Date: Wed, 08 Jan 1997 21:42:19 GMT Server: NCSA/1.4.2 Content-type: text/html
Distributed: Nov 11
Due: TA option
You should divide into groups of two to complete this lab assignment.
The logic analyzers with digital scope boards have BNC connectors on the rear panel for scope and trigger input. Probes are attached to the BNC connectors to make measurements. These probes provide extra impedance to the circuit to minimize disturbance to the circuit.
After the logic analyzer is powered up using the switch located on the rear panel, it will eventually display an opening screen. Press the <MENU> key to start operation. The figure at the top of the next page is a road map for using the oscilloscope that shows what commands are available in the different modes. You will find it helpful to refer to this figure as you read the rest of the lab assignment.
The main menu is divided into three sections: SETUP, DATA, and UTILITIES. Only two of these menus are used to access the scope capabilities of the system. The menu selection marked "Digital Scope" in the "DATA" section is used to display waveforms (<A>). The menu selection marked "Digital Scope" in the "SETUP" section is used to change setup parameters of the oscilloscope (<5>) that determine how the signals are sampled.
The <NOTES> key is used to display on line help about the available commands and options. A screen of reference material will be displayed on the screen when you press the <NOTES> key.
The arrow keys are used to move the cursors around the screen. In some cases, the cursor is a highlighted selection that can be moved from one item to another. The value of a highlighted selection is changed either by using the <0> and <2> to rotate through the available alternatives or by entering the proper value using the keypad.
This screen functions in two modes. One mode brings up a reverse video cursor which allows you to change some of the setup parameters listed on the left side of the screen. In the other mode, the reverse video cursor is not present, but you can change other parameters by commands accessed via keypad.
Change these three parameters until you get an easy to read trace of your clock signal. Change the frequency of your clock signal. Observe how this changes your trace.
To retain your changes, you must press the <ENTER> key after you have finished adjusting parameters. If you press the <DON'T CARE / X> key to leave the "change setup" mode, your changes will be discarded.
The cursor can be changed to a horizontal cursor using the <E> key. This is useful for measuring the voltages.
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".
The SOURCE of the triggering signal is typically the signal being sampled, although it can come from an external source.
COUPLING here means the same thing as for the signal itself. We will typically use DC coupling.
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.
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.
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.)
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:
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.
>
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?
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 <C> 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.
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.
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.
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.
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).