input pin 2 of the 7432. DCL on
DSTM2 and enter the Commandn values from the table above.
ƒ Connect wires from the as necessary to connect the DSTM1 to pin 1 of the 7432, and
DSTM2 to pin 2.
ƒ Add a wire about 1 inch long from the output pin3 to the right and stop.
ƒ Add the Net Alias A to the node of DSTM1
ƒ Add the Net Alias B to the node of DSTM2
ƒ Add the Net Alias Q to the output node of the 7432
Create a New Simulation Profile named lab1a for Time Domain (Transient) Analysis
using the following:
Run Time is 8m
Start Saving after 0
Maximum Step Size 0.8m
CL on OK
Enter ground referenced voltage
probes in order to nodes A, B, and
Q.
Your circuit should be similar to the
graphic on the right.
Run the Simulation.
In the top left corner, DCL on TM1:Pin1 From the list on the left, DCL on A
DCL on TM2:Pin1 From the list on the left, DCL on B
Your output should be now similar to the following graphic where A and B are the input
waveforms, and Q is the output waveform.
Note: in the previous instructions, it was not necessary to change TM1 and TM2 to A and
B. However, if we print the output and look at it later, the output waveform would be
easier to interpret, especially if the schematic was not available. The reason is beginning
letters of the alphabet are typically used for digital inputs, and Q is commonly used for
digital outputs in Boolean Equations and Truth Tables. By changing the labels, we can
readily create a truth table from the waveform diagram, which would help us identify the
logic function or Boolean Equation for the circuit.
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PSPICE HANDOUT BASIC DIGITAL LOGIC GATES
2.
3.
4.
Create a New Project named lab1b based on the project lab1a. If you do this, you will
not have to reenter the DSTM values. Modify the digital circuit as follows:
Delete the 7432 and enter a 7402. The remainder of the circuit and instructions for
step 1 are the same.
Create a New Simulation Profile named lab1b for Time Domain (Transient) Analysis
using the simulation time parameters from step 1.
After you run the simulation, change TM1:Pin1 to A and TM2:Pin1 to B
Create a New Project named lab1c based on the project lab1a. Modify the digital
circuit as follows:
Delete the 7432 and enter a 7411. Notice this is a 3-input gate.
Enter DSTM3 right 1 inch to the left and below input pin 13 of the 7411. DCL on
DSTM2 and enter the Commandn values from the table above.
Enter the Net Alias C to the new node.
The remainder of the circuit and instructions from step 1 are the same.
Create a New Simulation Profile named lab1c for Time Domain (Transient) Analysis
using the simulation time parameters from step 1.
After you run the simulation, change the input labels to A, B, and C if necessary.
Create a New Project named lab1d based on the project lab1c. Modify the digital
circuit as follows:
Delete the 7411 and enter a 7410. Notice this is a 3-input gate.
The remainder of the circuit and instructions from step 3 are the same.
Create a New Simulation Profile named lab1d for Time Domain (Transient) Analysis
using the simulation time parameters from step 3.
After you run the simulation, change the input labels to A, B, and C if necessary.
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PSPICE HANDOUT BASIC DIGITAL LOGIC GATES
A NOT gate performs the digital function of Complementation (or negation). This means
that whatever digital logic level is present on the input, the opposite logic level is present
on the output. If the input is Q, the output is QBAR or Q which is stated as NOT Q
5.
Create a New Project named lab1e based on the project lab1d. Modify the digital
circuit as follows:
Add a NOT gate, a 7404, to the output with the Net Alias Q
Add a wire about 1 inch long to the output of the 7404.
Add a Net Alias QBAR to the node.
Create a New Simulation Profile named lab1e for Time Domain (Transient) Analysis
using the simulation time parameters from step 1.
After you run the simulation, change TM1:Pin1 to A and TM2:Pin1 to B if necessary.
Assemble the hard copies in the proper order with a cover sheet on top, and submit to your
instructor.
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