DSAS Laboratory
no 4
Laboratory 4
OrCAD editing and simulation
5.1 Laboratory work goals
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•
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Presentation of the OrCAD package of programs.
Editing of a simple scheme.
Simulation of the implemented scheme functioning.
5.2 Editing a logic scheme
The OrCAD 9.1 programs package is used to edit and simulate
schemes containing digital circuits and electronic components. The
package contains a number of programs, among which:
Capture Release 9 for editing schemes and stimuli;
Simulate for schemes functioning simulation.
The OrCAD package can be installed on Windows 9x, NT, 2000
and XP operating systems.
The editing program can be started from the menu Programs→
OrCAD Release 9.1 → Capture CIS. The next step is to create a
new project or open a previously created one, then edit the desired
scheme as well as the necessary stimuli, and finally start the
simulation itself.
In the next paragraphs it will be presented one of the facilities
offered by the package, editing a scheme.
Capture CIS is started, and then it is created a new project like
this File →New → Project (Fig. 5.1).
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Fig. 5.1
In the Name edit box a name must be chosen for the project, and
then the option Analog or Mixed-Signal Circuit Wizard is to be
selected (Fig. 5.2). In the Location field it is introduced (by using the
Browse... button) the path to the file in which the project will be
saved on the hard drive. Creation of the project itself is done after
pressing the OK button.
The next step is to select the libraries containing the modules to
be used in the project. These can be selected from the left side
window list (Fig. 5.3) by pressing the Add>> button to add them to
the project. The libraries used in the project will appear in the list on
the right-side of the window, along other predefined libraries such as:
analog.olb, source.olb, sourcstm.olb, special.olb.
To edit the necessary laboratory schemes one will select the
following libraries: 7400.olb and 74ls.olb. At the end, the Finish
button must be pressed.
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Fig. 5.2
Fig. 5.3
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Next, the work area appears, on which the desired scheme is
edited. The work area is selected by clicking on it. Placing modules
or gates on the area is easy; the desired component (Tool part) is
selected from the Toolbar that is placed on the right side of the
window (Fig. 5.4 ->1). The same action may be accomplished if
pressing the key P or Shift+P meaning the option Place Part from
the Place menu.
As an example it will be edited the scheme implementing the
following logic function:
f(x, y, z) = (x + y) ⋅ (x + z)
1
2
3
Fig. 5.4
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For implementation there are needed two OR 2 gates and an
AND 2 gate, that is a 74LS32 module out of which there will be used
2 gates and a 74LS08 module and use one gate.
First an AND 2 gate is selected (Fig. 5.5). Place Part is pressed;
in the Part field it is introduce the name of the module, 74LS32.
From the underlying list the 74LS32 module is selected. In the
Packaging section, the gate is chosen by means of the Part combo
box.
At the end the OK button must be pressed.
Fig. 5.5
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The next step is to place the gate in the desired location in the
work area by a click of the mouse. A second click is necessary to
place the second gate as well. At the end the ESC key is pressed.
Each gate has two indicators: name of the module (for instance
74LS32), appearing in the bottom of the gate and an identifier of the
gate or a reference (for instance U1A), specifying that gate A is used
(identifier of the gate in the module) pertaining to module U1 which
is the first (representing an identifier of the module). On each pin of
the gate, either input or an output pin, it is written the number of the
pin, as described in the catalogue.
In order to modify the identifier of the second gate, the identifier
text must be selected (U2A), then right click with the mouse, then
Edit Properties or directly a double click on the current indicator.
The Value field must be edited, representing reference of the second
gate in the first module 74LS32; it will be modified to U1B, and
finally press the OK button (Fig. 5.6).
Fig. 5.6
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In a similar manner one must place one gate from a 74LS08
module. This module has the reference U2A.
Now all three gates belonging to the two specified modules are
placed on the work area (Fig. 5.7).
The next step is interconnecting all logic gates in the scheme
(Fig. 5.8). This aspect is materialised by means of interconnecting
wires. It is chosen Place Wire from Tool Part (Fig. 5.4 ->2) or the
key W or Shift+W is pressed or from the Place menu the option
Wire is chosen. Placing the links is done by clicking with the mouse
in the starting point and then in the end point. The ESC key must be
pressed to exit the Wire mode.
Because on pin 4 input from gate U1B it is connected the same
signal as on pin 1 input from gate U1A (the variable x), one must
trace a wire from pin 1 input of U1A to input 4 from U1B.
Automatically it will appear a junction or a wiring point.
U1A
1
3
2
U2A
74LS32
1
3
2
U1B
74LS08
4
6
5
74LS32
Fig. 5.7
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U1A
1
3
2
U2A
74LS32
1
3
2
U1B
74LS08
4
6
5
74LS32
Fig. 5.8
The next step is to name the inputs and outputs from the scheme.
This is done by placing Net Aliases (Fig. 5.9) or if pressing the key
N or Shift+N or selecting the option Net Alias from the Place menu.
Then some name must be written in the Alias field and finally the
OK button is pressed. To position the alias on the desired wire (for
an input or an output of a gate), it is used the mouse.
Fig. 5.9
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For the scheme given as example, the input 1 of U1A will be
denoted x, input 2 of U1A will be named y, z for input 5 of U1B and
f for output 3 of U2A (Fig. 5.10).
U1A
x
1
y
2
3
U2A
74LS32
1
3
f
2
U1B
74LS08
4
6
z
5
74LS32
Fig. 5.10
5.3 Scheme functioning simulation
In order to simulate the functioning of a scheme, it is necessary
to connect stimuli on the inputs of the circuit, signals that can be
configured to generate the desired input combinations. To place a
stimulus, one must open Place part, select the SOURCSTM library
and choose DigStim1, which is one line digital stimulus.
One such stimulus must be placed on each input of the scheme.
Next, right click with the mouse on the Implementation field and
select Edit Properties or directly double click on the
Implementation field and edit the Value field – put in the name of
the input, x, y or z. The scheme must comply with Fig. 5.11.
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Fig. 5.11
The last step is to edit the stimuli.
One of the scheme stimuli must be selected with a click on its
symbol, then the Edit menu must be accessed and the PSpice
Stimulus option has to be selected.
As an example, in the next paragraphs the stimulus for the signal
x is edited. The Signal option from the Digital section is to be
selected. If 0 is to be chosen as initial value, then the Initial Value
field must not be modified.
In order to add the rest of the stimuli – for signals y and z, from
the same application Stimulus Editor one must choose the New
option, from the menu Stimulus or Alt+N or press the New
Stimulus toolbar button (Fig. 5.13 –>1). The names of the stimuli
are edited, y and then z. At the end, the file containing these stimuli
must be saved, the file being named like the project and with the .stl
extension.
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Fig. 5.12
To be able to apply all possible combinations on the inputs of
the scheme, as shown by the truth table, the stimuli must be
modelled, because initially they have the value 0 for the whole
simulation period. This is done by accessing the Edit menu, the
option Add or Alt+A or the toolbar button (Fig. 5.13 –>2). Then
with the mouse, which changes cursor from regular shape to a pencil
like shape, click on the stimulus wave form in the point where one
wants to change value from 0 to 1 or from 1 to 0.
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1
2
Fig. 5.13
The x, y and z stimuli, with values according to the truth table,
are depicted in Fig. 5.14.
Fig. 5.14
The stimuli file, containing the new values, has to be saved, then
closed and the attention focuses on the work area again.
From the PSpice menu one must choose the New Simulation
Profile option, in the window that appears (Fig. 5.15) the name of
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the simulation must be edited and in the end the Create button must
be pressed.
Fig. 5.15
A new window appears but for now nothing needs to be
changed, so the OK button is pressed. Next, select Run from the
PSpice menu, in order to enter the simulation mode. To visualise the
scheme evolution in accordance to the stimuli applied on the inputs,
from the Trace menu the Add Trace option has to be chosen. Then,
from the next window (Fig. 5.16) one must select, one after the other,
the inputs and outputs.
At the end, press the OK button.
The wave forms of the signals selected for visualisation are
shown in Fig. 5.17; one can also notice the delays at the outputs with
respect to the inputs (at this moment the delays are not a concern).
The wave forms in Fig. 5.18 are identical to those from Fig. 5.17
and they are obtained selecting the option Print Preview from the
File menu. This kind of representation is specific to printing the
results and is recommended for capturing images when printing in
black and white.
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Fig. 5.16
Fig. 5.17
Fig. 5.18
The simulation is to be executed again, but one must place
aliases for gates U1A and U1B also, to be able to visualise them, in
order to observe the delays induced by each gate.
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5.4 Lab activity progress
Practice editing and simulation of a scheme in OrCAD
through all steps described in the previous paragraph.
It is considered the scheme from Fig 5.19 and it must be
analysed in order to determine the truth table.
The scheme is edited in OrCAD as presented in the previous
paragraph.
Stimuli are chosen so as to be taken into account all possible
combinations of values for input variables. Between two
successive commutations of a stimulus there must be at least
250ns.
It is simulated the functioning of the scheme to determine the
truth table of the functions performed by it. The simulation
will be performed with minimum delays on an interval of
2000ns.
It must be established the significance of the functions S and
T.
a
b
S
Ti
T
Fig. 5.19
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It is considered one of the functions proposed in paragraph
5.5 and it is determined the truth table corresponding chosen
expression.
It is determined the scheme for the corresponding chosen
expression.
It is edited and simulated the scheme using the same stimuli
like previous scheme.
The results obtained by simulation must be analysed and
compared with the truth table previously deducted.
5.5 Proposed problems
1. Implement and simulate the scheme for the following
function in OrCAD:
f(x, y, z) = [(x ⊕ y) ↑ (x + z)] ↑ ( y ⋅ z)
2. Implement and simulate the scheme for the following
function in OrCAD:
f(x, y, z) = (x ↓ y) ↑ (x + z ⋅ y)
3. Implement and simulate the scheme for the following
function in OrCAD:
f(x, y, z) = [( x ⊕ y) + z ] ↑ (x ⋅ y)
4. Implement and simulate the scheme for the following
function in OrCAD:
f(x, y, z) = [ x ⊕ ( y ⊕ z )] + (x ↓ y)
5. Implement and simulate the scheme for the following
function in OrCAD:
f(x, y, z) = ( x + y ⋅ z ) ↓ (x ⋅ y)
6. Implement and simulate the scheme for the following
function in OrCAD:
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f(x, y, z) = (x ⋅ y + z) ↑ ( x ⊕ y)
7. Implement and simulate the scheme for the following
function in OrCAD:
f(x, y, z) = [(x ↓ y) ⋅ (y + z)] ↑ (x + y)
8. Implement and simulate the scheme for the following
function in OrCAD:
f(x, y, z) = ( x ⋅ y + z) ⊕ (x + y)
9. Implement and simulate the scheme for the following
function in OrCAD:
f(x, y, z) = (x ⋅ y ↑ z) ⋅ (x + y)
10. Implement and simulate the scheme for the following
function in OrCAD:
f(x, y, z) = (x ⋅ y ⋅ z) ↑ [(x ↓ y ) ↓ z)]
Remark! At this stage the delays in signal propagation are of no
concern, although these delays may sometimes dramatically alter the
results. This is why the simulation result must be read from the
middle of each interval between any two commutations.
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