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that is superimposed on a large DC voltage.  We will
typically use the DC setting.
<H4>Triggering</H4>
Triggering determines when the scope samples the signal.  In
"free" triggering mode, the scope starts sampling a new
trace as soon as the previous trace is finished being
captured.   This means that the position of edges in a
periodic signal will be placed randomly depending on when
the scope starts sampling the signal.  Free triggering is
useful to get a continuous monitor of a signal, but is not
used all that much.  Set the triggering to "FREE".  Notice
that the waveform seems to "move" as it is redrawn because
the transitions are not redrawn in the same places.  If you
want to freeze the image press &lt;STOP&gt;.  Press &lt;START&gt; to
resume sampling.
<P>
Continuous triggering mode (CONT) is like free triggering
except that a synchronizing signal is used to determine when
to start sampling a signal.  This signal is determined by
the parameters called "SOURCE", "COUPLING", "SLOPE" and
"LEVEL".
<P>
The SOURCE of the triggering signal is typically the signal
being sampled, although it can come from an external source.
<P>
COUPLING here means the same thing as for the signal itself.
We will typically use DC coupling.
<P>
The trigger is defined by some edge in the signal you are
observing.  You indicate whether you want to trigger on the
rising or falling (+/-) edge using the SLOPE parameter.  You
indicate at what point on the edge you want to trigger by
specifying the voltage LEVEL.
<P>
Change the triggering mode to "CONT" and observe the
waveform again.  The waveform should be stable now.  This is
possible because continuous triggering uses the trigger
signal to start sampling at the same point of the waveform
each time.  If the signal is not stable, then you must
adjust the trigger parameters described above.
 The final parameter, "POSITION", specifies where the
triggering event is displayed on the screen.  By changing
this parameter, you can examine events before or after the
triggering event.
<H4>Switch Debouncing and Single Traces</H4>
Since this is a digital storage scope, you can sample a
single trace and examine it at your leisure.  This is SINGLE
triggering mode.  First setup the trigger and then press
&lt;START&gt;.  This will cause just one sampling to take place
when the first trigger event is detected.  Use this to
display the bounce on your push-button switch as connected
below.
<p><img src=SPST.jpeg><p>
Connect the oscilloscope to the output, depress the
pushbutton, and capture the waveform showing what happens as
the switch is closed.  (Note: you will need to set a trigger
for the oscilloscope.)  You will notice that the output does
not make a clean transition but bounces around a bit. How
long does the bounce last?  Repeat the experiment several
times. Also repeat the experiment observing the output when
the pushbutton in released. (You will need to change the
trigger setting.)
<P>
We will now debounce the pushbutton.  To debounce this type
of switch (single-pole single-throw or SPST) we will require
a special analog circuit consisting of a resistor,
capacitor, and a Schmitt trigger (your 'LS14).  You already
have a resistor connected.  The circuit should be configured
as:
<p><img src=SPST-debounced.jpeg><p>                              
When the button is depressed, the capacitor is shorted to
ground and is quickly discharged.  This makes the output of
the Schmitt trigger inverter immediately go high.  If the
button momentarily disconnects, the capacitor will start
recharging but only slowly due to the large RC time
constant.  If the time constant is longer than the bounce
time there won't be time to charge the capacitor enough to
trip the Schmitt trigger and the output will stay high.  The
internal hysteresis of the Schmitt trigger  will prevent it
from switching again until the input gets very high.  Once
the pushbutton is released, there will be enough time and
the capacitor will eventually charge to 5v and the output of
the Schmitt triggers will be low.  The R and C values should
be chosen so that R times C is more than 10 times the
duration of the longest bounce in the previous experiment.
Also, R should not be larger than 100 kiloohms or the 'LS14  will
not work correctly (its input will sink more current than is
being provided through R).  Compare the waveforms on the
pushbutton output before and after you add the debouncing
circuit.
<H4>Measuring Propagation Delay</H4>
Propagation delay is the time for a change in the input of a
gate to be reflected at the output.  Propagation delays are
measured from the 50% points of the transitions, the points
in the transition that are halfway between the high and low
values of the waveform.  For an inverter we can define two
propagation times: tphl, the transition time for a rising
transition to be propagated, and tplh, the time for a
falling transition to be propagated.
<p>><img src=prop-delay.jpeg><p>
We will measure the delay of one of the 74LS04 inverters in
your kit by connecting the clock generator to one input and
comparing it to the output of the inverter as shown above.
Unfortunately, the scope boards do not permit us to
superimpose two traces on the same display.  Therefore, the
procedure for making these measurements is somewhat
complicated.
Connect another scope probe to channel two of your scope.
Use one channel of the scope to examine the clock signal and
the other channel to examine the signal coming out of the
'04 chip.  To make the comparison honest, you should load
the '04 output with a TTL input (another inverter will do
for this).  Are the propagation delays within the range you
expected?  Does the delay change if the fanout of the
inverter is increased?
<H4>Dual Screen Mode</H4>
From the scope display screen, press &lt;6&gt; to go into dual
screen mode.  The dual screen mode works like the single
screen mode you used before except that two traces are
displayed.  You can only access one half of the screen at a
time.  To toggle between the upper and lower halves of the
screen, press the &lt;9&gt; key.  The grid overlay you obtained by
pressing &lt;3&gt; is disabled in this mode.
<P>
Adjust the parameters to the two traces until you can see
the signal going into the inverter and the signal coming out
of the inverter.  To take measurements between these
screens, you must ensure that the signals are comparable -
both traces must have the same timebases and zoom factor.
You have already learned how to adjust the timebases.  To
ensure that both traces trigger from the same source, go to
the scope setup menu.  Adjust the triggering parameters so
that both waveforms are in continuous trigger mode with the
same triggering parameters.  (You can use the &lt;C&gt; key to
copy a parameter from one channel to another.)  Channel one
should have "INT" as the triggering source.  Channel two
should have "CH1" as its triggering source so that they both
start sampling at precisely the same time.
<P>
Return to the scope display and examine the displays.
Adjust the timebase parameters and if necessary your clock
generator to obtain traces from which you can accurately
measure propagation delay.  Use the cursor and reference
cursors to measure the delay from one signal to the other.
Make sure the that the zoom factor is identical for both
traces and that the horizontal trace position is identical.
<H3>Hand in: (Due May 10)</H3>
<oL>
<LI>Do the frequencies you observed on the oscilloscope match the
setting of your clock generator?  How far off are they?
<LI>How long does the bounce on your push-button switch last?  How
many edges are there on average?  Does the debouncing circuit
eliminate the bounce?
<LI>What is the delay of a signal through a 74LS04 inverter?  Is it
what you expected given the information in the data sheets?
</oL>

<H3>II.  Logic Analyzer</H3>
Debugging sequential circuits is much more difficult than debugging
purely combinational circuits.  Logic analyzers sample and collect data
and display the waveforms on their screen.  This will enable you to
observe the behavior of your circuit during an entire time interval
rather than at just one point in time as is possible with your probe.
<P>
In the lab, there is documentation that describes the operation of the
Tektronix 1230 logic analyzer.  You may find using this logic analyzer
confusing at first, so we suggest you read carefully pages 7-9 of
section 1 which covers general information about using the logic
analyzer.  You should also read the rest of this section, but some of
the information will not make sense until you actually try to use the
logic analyzer.  The next step is to try out the logic analyzer using
the exercises from section 2 in the documentation.  The best way to
learn is to try things out and ask questions if you don't understand
something.
<P>
Section 2 contains two exercises which rely on a test card (available
in the lab) that contains a simple counter circuit.  The first
exercise covers asynchronous operation which uses a free-running
internal clock to sample data.  With this type of sampling, the logic
analyzer will not generate crisp waveform and you'll need to
oversample (multiple samples per clock edge) in order to get evenly
spaced transitions on signals.  The second exercise covers synchronous
operation which uses a clock that you supply to sample data.  This
makes sense when you want to see the value of signals precisely at the
active edge of the system clock and not deal with the jitter typically
seen in asynchronous operation.  By sampling more wisely, you'll be
able to avoid oversampling and collect a longer history into the logic
analyzer's memory.
<P>
After you have done these two exercises, use the logic analyzer to
observe your 3-bit counter from the previous lab assignment, using the
pushbutton switch to generate the clock signal.  Connect
leads of the logic analyzer to the switch controlling the counter and
all its outputs.  Have the analyzer trigger on a transition on the
debounced switch.  Then do it again without the debounce circuitry
(with a switch you know bounces) and see if you get a waveform that
shows the counter counting more than one for a single (bouncy) switch
press.  Lastly, try triggering on a counter value of 6 making sure to
collect some data before the trigger (so that you can see the switch
event that caused the counter to advance to 6).

<HR>

<H3>Hand In: (Due TA option</H3>
<OL>
<LI>Demonstrate your mastery of the logic analyzer to the TA using the
counter circuit.  Your TA should sign your turn in sheet from Part I
to show you have completed the demo.
</OL>

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