By: Cintia Margi & Bob Vitale. Modification of original exercise developed by Pak Chan and Kevin Karplus and used with permission.

Transcription

1 Laboratory Exercise #5 Using an Oscilloscope to see the real behavior of a TTL gate By: Cintia Margi & Bob Vitale. Modification of original exercise developed by Pak Chan and Kevin Karplus and used with permission. Report is due Monday October 28, 2002 or Tuesday October 29, 2002 depending on your section. Figure 1: Oscilloscope I. Objectives In this lab we will look at some of the real behavior of the transistor-transistor-logic (TTL) gate, particularly the dynamic behavior of digital circuits, by learning how to use the oscilloscope. The oscilloscope is a tool that allows you to look at time-varying analog voltages. Use of the oscilloscope is a fundamental part of any real digital or analog circuit analysis and debugging. II. Lab 5 prelab Read and understand the use of the oscilloscope from the following link: Answers the following questions prior to lab: 1. Suppose you want to view a square wave (clock-type signal) that is 5V peak-to-peak with a frequency of 1 MHz. What would be a good setting for the Time/Div and V/div to view the waveform? 2. What is the difference between CHOP and ALT? 3. What is the x10 switch on the scope probe used for? 4. What is the difference between AC and DC coupling? Page 1 of 9

2 III. Lab Work 1. Setting up the Oscilloscope Here are a few settings for the Oscilloscope to get you started: HOR DISPLAY: set to A---chooses an automatically generated sweep across the screen. TRIGGER SWEEP MODE: set to AUTO or NORM SOURCE Trigger: set to INT TRIGGER COUPLING: set to + SLOPE and AC TRIGGER SOUCE: set to INT INT TRIGGER (Lever between Ch1 & Ch2): set to Ch1 (set to Ch2 when you view Ch2) VERT MODE: Set to ALT or CHOP for displaying two signals (set to CH1 or CH2 if only viewing one channel or other). INPUT COUPLING: Vertical lever with AC GND DC markings next to it. Set to GND for both Ch1 & Ch2. HOR SWEEP SETTING: set to 0.2ms/div Display the traces on the screen. You may need to find them by adjusting the intensity knob and the focus knob. Also you may need to move the trace (sometimes called the beam) to the center of the screen using the horizontal or vertical position knobs. With the probes connected to nothing, you should see two illuminated horizontal lines across the screen. The lines are caused by electrons bombarding the phosphorus screen (same as TV screen). The stream of bombarding electrons is moved across the screen by the scope timing and up/down by the magnitude of the incoming signal. The effect is similar to an etch-a-sketch, except in electronic form. The phosphorus screen is briefly illuminated by the bombarding electronics and then the light from electrons hitting the screen dies out. A trace is continuously displayed by the trace being continuously redrawn over time. Lets now check the calibration of the oscilloscope. Connect the Ch1 probe to the calibrated output (at the lower-left-hand corner of the oscilloscope), and adjust the vertical gain and horizontal sweep to display a stable square wave. You may need to adjust the trigger value to keep the square wave from moving about. Inspect the horizontal portions of the square wave and see if they are truly horizontal, or if they slope up or down. This may be especially noticeable at the corners. If the calibration squarewave is displayed with horizontal lines with up/down slope, then the scope probes need adjustment. This is done, for the probe, by adjusting the flathead screw located at the probe connector (where it attaches to the scope). Check each the calibration of each channel s probe (Ch1 & Ch2). If you cannot display a stable square wave, you may need to adjust the triggering settings. Trigger refers to when the scope starts drawing probe data on the screen. Refer to the following instructions for additional information or discuss with your lab staff. Page 2 of 9

3 Additional Background on Oscilloscope Display and Settings The oscilloscope displays a magnitude (typically voltage) of a time-varying signal. Much like a TV, it has a cathode-ray tube (CRT) and input source. Unlike a TV, where the input signal comes from a TV cable or antenna, the input source of an oscilloscope comes from the measured signal of your Device Under Test (DUT). To display a stable waveform for a periodic signal, you must ensure that controls are properly set up. The following is a description of some of these controls -Horizontal sweep rate: This is the large knob in the middle upper sections of controls. This control determines how fast the beam sweeps across the screen. This in turn determines how much time each horizontal division of the screen represents. Typically O-Scope time knobs are labeled in some time per division (example: 2 ms/div; 5ms/div, etc). See Figure 2.0 Figure 2: sweep rate Vertical gain: There are two knobs, one for each probe or channel. This adjustment is a multiplication factor for the magnitude of your signal. If you have a small signal, you would increase the vertical gain or amplification so that the signal fills most of the screen, thereby making it easier to see and measure. Signal amplitude can then be read from the vertical divisional marks on the screen. An example setting would be 2 volts/division for a 5 volt maximum signal. Vertical Position: Two small knobs provide allow you to reposition the signal up/down on the screen. It does not increase the value of the signal, but rather just moves the entire signal up/down so that you can align it with the grids on the screen or to move it out of the way. Probe Setting: Some O-scope probes have a switch on the body that allows you to set it to either 1X or 10X. A 10X probe setting is like a zoom-in feature (effectively reducing the voltage seen by Page 3 of 9

4 the scope by a factor of 10). In the previous case were setting was 2 volts/div, a 10X probe setting would result need a vertical gain of 0.2 volts/division. Investigate and determine if your lab probes have this feature. Trigger level: This is a small brown knob in the sweep mode section of the scope. This allows you to adjust the magnitude level for triggering a sweep. The sweep is triggered (started) from a signal that is identified by the Source Trigger lever. See Figure 3.0 Figure 3: trigger level Trigger slope: This adjustment allows you to start drawing the waveform using a rising or falling edge of the triggering signal (identified by the source trigger lever). Trigger holdoff: This adjustment is for the time interval after the sweep is done, before the scope starts looking for a new trigger. See Figure 4.0 Page 4 of 9

5 Figure 4: more on trigger Trigger source: This level determines what signal is used to start the display of traces. You can choose from probe channels 1 or 2, or an externally applied signal connected to the BNC connector just below this lever. A BNC connector is the coaxial connector the probes use to connect to the Oscilloscope. BNC is an abbreviation for Bayonett-Neil-Concilmen. Bayonet is the style of connection tabs from one connector insert to the other rather than threads; Neil & Concilmen invented this connector this widely used Radio Frequency (RF) Connector at Bell Labs in the first half of the 1900s. Page 5 of 9

6 Figure 5: more adjustments 2. Oscilloscope Probe: The oscilloscope probe is the measuring device for the oscilloscope. The probe picks up signal voltage from the device under test and transfer the signal to the oscilloscope for display. The probe has three parts: a BNC coaxial cable (two wires are inside this cable), a grabbing metal pointed tip with a plastic cap (signal connection), and an alligator clip (reference ground connection). Typical signal measurement involves (Refer to Figure 6) connect the alligator clip to the DUT's (Device Under Test) ground or the ground connector on the Oscilloscope. Do not leave dangling otherwise your displayed signals may become corrupted by noise. Connect the probe tip to the output of measurement location on the DUT. The oscilloscope should then display the measured time varying signal of the DUT with respect to ground. Page 6 of 9

7 Figure 6: Probe A common misconception is that a probe is just a piece of wire with a clip. It is not, a probe is a carefully crafted transmission line. It is carefully designed to pick up the signal (voltage) with almost minimal impact (loading) to the circuit (DUT) that you are testing. A simple wire with a clip connected to your circuit would load your circuit with resistance and capacitance of the Oscilloscope and result in a different measurement. The goal here is to make a measurement of the circuit without disrupting it. Measure the relationship between earth ground and the probe ground. Two probes, and therefore two alligator clips, should be attached (one each) to Channel 1 and Channel 2 of your oscilloscope. Disconnect the power plug of the oscilloscope from the main outlet. Measure the resistance (using the multimeters) between the earth ground prong of the power plug and the first alligator clip. Report the resistance. Measure the resistance between the earth ground prong of the power plug and the second alligator clip. Report the resistance. Based on the above measurements, state the basic relationship between the earth ground and the alligator clips. 3. Getting clock signals out to the protoboard Signals that vary as a function of time are developed using the Xlinx demoboard and then connected to the plugboard. You can view them on the oscilloscope. Review the schematic for the demoboard. There a clock oscillator hooked up to pin 13 of the XC4003A. The clock oscillator appears to be an MHz oscillator. We will use this clock and a counter to generate some time-varying signals. Use a 16-bit binary counter from the Xilinx library (either CB16RE or CC16RE---they are equivalent for this lab), connect pin 13 to the clock input and tie the clock-enable line high. This setup should result in a counter that increments on every clock cycle. The little board that connects the demoboard to the protoboard connects and makes FPGA pins 1-11 and available for input/output to the protoboard (pins are dedicated to the 4 pushbuttons). Page 7 of 9

8 Figure 7: Connections from demoboard to protoboard Use pins 81, 82, 83, 84, 3, 4, 5, and 6 to output bits 15,13,11,9,7,5,3,1 of the counter output. Download your design to the FPGA. Connect the Ch1 oscilloscope probe to bit 15 of the output and set oscilloscope to trigger off channel 1. Connect the Ch2 oscilloscope probe to the other bits of the counter and investigate the other output bits in turn. Draw in your lab book the resultant waveforms relative in time and magnitude to each other for the bits investigated. What is the period (time until the signal repeats) of each of the outputs, as measured on the scope? Calculate the expected periods assuming that the oscillator for pin 13 is indeed MHz. 4. Dynamic behavior Build the circuit shown in Figure 8 using LS-TTL chips and connect the input to bit 11 of the counter output. Remember to connect power (V cc ) and ground (GND) for the SSI part. An inverter can be made from a spare XOR gate, so you only need one LS-TTL package (74LS86). The capacitor should be either a brown disk body or a rectangular small plastic-like body with two wires coming out the same direction. It should be labeled 103 (where 1 is the 1 st digit, 0 is the second digit and 3 is the number of zeros after the two digits). This number is then multiplied by pico-farads (10-12 ) to get the value of capacitance. Figure 8: TTL circuit Page 8 of 9

9 Hint: Prior to assembling your circuit, diagram out the chip, capacitor and connections. Label each wire, chip and input and output in the circuit with a pin number. List of all the wires that need to be connected---remember to connect Vcc and GND wires! Answer the following: 1. How do make an inverter out of an XOR gate? 2. What is the truth table for the given circuit? 3. What do you expect the circuit to do under dynamic conditions? Drive the circuit using a square wave from the Xlinx demoboard connected to the input of the circuit, view the result on the scope. Use both channels of the scope, Ch1 for the input waveform and Ch2 for the output waveform. You should trigger off of the input channel. What is the dynamic response to the circuit when it is driven by the oscillator? Look at the signal at the output of the inverter with and without the capacitor connected to the inverter. What is the capacitor doing? Give an estimate of the equivalent resistance of the inverter when pulling up and when pulling down, based on what you see on the scope. What is the measured duration of the glitch in the output with the capacitor connected? Estimate the duration of the glitch without the capacitor (explain your assumptions and method for this estimate). Page 9 of 9