EE 231 Lab 1: Introduction to the HCMOS Logic Family

Begin by browsing the section on the 74HC02 in your data book before you come to the lab. Try to understand the symbols VIH and VIL before coming to the lab.

1. Put a 74HC02 IC in your breadboard. Look in your data book to determine the pinout. (Make sure power is off when you put components in and connect wires.) Connect pin 7 to ground and pin 14 to VCC (+5 volts). When both inputs of a NOR gate are low (below some threshold called VIL, Input Low Voltage), the output will be high. When either or both inputs are high (above VIH), the output will be low. The truth table for the NOR gate is shown in Figure 1 below.

Complete and confirm this truth table by connecting pins 2 and 3 (inputs) of your 74HC02 to VCC and GND in all possible combinations and recording the input and output voltages (pin 1) in Figure 1, using a voltmeter. Using a voltmeter to measure logic levels is tedious, and sometimes can be misleading. For example, an unconnected input will read 0 volts, leading one to think that it is connected to a logic low when it is not. A logic probe is a device designed to give quick, accurate readings of logic levels.

Look at your logic probe. Make sure the TTL/CMOS switch is set to CMOS. Connect the black lead of the probe to GND, and the red lead to VCC. Repeat the above measurements, recording the color of the light on the logic probe when you touch it to the inputs and outputs of the NOR gate. Use the Figure 1 to record your findings.
 Input A Input B Output Y Logic Level Pin 2 Vin (volts) Logic  Probe Color Logic Level Pin 3 Vin (volts) Logic  Probe Color Logic Level Pin 1 Vout (volts) Logic  Probe Color L L H L H L H L L H H L

Figure 1: Truth table for a NOR gate.

Use your voltmeter to measure the voltage on an unused input of your 74HC02, e.g., pin 5. Record your result. Now use the logic probe to measure the logic state. What color is the light? What is the logic level? What are the advantages and disadvantages of using voltmeters vs. logic probes for digital measurements?

2. When one input of a NOR gate is low, the output depends solely on the other input. When the second input is high (Vin > VIH), the output will be low (Vout < VOL), and when the second input is low (Vin < VIL), the output will be high (Vout > VOH).

Let us determine the circuit response as Vin is varied. Connect pin 2 of your NOR to GND, and pin 3 to the wiper (arrow) of the 10 kilohm pot on your breadboard to generate a variable voltage between 0 and VCC volts. Use your voltmeter to measure Vout as a function of Vin. Make your measurements closely spaced in the region where Vout changes logic levels. Sketch your results in your lab book. At what input voltage does Vout change states? Compare this voltage to VIL and VIH listed in your data book.

3. Connect pin 2 to GND, and pin 3 to the output of the function generator on your breadboard. Set the function generator on your breadboard for a 10 kHz TTL square wave. Examine the input and output with both the voltmeter and logic probe. Increase the frequency. How do the instruments respond as the signals change faster? Which light illuminates on your logic probe? (Keep the inputs connected for the next part of the lab.)

For most operational digital systems, the inputs and outputs change so fast that you will be unable to follow them with a voltmeter or logic probe. The logic analyzer is a tool to visualize changing logic signals. In the lab, we have a 48 channel logic analyzer (48 channel means it can simultaneously look at the logic state at 48 separate points). Connect lines 1 and 2, of your logic abalyzer, to the two inputs of your NOR gate, and line three to the output. Connect a line labeled GND from the logic analyzer pod to Ground on your protoboard.

Start the logic analyzer by double-clicking on the PA485 Logic Analyzer icon on the Windows desktop. Do the following:

1. From the Edit menu, choose Clock Setup. Set the internal clock for a 100 kHz sampling rate.
2. Double-click the Fields Editor icon. Give the top line the name InputA, with only line 1  selected. Give the second line the name InputB, with only the second line selected.  Give the third line the name Output, with only line 3 selected.
3. Click on the green GO button. The logic analyzer will sample the logic levels on lines 1, 2 and 3.
4. Observe the Waveform window. You can use the Magnifying Glass icons on the menu bar to zoom in and out. Does the waveform make sense to you? Print a sample of the waveform, and put it in your lab notebook.
5. Another way of displaying logic data is as numbers. Double-click on the Numeric icon. Insert or Delete columns until you have two columns in the window. Double-click on the header for the first column. Choose Trace Data, INPUT, and set Column Base to Binary. Does the numeric data make sense to you? How does it compare to a truth table format?
6. Save this logic analyzer setup. Choose Save Setup under the File menu item. Save it with a name such as u:\ee231\lab1.set. (You need a subdirectory name ee231 for this to work. If you don't have such a subdirectory create one now.)

What is the relationship between the input and the output signals of the NOR gate when pin 2 is L? What happens if pin 2 is connected to VCC? What could this be used for?

What was the effect of changing the numeric window from HEX to binary?

When you are satisfied that you have a good measurement on the logic analyzer, make a print out of your screen.

August 2000