Basic Lab Instrument Operation

Each work station is equipped with a set of basic lab instruments that you will use in conjunction with the microprocessor development system tools.   Each station has an MSO (mixed signal oscilloscope), signal generator, digital multimeter and triple output power supply. The lab also has several portable USB based 8 channel logic analyzers that you can check out of the lab for project work at home or other purposes.

The Mixed Signal Oscilloscope (MSO)

Each work station is equipped with a Agilent MSO-X 3012A mixed signal oscilloscope. This is probably the most useful and complete instrument on the workstation. The MSO-X is capable of observing 2 analog signals and 16 digital signals simultaneously. So it can behave like a basic analog signal scope and 8 channel logic analyzer. It also has a fairly good signal generator and digital voltmeter built in too. 

For starters, we will just use the scope to observe some simple analog signals. In the following section we will observe a basic sine wave generated from the signal generator so you can get a quick introduction to each instrument.

For more detail on the scopes operation, you should try the following exercises in the Agilent's training guide manual at  Training Guide.


It is not necessary to do this in the lab today, but you should consider coming into lab when you have some free time to run through them. Of course, reference the manual as need be. The scope also has an excellent help function. Simply push the help key (lower left) and you will be provided instructions.

In-Lab Part 4: Observing a Sine Wave with the Oscilloscope

Step 1: Agilent 33220A Waveform Generator Setup

Next we will use the Waveform generator to generate a sine wave and view it on the scope. First turn on the Waveform generator with the power button. Next, select Utility, then System Output using the blue buttons. Select High Z for load and hit Done.

1.1) Next, hit the Graph button. Then tap the Ampl button twice until it changes to HiLevel.

1.2) Next, select 3 V using the number pad on the right and the blue button.

1.3) Finally, set the LoLevel to 0 V using the number pad. Don't turn on the output yet.

Step 2: Settuing up the Scope


2.1) Connect a three way BNC adapter to the function generator. Connect one BNC cable to the scope's channel one. Connect another one to the Red/Black pin adapter, which will be used with the Logic Analyzer later. Now turn on the Function Generator's output.  Then hit Auto-Scale. You should see the following trace.



2.2) This is a screen shot  from the scope. If you have a USB flash drive, you might want to try saving. For this post lab and others you are asked to provide screen shots.

2.3) Try Some Basic Triggering

Auto Scale will put the scope in Auto trigger mode or will cause the scope to constantly trigger or display what is applied to the input. Turn off the signal by toggling the signal generator OUTPUT button. You should notice that the scope shows a flat line at the zero volts or ground level.

Sometimes it is desirable to capture a single event. For the post lab will observe the electrical signal that occurs as a consequence of mechanical switch bounce. This a transient condition and will only be available for short time. If we were to use Auto trigger mode the display would soon replace transient condition with a steady state signal. We can capture the transient with the normal trigger mode. In the trigger functions box, select Mode/Coupling. On the lower left you will see the menu Mode Auto menu appear. Select with the soft key below. You can choose the setting by rotating the Push to Selet Knob (center with grey background) pushing to select.



Try turning the signal on and off again. You will see the sine wave stays on the screen. This is because in normal  trigger mode the screen is not refreshed automatically. It is only refreshed when a signal is present. In this way you can capture transients like the switch bounce we will observe in the post lab assignment.

The Saleae Logic Analyzer

What is a Logic Analyzer?

Logic analyzers are used to verify logic functions by observing logical signals as we did with the oscilloscope. The big difference is that you can observe many more signals with a logic analyzer then you can with a basic 2 channel scope.  The problem is you cannot observe the signals with the same vertical (voltage amplitude) or horizontal resolution (time). The logic analyzer classifies and then stores the input signal's amplitude as either a high or low.  The Saleae's maximum sample rate is 24MHz providing a time measurement resolution of about 0.1us. Remember, with the scope we could resolve propagation delays to about 1ns. Depending on the application, these measurement resolutions are tolerable when observing basic logic function and when time measurements are much greater than 0.1us.

Saleae Kit Installation

The Saleae DLA is powered via the USB connection, so simply plug it in. Note: the analyzer does not work thru the display USB port. The analyzer is quite compact with one USB connection and a probe pod consisting of 8 signal wires and a reference or GND connection. Each probe has a socket connector on the probe end that be connected into common circuit board headers or grabber clips supplied with the kit. 

probes

Saleae Application Program

The Saleae application program can be evoked from the following start menu ICON or from the program list.

A following window should appear.

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If the USB connection was successful, you should see a "Connected" message in the upper window border. The first user drop down window allows you to set the sample buffer size. Generally, 1 M samples will do. The next window allows you to set the sample rate. For this lab we will use the maximum rate providing a time resolution of about 01.us. The  "Start" button starts a measurement. Note, each channel is color coded to follow the probe color.

Simple Sampling
Give the analyzer a try by connecting it to the Signal Generator. Provide a nice simple digital signal by changing the sine wave to a square wave  1kHz, 3Vpp signal. Making sure that the signal is 0V at it's lowest and 3V at it's highest rather than +/- 1.5V. Connect channel zero to the source output and don't forget the ground reference. With the above setting take a sample by hitting the "Start" button. You  should see something like this.

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You can adjust the horizontal time scale by holding the mouse over the waveform display area and scrolling up or down. Holding the mouse over the waveform will provide a period, frequency and pulse width measurement if they are enabled in the measurements window. 

Notice that there is no vertical scale or voltage input gain adjustment. The DLA displays the input signal as either a logical 0 or 1 in much the same manner as typical digital logic would interpret an input signal.  If the input value is -0.5V to 0.8V the value is displayed as logical 0. If it is in the range 2.0V to 5.25V it is displayed as logical 1. Values in between 0.8V and 2.0V can either be 0 or 1.  Note you should take care not to exceed the input range -0.5V to 5.5V when making measurements or you may damage the DLA input circuitry.

Triggering the DLA

You will notice that if you take multiple samples of the scopes 1kHz signal, the waveform will appear to shift or start at different points in time. When you hit the start button, sampling starts at different points along the waveform. Quite often we want to trigger on a specific signal event much like we did with the oscilloscope. A particular channel can be triggered for either rising edge, falling edge, logical low or logical high level. The conditions are set with the 4 button set for each channel. To trigger the  channel 0 for rising edge, select the rising edge and hit the start button with the scopes 1kHz ref signal attached.  The display will look something like this.

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Notice that the display time base is now roughly centered at 0.0ms with time decreasing before and increasing after the 0.0ms time marker. The 0.0ms marks the trigger point. The signal is sample before and after the trigger and stored in the sample buffer for observation about the trigger point.  You can adjust the display to see more waveform cycles.

Selecting the negative edge likewise causes the trigger to occur accordingly.

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Likewise, triggering can be logical level 0 or 1 sensitive. This is the display for a positive level trigger.

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Triggering on Multiple Conditions

Sometimes we wish to trigger on more than one condition. For example, consider triggering on channel 0 with a rising edge and channel 1 at a logical 1. You can attach channel 1 to the 3.3V test pin (near push button switch 2) on the SmartFusion Kit. The display will look something like this after a start measurement.

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However, if we attach the channel 1 to the gnd pin next to the 3.3 volt pin and we attempt to sample, the DLA will continue to monitor, but wait for the trigger condition to come true before recording. The following message will appear.

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Move the probe to the 3.3 volt pin and you will see the DLA trigger. Note, once you lift the probe from the gnd pin the input will be interpreted as logical one and the DLA will trigger.

Note, if you let the analyzer run long enough at 24 MHz it may stall. It will report that the analyzer cannot sustain the current sample rate.  Simply hit the "Start" button again.

Observing a Sine wave with Logic Analyzer?

Connect the logic analyzer to the signal generator like so:



Switch the signal generator back to sine wave and observe the signal on the logic analyzer.  Be prepared to answer the following question for the post lab: How does the sine wave look on the LA compare to the square wave?  Why if at all are they different?

More on the Saleae DLA

The Saleae DLA also has the ability to monitor and decode serial IO protocols such as RS232 (UARTs), I2C and SPI. We will explore these capabilities in later labs. Go to the Saleae website and read the manual for more detail. 

The MSO has all the digital functionality of the Saleae DLA, but we will not cover that for present.

Measurements with the Digital Multimeter and Lab Supply

Each workstation is equipped with an Agilent digital multi-meter (DMM). The DMM is used for general measurements including: voltage, current, resistance and capacitance. These should be relatively familiar to you from introductory lab courses.

For a simple application, let's measure the power drawn by the kit. We can model the kit as a load resistor for this measurement. By measuring the voltage across the kit and the current supplied we can determine the power load. Voltage to the kit is provided through the USB cable. We could cut the USB cable to measure the power, but that would be just a tad messy. Instead, let's use the lab power supply.

Set the Power Supply Output

Before hooking anything up, adjust the power supplies 6V output to be 5.00 volts. Note you have to have the Output turned on.

Connect the Kit to the Power Supply

Disconnect the USB cables from your kit. Get a BNC cable, BNC grabber adapter and BNC to banana adapter.

TURN OFF THE POWER SUPPLY OUTPUT

TAKE CARE TO CONNECT THE BANANA TO BNC CONNECTOR WITH THE GND TAB ON THE GROUND TERMINAL OF THE POWER SUPPLY. FAILING TO DO THIS WILL APPLY NEGATIVE VOLTAGE AND FRY THE KIT!!

Next, connect the red clip to the kits 5 volt supply test pin and the black clip to a GND test pin.

Turn the power supply kit output on and you should be able to observe the current supplied to the kit. The power supply current meter is accurate for currents >10ma. Let's check it. Get another cable like the one you used to hook the power supply to the kit. Attach the cable to the DMM's current terminals.

The adapter ground ref tab orientation is not critical. It will only effect the current flow reading direction (+/-). Next place this cable in the ground path of the power supply cable like this.

Observe the difference. At this current level the power supply meter should be very accurate. In most cases the power supply meter is adequate.