|
ECE171: Introduction to
Digital Circuits |
Fall 1999 |
Rev: 11.16.99 |
MENTOR TOOLS TUTORIAL SESSION #3
Douglas V. Hall
Craig Files
Electrical Engineering Department
Portland State University
P.O Box 751
Portland OR, 97207
Version C2
September 1998
INTRODUCTION
In the preceding tutorial sessions we showed you how to access
the Mentor manuals and how to draw the schematic for a circuit.
In this session we show you how to verify that the logic and timing
of the circuit are correct by simulating the circuit.
Simulation is very similar to the method you use to verify the
hardware prototype of a circuit. To verify a hardware prototype
you apply test signals to the inputs of the circuit, then use
a scope or logic analyzer to observe the input and output signals.
Likewise, when you simulate a circuit with a simulator program,
you specify a set of stimulus signals that you want applied to
the inputs of the circuit. The simulator program then evaluates
the effect these signals will have on your circuit and displays
the results as waveforms or a list. From these waveforms you can
determine if the logic and timing of your circuit are correct.
Using a simulator is relatively easy. Most of the work involved
in simulating a circuit is the same as it is for testing a hardware
prototype: first determining a set of input stimulus signals that
will thoroughly test the circuit; and second, predicting the correct
response for this set of input signals.
At the conclusion of this session you should be able to:
- Create the Design Viewpoint for a schematic design.
- Bring up the QuickSimII simulator and the appropriate windows.
- Generate input stimulus signals for a circuit.
- Run a simulation and scale the generated waveforms as desired.
- Add probes to a circuit and rerun a simulation.
- Print a hardcopy of the simulation trace window or list window.
- Generate a "do" file which contains a set of input
stimulus signals and use this file for a simulation.
INVOKING THE DESIGN MANAGER
- Carefully bring up the Design Manager as described at the
start of Tutorial #2. Don't forget to set the MGC_WD environment
variable to the directory which contains the spike1 circuit you
drew in Tutorial #1.
SETTING UP THE DESIGN VIEWPOINT
According to the Mentor manual a Design Viewpoint is a set
of rules used to establish the configuration of your design. The
following sequence of steps will create the proper design viewpoint
references for the QuickSimII simulator and the Logic Modeling
Corp. models.
EXAMPLE DISPLAY
- In the Design Manager Tools window find the Design Viewpoint
Editor(DVE) icon and double click on the icon to open it.
- When the Design Viewpoint Editor window appears, find the
OPEN VPT box in the Session window on the right and click on
it.
- In the Open design Viewpoint form that appears, enter spike1
and click on OK.
EXAMPLE DISPLAY
Note: QuickSim will only work if your schematic name does
not have any space characters in it. For example, if you try
running quick sim on "Project 1" it won't work. To
fix this resave your project using a new filename without any
spaces and then run QuickSim.
- Find the SETUP VPT in the Session window on the right and
click on it.
EXAMPLE DISPLAY
- When the menu comes up, select the menu item: Quick_Sim_Fault_Path_Grade,
then press OK.
EXAMPLE DISPLAY
- When the check is completed, pop up the banner File ->
Save Design Viewpoint menu and choose With same name.
- Quit the Design Viewpoint Editor window. Then put the cursor
in the orphan command shell window, hold down the Control key,
and press the C key to get rid of this orphan.
STARTING QUICKSIM AND SETTING UP WINDOWS
- In the Design Manager Tools window, find and double click
on the QuickSimII icon to start QuickSim.
- After a short pause a form will appear. In the top box type
the path to your schematic file. (If you have set MGC_WD, all
you have to type in is spike1, otherwise you enter something
like /u/user_name/spike1).
EXAMPLE DISPLAY
In the line labeled Timing Mode click on Delay and in the line
labeled Detail of Delay Timing Mode click on Visible.
When the form expands, note that the default is typical, then
click on Max and click on OK.
- It takes the simulator a couple of minutes to generate the
netlist file, and do all the other work needed to get your schematic
ready for simulation. Eventually the QuickSim window will appear.
Grab the lower right corner of the window with the left mouse
key and drag it down and to the right to enlarge the window.
- Read the contents of the small Model Messages window. It
should contain just a short message identifying Logic Modeling
Corporation.
- Move the cursor to a blank region of the window and pop up
the QuickSim menu. You will be using this menu a lot, so read
through it to get an overview of the available commands, then
choose Open->Sheet. Eventually, an old friend should appear
in a window on the screen.
- To move the sheet view window to a more convenient location
on the screen, put the cursor in the title box containing /:sheet1,
hold down the left mouse key, use the mouse to move schematic
window to lower right corner of the QuickSim window, and release
the mouse key. (Note: If you want to move a window that is not
marked as active by a solid border, first click the left mouse
key in the title box to activate the window, then hold the left
mouse key down to move the window.)
- Activate the Model Messages window and move it down to the
lower left corner of the QuickSim window.
- The next step is to add a window which will display logic
analyzer type traces of your input stimulus signals and the resulting
output signal(s). To start, pop up the Add -> Traces menu.
- When the Add Traces form appears, click on Named signals
then type D in the top signal name box. Type C in the next signal
name box, B in the next, A in the next, and Y in the next. Move
the cursor down until the OK button appears and click on it.
EXAMPLE DISPLAY
EXAMPLE DISPLAY
- When the Trace window appears, move it to the top of the
QuickSimII window. Note signal name labels along left side of
trace window and the time scale along the bottom. As you can
see, this window is very similar to the timing display of a logic
analyzer.
- For some circuits it is helpful to have the input and output
signals displayed in a list format as well as in waveform format.
To produce a list window for this circuit pop up the Add ->
List menu and choose Specified. When the form appears, click
on Named Signals, fill in the Signal name boxes with A,B,C,D,Y,
then move the cursor down until the OK button appears and click
on it.
EXAMPLE DISPLAY
- When the List window appears, move it to the left side of
the QuickSim window just below the trace window, then note the
labels in the List window.
This completes the basic setup. The next step is to create
the input stimulus signals.
CREATING INPUT STIMULUS SIGNALS
As we mentioned above, a simulator allows you to verify the
logic and the timing of a circuit. We will first show you how
to verify the logic of a circuit.
Figure 1 below shows the truth table for the spike1 circuit you
drew in session #2. To thoroughly test the logic of the circuit
you need to apply each of the possible combinations to the inputs
and check if the output is correct for each. The sequence of input
combinations is a binary count sequence, so all you need to generate
for the sequence of input stimulus signals is a 4-bit binary counter
with A as the MSB and D as the LSB.
The most common way to generate input stimulus signals in QuickSim
is with Force commands. You can force a single value on a signal
line, force a sequence of values (multiple) on a signal line,
or force a repetitive signal called a "clock" on a signal
line. The signals you need for the inputs of this circuit are
repetitive, so you will use the Add -> Force -> Clock method.
A B C D Y
---------------------
0 0 0 0 0
0 0 0 1 0
0 0 1 0 0
0 0 1 1 0
0 1 0 0 0
0 1 0 1 1
0 1 1 0 1
0 1 1 1 1
1 0 0 0 1
1 0 0 1 1
1 0 1 0 0
1 0 1 1 0
1 1 0 0 1
1 1 0 1 1
1 1 1 0 1
1 1 1 1 0
Figure 1
EXAMPLE DISPLAY
- To generate a square wave signal with a 200 ns period for
the D input signal, pop up the Add -> Force -> Clock menu.
When the Force Clock form appears, type D in the Signal Name
box, type 200 in the Period box, and click on OK.
Note: The stimulus signals you create are stored in the Waveform
Data Base, but they do not appear in the trace window until after
you run the simulation.
- Repeat the procedure in #1 to generate a 400 ns square wave
on the C input, an 800 ns square wave on the B input, and a 1600
ns square wave on the A input.
RUNNING A SIMULATION AND VIEWING THE RESULTS
- Now that you have specified the input stimulus signals, you
can run the simulation and observe the results. To run the simulation
pop up the Run -> Simulation menu and choose Until Time. A
small form should appear in the lower left corner of the screen.
One complete cycle of the input waveforms you specified takes
only 1600 ns, but to show a little extra time, enter 2000 in
the Until Time box in the form and click on the OK button.
After a brief pause the input and output waveforms should appear
in the trace window and a truth table type display should appear
in the List window. Let's play around with the trace window first.
EXAMPLE DISPLAY
- Note in the Trace window that the specified simulation stop
time is at the right side of the window. Move the cursor to the
trace window and click the left mouse key to activate that window,
then use the scroll along the bottom of the trace window to scroll
the display back to time 0.0
- Study the waveforms and observe how they correspond to the
entries in the truth table in Figure 1. Note that the value of
the Y output is indeterminate(neither high nor low) until the
applied input signals have had a chance to propagate through
the circuit. Also, note that there are a couple of glitches displayed
on the Y waveform. We will come back to these a little later.
- As with a logic analyzer display, you can change the view
and the scale of the traces. To try one of the options put the
cursor in the Trace window, pop up the View menu, study the choices,
and then choose Zoom Out -> 2.0. You should now see more of
the 2000 ns. Repeat the procedure to see the entire 2000 ns of
simulation displayed at once in the trace window.
- Use the View -> Zoom In -> 2.0 command to expand the
display until the time scale reading across the bottom has labels
for every 25 ns. Then invoke the View -> Zoom In -> As
Specified command and when the form appears in the lower left,
enter 2.5. The time scale reading across the bottom of the screen
should now have labels for every 10 ns.
- Now, suppose that you want to take a closer look at the first
glitch on the Y waveform which is at time 630 ns. You could use
the keyboard arrow keys to scroll across to that time, but an
easier way is to use the View -> Time -> Specified command.
In response to this command a small form will appear in the lower
left corner of the screen. Type 700 in the Time box of the form.
Move the cursor to one of the small arrows to the right of "Mode
relative" in the form and click the left mouse key until
absolute replaces relative in the form. Then click on the OK
button. The display should now be roughly centered on the first
output glitch. Note that the specified time of 700 is at the
right side of the screen.
Study the waveforms to determine the width of the glitch and
scroll the display so you can see the input signal transition
which causes the glitch.
- To help in examining waveforms you can add one or more vertical
cursors to the display. To do this, put the arrow cursor in the
trace window and pop up the QuickSim menu (Using the right mouse
button) and choose the Cursors -> Add command. When the form
for this command appears in the lower left, enter start as the
Cursor Name. Move the arrow cursor to the Location box, click
the left mouse key, and enter 690. Click on OK. Note that the
cursor time position is displayed at the bottom of the cursor.
EXAMPLE DISPLAY
- Once you place a cursor you can move it around anywhere you
want. Click on the vertical cursor to select it. Then pop up
the QuickSim -> Cursors menu and choose Slide. When the command
executes, a ghost of the cursor will appear. Use the mouse to
move the ghost cursor to the desired position and click the left
mouse key to place it there. Then move the arrow cursor down
to the form at the bottom of the screen and click on Cancel to
terminate the command. For practice, use a couple of cursors
to determine the time between the input transition which causes
the glitch and the end of the glitch. This is the time you have
to wait for a valid output from this circuit.
- Now let's take a look at the List window. Move the cursor
to the List window and click the left mouse key to activate the
window, then use the keyboard arrow keys to scroll to time 0.0
in the list. Note that the list shows an entry for each time
where an input or output signal changes. Use the list entries
to determine the maximum propagation delay for the circuit.
EXAMPLE DISPLAY
ADDING PROBES TO A CIRCUIT AND RERUNNING A SIMULATION
- Suppose you want to add a probe to output of the upper 74LS10
in the circuit so you can see a trace of the signal on that line
when you run the simulation from time 0 again. First, let's place
the probe and add a trace for it.
Move the cursor to the schematic view window and click the left
mouse key to activate the window. Put the cursor on the output
net from the upper 74LS10 and click the left mouse key to select
it. Pop up the QuickSim -> Add menu and choose Probe.
EXAMPLE DISPLAY
When the form pops up, name the probe P1, click on diamond shaped
box named Net, and then click on the OK button. When the command
executes you should see a small flag appear on the signal line
in the view window.
- To add a trace for the probe, pop up the QuickSim -> Add->
Trace menu and choose Specified. When the form pops up, click
on Named Signals, enter P1 as the signal name and click on the
OK button. An entry for P1 should appear at the left edge of
the Trace window, but there will be no trace for this probe,
because it was not present when you did the simulation.
EXAMPLE DISPLAY
NOTE: You can use the QuickSim -> Delete menu to delete a
trace and/or probe at any time.
- Now that you have the probe and trace set up, the next step
is to reset the simulator to time zero and rerun the simulation
so you can see the signal present at the point where you put
the probe.
EXAMPLE DISPLAY
Pop up the QuickSim -> Run menu and choose Reset. When the
first form appears, click on the "state" box to choose
that option. Note: an option is chosen if the box in front of
the name contains a solid square. When the second form appears,
click on the "save results waveform DB" button to toggle
this default off, then click on the OK button. The waveforms
and the cursors in the Trace window should disappear. Also, the
list in the List window should disappear.
- To rerun the simulation, pop up the QuickSimII -> Run
-> Simulation menu and choose Until Time. When the form appears
in the lower left corner of the screen, enter 2000 as the time
and click on the OK. After a short pause new traces and a new
list should appear. Use the QuickSim -> View -> View All
command to see the traces for the entire simulation time.
EXAMPLE DISPLAY
MAKING A HARDCOPY OF THE TRACE AND LIST WINDOWS
- You must configure the printer as described in Mentor
Printing, if you have not already done so.
- About now you would probably like to see some tangible results
from all your work. Let's start with a hardcopy of the trace
waveforms.
If it is not already activated, move the cursor to the trace
window and click the left mouse key to activate it. Move the
cursor to the File entry in the banner at the top of the QuickSim
window, pop up the File -> Print menu, and choose "Active
Window".
- When the form appears, you can use the default printer name
already in the printer box, or you can enter one from the list
given in Tutorial Session #2. Enter 0 as the Begin domain and
2000 as the End domain. The default scale is 1.0 which is appropriate
here, so you can leave that box blank. If you want to show the
cursors in the printout, click on the cursors box in the form.
Click on the OK button to send the trace waveforms off to the
print spooler. If the attempt is successful, you should receive
a confirmation message at the bottom of the screen.
- To make a hard copy of the list window, activate the list
window, pop up the File -> Print window and choose Active
Window.
When the form appears, enter 0 as the Begin time, 2000 as the
End time, then click on OK.
RUNNING A SIMULATION WITH A FORCEFILE
The design-simulate cycle is usually repeated several times
before a final design is achieved. To save entering the force
commands individually for each simulation you can write them to
a file and then just use the file to apply the force commands
when you resimulate. Here's how you generate and use a forcefile.
- Put the cursor on the MGC entry in the banner, pop up the
Notepad menu, and choose New. NOTE: If you want to edit an existing
file, choose Open and when prompted, select the file you want
to edit from the list shown.
EXAMPLE DISPLAY
- When the notepad appears, type in the following force commands.
set clock period 200 force D 0 0 -R force D 1 100 -R set clock
period 400 force C 0 0 -R force C 1 200 -R set clock period 800
force B 0 0 -R force B 1 400 -R set clock period 1600 force A
0 0 -R force A 1 800 -R
- To write this file to disk, put the cursor on the File entry
in the banner, Pop up the file menu, and choose Save As. When
the form appears enter spike1.do as the document name and click
on OK.
- To close the notepad window, put the cursor on the small
box in the upper LEFT corner of the notepad window, pop up the
menu there, and choose close.
- To reset the simulator state pop up the QuickSimII ->
Run menu and choose Reset. When the form appears, click on state,
make sure the Save waveform DB is toggled off, and click on OK.
- To eliminate the old forces from the waveform Data Base,
pop up the QuickSim -> Force menu and choose Delete. When
the form pops up, click on All Signals and click on OK.
- To apply the forces you specified in the forcefile, pop up
the QuickSimII -> Forces menu and choose From file. When the
form appears, type spike1.do, and click on OK.
- To run the simulation with these forces, pop up the QuickSim
-> Run -> Simulation menu and choose Until time. When the
form appears in the lower left corner, enter 2000, and click
on OK. After a short time the familiar simulation waveforms should
appear.
OPTIONAL PRACTICE
If you are having fun with all this and time permits, here's
a little experiment you can do for extra practice.
Using a common Karnaugh map method an engineer determined that
the ABCD input transitions 0111->0110->0111, 1110->1100-
>1110, and 1101->0101->1101 may cause static 1 hazards
on the output of the spike1 circuit you drew.
- Generate a do file which will apply these three input sequences
to the circuit.
- Reset the simulator state.
- Delete the old forces in the Waveform Data Base.
- Rerun the simulation with your new do file.
- Make a hard copy of the resulting traces
- Determine if these transitions actually do cause hazards(glitches)
on the output of the circuit.
EXITING FROM QUICKSIM
- Put the cursor in the Quicksim banner at the very top, pop
up the menu there, and choose Quit. When the form appears click
on Without Saving and click on OK.
After some time QuickSim will disappear but its command shell
window may still be present in the center of the screen. To get
rid of this, put the cursor in it, hold down the Control key
and press the C key.
- Quit from the Design Manager.
