Lab Project #2: Small-Scale Integration Logic Circuits Duration: 2 weeks Weeks of 1/31/05 2/7/05 1 Objectives The objectives of this laboratory project are to design some simple logic circuits using small-scale integration (SSI) logic chips from the 74xx family. You will also investigate and demonstrate some of the input-output electrical characteristics of these gates. You will have the opportunity to physically measure some of the parameters of these devices and compare them to specified values on the device data sheets. Another objective is to become familiar with the multimeter, the oscilloscope, and the PencilBox Logic Designer. 2 Background An integrated circuit (IC) chip can come in several different types of packages. Figure 1 below shows the physical package outline and pin numbers for a 14-pin dual-inline pin (DIP) package. The 74LS86 is a quad 2-input exclusive OR gate. Refer to the attached copy of the data sheet for the SN74LS86. 0.25 1 2 3 14 13 12 0.1 4 5 6 7 11 10 9 8 0.7 Figure 1 Physical dimensions of a 14-pin small scale integration (SSI) chip. 8 9 Other chips in the 7400-series 1 implement other logic functions such as AND, OR, and NOT. For example, a 7404 implements the NOT function, a 7408 implements the AND function, and a 7432 implements the OR function. Usually, we can think of digital circuits as implementing ideal Boolean functions, with signal values being either abstract 0 s or 1 s. However, sometimes we need to be concerned with electrical details, and be aware of the actual s and s for the inputs and outputs of these devices (Figure 2). Source (gates or devices that are driving) V IN, I IN Digital Logic Gate V OUT, I OUT Load (gates or devices to be driven) Figure 2 Voltage and interfaces for digital logic gates. 1 The prefix 74 indicates a commercial product, whereas a prefix of 54 would indicate a chip that is functionally equivalent except that it meets military specifications. 1
The definitions of and electrical parameters are given in Table 1. All parameters (except for the propagation delay parameters) refer to steady-state, or DC, operation. Table 1 Electrical Parameters of Digital Devices Symbol Name Definition V IHmin V ILmax V OHmin V OLmax I IHmax I OHmax I ILmax I OLmax t PLH t PHL High-level input Low-level input High-level output Low-level output High-level input High-level output Low-level input Low-level output Propagation delay time, low-to-high output Propagation delay time, high-to-low output The minimum input to a logic element such that it will recognize it as a logic high. The maximum input to a logic element such that it will recognize it as a logic low. The minimum output in the high state. If the device is supposed to be outputting a logic high, the output is guaranteed to be this value or higher. The maximum output in the low state. If the device is supposed to be outputting a logic low, the output is guaranteed to be this value or lower. The maximum into 1 an input when a high level is applied to that input. The maximum into 1 an output when the device is outputting a logic high. The maximum into 1 an input when a low level is applied to that input. The maximum into 1 an output when the device is outputting a logic low. The time for a change in the input signal to propagate across to the output signal 2, where the output is changing from the low level to the high level. The time for a change in the input signal to propagate across to the output signal 2, where the output is changing from the high level to the low level. Notes: 1. By convention, flowing into a device is always positive; flowing out is negative. 2. Propagation delay time reference points are typically specified to be the point where the crosses the 50% or midway value from high level to low level, or vice versa. Every digital IC has a data sheet that gives not only its pinouts and a description of its function, but a specification of its electrical parameters, including those above. For example, the data sheet for the 74LS86 gives a value of 2.0 volts for V IHmin, and 0.8 for V ILmax. Notes on workmanship: It is a good idea to practice good workmanship when wiring up circuits in this lab. Cut wires to as short as needed and have them lay flat on the board. This will not only make your life easier when debugging your circuit, but also limit the effects of stray capacitance and inductance when we are working with high speed circuits. Use a consistent color scheme for wires - red for +5V, black for ground, and other colors for other signals. We will take off for excessively shoddy workmanship! Other notes: Handle IC s with care to avoid destruction due to electrostatic discharge (ESD) - keep IC s in electrostatic foam whenever possible, keep yourself grounded when handling them, and don t touch the pins. This is especially important in cold weather when the air is very dry. Have power off when wiring up circuits, and double check your wiring before turning power on. 2
3 Tasks 3.1 Introduction to Lab Equipment 1. Using the multimeter, test the potentiometer and identify leads. Draw a schematic of the potentiometer. 2. Use the oscilloscope to display the 1 KHz clock signal from the PencilBox. Refer to the oscilloscope manual and the PencilBox manual as necessary. Record the actual frequency as measured by the oscilloscope. 3.2 Design of Logic Circuits Using SSI Components Consider the circuit in the figure below. Note that each gate is labeled with its IC type (e.g., 74LS86), pin numbers, and reference designators, or unit numbers. The schematic diagram shows that the two XOR gates reside on the same IC chip (U1). Figure 3 XOR parity circuit. 1. Analyze the circuit and write the truth table for F(A,B,C). In digital vocabulary, the word parity means whether the number of ones is even or odd. Why is this circuit called a parity circuit? 2. Extend the circuit above to show four XOR gates linked together. There should be 5 input signals (A, B, C, D, E), and one output signal F. Draw the circuit, indicating IC type, pin numbers, and unit numbers. What does this circuit do, in terms of parity? 3. Implement this circuit on the Pencilbox. Input signals will come from the switches on the Pencilbox, and output signals will go to the LEDs. Show it to the TA and get his or her approval before turning on power. Note that the logic chip must be supplied with power (+5V) and ground. Tie any unused inputs to low (ground). Demonstrate your circuit to the TA when it is working. 3.3 Electrical Properties Propagation Delay For this section, use the 5-bit parity circuit that you designed above, containing four XOR gates chained together. Connect the Pencilbox clock signal to signal A, leaving switches connected to the other inputs. 1. Using the oscilloscope, measure the propagation delay from input A to the output F. You will have to display two channels on the oscilloscope simultaneously. 2. The propagation delay for the entire circuit is the sum of the delays through each of the four XOR gates. Estimate the propagation delay for a single XOR gate by dividing your result by four. Is your result within the allowable times as indicated on the data sheet? 3
3.4 Electrical Properties Input Voltage Levels In this section, you will analyze the actual input s of the 74LS86, and compare them to the values on the datasheet. 1. Using a single XOR gate, design a circuit to implement a NOT function. The circuit should have a single input A and a single output F. Connect the potentiometer to the input A in such a way that it allows you to vary the input to A, between 0 volts and 5 volts. The input can be measured with the multimeter. Draw the schematic, and check it with the T.A. before proceeding. Hook up the circuit and verify that it performs the NOT function. With the potentiometer set to provide an input signal with 0 volts, the output should be a logic high. When the input signal is +5 volts, the output should be a logic low. 2. What is the actual minimum input to your IC such that it recognizes it as a valid logic "high" (meaning that the output of the NOT gate is a logic low )? Compare this to the value on the datasheet your minimum should be at least as good as the specified value (it should be equal or less than V IHmin ). 3. What is the actual maximum input to your IC such that it recognizes it as a valid logic "low" (meaning that the output of the NOT gate is a logic high )? Compare this to the value on the datasheet your maximum should be at least as good as the specified value (it should be equal or greater than V ILmax ). 3.5 Electrical Properties Output Voltage and Current In this section, you will analyze the actual output and of the 74LS86, and compare them to the values on the datasheet. Note: Be especially careful in this section! Do not allow the into or out of the output to exceed 25 ma, or the part may be destroyed! Consider the circuit below, that has a variable resistor connected to the output of an XOR gate. An ammeter is connected so as to measure the into or out of the output. By varying the value of the resistor, you vary the load on the output. Figure 4 Circuit to measure output, when output is a logic high. Hook up the circuit above, and check it with the T.A. before turning power on. 1. Plot a few values of the output as a function of output, between 0 and 4 ma. On the datasheet, note the value for V OH. This says that the output at I OH = -0.4 ma should be a minimum of 2.7 V. Compare your value of the output at this. Is it at least 2.7 V? 4
Modify the circuit to measure output with the output at a logic low. Check it with the TA before turning power on. 2. Plot a few values of the output as a function of output, between 0 and 12 ma. On the datasheet, note the two values for V OL. Compare your values of the output at these two s. Are they within the specified values on the datasheet? 4 Additional Questions to Answer Answer the following additional questions in your report. Ask the TA or lab instructor for help if needed. 1. There are many IC s in the 74xx series, with many possible different functions. Look up the 74LS27, describe its function, and say how you found it. You can check the data books in the lab or check on the internet. You can search the "chip directory" (see link on the course web page) or a semiconductor manufacturer like Texas Instruments. 2. If you were designing a little circuit to go in your car that would display the temperature outside, would you use 74xx series parts or 54xx series parts, and why? 3. If you want to drive a light emitting diode (LED) with the output of a digital gate, you can either connect it to ground or to logic high through a resistor, as shown below. An LED requires 5 or 10 ma to be adequately illuminated. Why is the resistor necessary? Which of the two configurations would be better for the 74LS86? Figure 5 Driving an LED with a digital gate. Note that if have a device that draws a lot of, you cannot drive it directly from a digital output. For example, most DC motors (even small ones) will draw a minimum of hundreds of milliamps. You will need a separate driver circuit such as a power transistor. The digital output can control the power transistor, which can then drive the motor. 4. The instructions in this lab said to tie unused inputs to ground why? Should you tie unused outputs to ground why or why not? 5