CIS 351

Lab 2: Introduction to Digital Logic

Fall 2018

Pre-lab

Questions 1 through 4 are due when you arrive at lab.

Overview

The purpose of this lab is to provide a simple introduction to basic electronics, and to investigate the principles of Boolean algebra. Please pay special attention to terminology, safety issues, and standard lab practices.

Activities

Work your way through the following brief tutorial.
Submit a detailed lab report (one per group) that includes your answers to the numbered questions.

Resources

Chapters 1 - 3 describe the basics of digital logic. (See also Appendix C located on the companion CD of the Patterson and Hennessy textbook.)
This site explains how to determine the number of Ohms a resistor provides based on the colors of its bands.
Pinouts for the chips in your breadboard kits.

Equipment

Familiarize yourself with the following components of your lab equipment toolbox:

Breadboard: This device is designed for rapid prototyping and construction of circuits.
- There are three power terminals (or binding posts) near the top of the board.
- The board has a series of isolated tie points (holes or sockets aligned in a grid) that are connected together to provide continuous bus structures or strips. Component leads or wires are inserted into these sockets and held firmly enough to create a solid contact without requiring soldering.
- There are four horizontal strips of connected tie points (two each across the top and bottom of the board). These sockets are connected into a distribution bus and are typically used as power strips or as common ground. All the holes in each 1/4 of a strip are connected together.
- There are 128 vertical strips of 5 holes - each of the holes in a strip is connected. These are used to wire together discrete components.
Make sure you understand the built-in connections on the breadboard and how to use them effectively and correctly. It will be used to wire together the digital circuits used in this lab.
Power supply: We will be using a conditioned 5V/adjustable power supply. We will use the fixed 5V lead, not the.1.5-12V adjustable range. Note that the power supply must be turned on, even when using the fixed lead.
Power leads: these have banana clips on each end. Use them to connect the power supply to the terminals on the breadboard - then connect the terminals to your distribution buses.
LEDs (Light Emitting Diodes): LEDs are simple optical devices made of gallium-arsenide-phosphide that are used for signal detection. Note that the longer lead represents the anode side of the device. (In other words, the longer wire goes on the side attached to the positive terminal.)
Resistors: Various resistors are provided, primarily to limit the current to the LEDs. Most LEDs will burn out quickly if their current exceeds 50 mA; we will use much less than that in these labs. Each resistor has a set of color-coded stripes. These stripes indicate the amount of resistance in Ohms. See http://xtronics.com/kits/rcode.htm for information on how to read the color code.
Wire kit: Various length wires with pre-bent ends to facilitate circuit wiring.
Wrist straps: These are provided to guard against static electricity (CMOS chips can be damaged by excess static). Ground these to the breadboard or negative terminal and wear one whenever you are handling IC chips.
Logic chips: Our circuits will be composed of gates implemented in logic chips of the HCT family, which stands for High-speed CMOS TTL-compatible (CMOS is Complementary Metal-Oxide Semiconductor and TTL is Transistor-Transistor Logic). They are pin compatible with the 74LS (Low-power Schottky) family of logic chips. Although CMOS chips are vulnerable to static, they consume considerably less power, fit more functionality per chip, and can be powered by a wider range of voltage than TTL or TTL/LS chips.

Some important guidelines when working with CMOS:

Avoid touching the pins.
Never connect an input signal to a CMOS chip when the power is off. In other words, make sure there is power flowing through pins 7 and 14 before connecting input wires to a power source.
Do not remove power if an input signal is still present. In other words, disconnect power to your inputs before disconnecting power to the chip or breadboard.
Unused inputs may pick up stray signals. Stray signals can cause rapid oscillation in logic state, consuming excessive power. If chip behavior is erratic, or the chip becomes very hot, connect unused inputs to ground.

To use integrated circuits with a breadboard:

Align the leads (pins) of the chip with the contact points on each side of the center, such that each pin is isolated.
Gently press the center of the IC until it drops into position.
Remove the IC by using the chip extractor to carefully pry it up until it can be lifted free.

Creating a Simple Circuit

Before creating your first circuit, there is something you need to understand about LEDs: Normal, incandescent light bulbs produce light by resisting the flow of electricity. This resistance produces heat, which causes the filament to glow. When you drain a flashlight battery, most of the energy in the battery has been converted to heat.

In contrast, LEDs produce light by exciting electrons causing them to release photons ("light particles"). This process uses use very little energy. Consequently, LEDs do not resist the flow of electricity. When there is nothing to resist the flow of electricity, the amount of current (think "number of electrons per second") increases. You may remember the following formula:

V = IR

Or, in other words, voltage equals current (in amps) times resistance (in Ohms).

Suppose you hooked up a red LED with a resistance of 1 micro Ohm (1x10^-6 Ohms) directly to a 5 volt battery. Using the formula above, how much current will be drawn from the battery?

The power supply cannot produce that many amps. However, the 1 amp that the power supply can deliver is enough to damage the LED and (more importantly) the breadboard. Therefore, always add a resistor as shown in Figure 1.1 to limit the amount of current through the diode to about 10mA.

LEDs typically use a fixed amount of energy, referred to as the "voltage drop". A red LED has a voltage drop of 1.7V, leaving 3.3 volts to "push" the current through the resistor. How strong of a resistor should you add to the circuit to limit the current to 10mA?
What value resistor should you use for a green LED with a voltage-drop of 2.1 volts?
What are the color codes for the resistors in the preceding questions?

Now that you understand the importance of using resistors, construct the circuit shown in Figure 1-1 and described below. Simple LED

Connect the power supply to the positive and negative terminals on the breadboard.
Connect each terminal to a distribution bus (one of the horizontal rows).
Insert the appropriate resistor into the breadboard and connect it to positive.
Insert an LED and connect the resistor to the LED.The LED is a diode, and will only work when the longer wire is connected to a path to positive voltage.
Connect the LED to ground.
When the circuit is constructed correctly, the LED will glow visibly whenever power is applied. If it glows dimly, you chose the wrong resistor.

Using Integrated Circuits

Integrated circuits typically contain multiple logic gates and hence provide multiple input and output connections (called pin outs). Find an IC labeled 74HCT08 - this is a quad 2-input AND gate. That means there are four AND gates within the chip, each of which takes two inputs. Each AND gate implements the Boolean AND function.

If the chip is positioned with the notch to your left, then pin number 1 in the lower-left corner. The remaining pins are numbered sequentially and counterclockwise.

Integrated circuits need power in order to open and close the transistors that implement the gates they contain. Pin 14 (in the upper-left corner) is called V_CC. This is where you connect the source voltage. Pin 7 (in the lower-right corner) is where ground voltage is applied. In other words, Pin 14 must be connected through a breadboard terminal and power distribution strip to the positive terminal of the battery. Likewise pin 7 must be connected to the negative terminal of the battery. Never place a load (current and voltage) onto a pin unless the chip is also powered through pins 7 and 14.

The figure below shows the pinout for chip 74HCT08 --- a chip containing four 2-input AND gates. Pins 1 and 2 are the input to the first of the four AND gates, pin 3 is the output. Likewise, pins 4 and 5 are inputs to an AND gate with output pin 6, pins 9 and 10 (input) connect with pin 8 (output), and pins 12 and 13 (input) go with pin 11 (output).

Your kits also include the following integrated circuits:

Designation	Operation
74HCT00	NAND
74HCT02	NOR
74HCT08	AND
74HCT32	OR
74HCT86	XOR

They are all quad 2-input ICs - meaning there are four gates within each chip, where each gate takes two input. This page shows the pinouts for these chips.

To investigate the operation of an integrated circuit that implements the AND function, construct the following circuit (Figure 1-3 gives the schematic):

Carefully insert a 74HCT08 IC into the break in the breadboard so that each pin sits in its own strip.
Connect the power supply between pins 7 and 14, with the positive terminal on pin 14.
Connect a 330-ohm resistor from output pin 3, then to an LED, which is then connected to ground.
Connect two wires to the input pins 1 and 2. Using these wires, connect both inputs to 0V (ground) and record the logical state of the LED. (In other words, is it on or off?) Then connect both inputs to 5V (HIGH) and record the logical state of the LED. Then connect pin 1 to LOW and pin 2 to HIGH and note the LED. Finally, swap the input wires and record the state of the LED.

Perform the following:

Construct a truth table from your observations.
Take the wires attached to pins 1 and 2 and leave the other ends floating in the air. Describe the effect of having these "floating inputs". (To get some more interesting behavior, grab the floating ends with your fingers and wiggle them.)

Switches

You may find it convenient to use switches to control whether an input wire has a "1" or a "0" on it; however, as you noticed, floating inputs can cause erratic behavior. In the Resources section of the course web page, the page Using Switches explains how to correctly use the two types of switches in your kit.

Add switches to the circuit you built (the one shown in Figure 1-3). You may use either dip switches or momentary switches. Demonstrate to the instructor or lab assistant that your circuit works correctly, then attach a photo of your circuit to your lab report.

Experiment

Get a random IC from me. Replace your 74HCT08 with the new chip. Construct a truth table from your observations. Identify the type of gates the chip contains. (Your writeup needs to include the letter on the chip.)

Creating a more complex combinatorial circuit

Digital logic is implemented by using basic logic gates as building blocks to create more complex circuits. A combinatorial circuit is a set of gates whose output function depends only on the values of its current inputs (i.e., there is no storage or feedback involved in the system). The following circuit is a common combinatorial circuit.

You will need 2 quad NAND IC's (74HCT00), 2 resistors, and 2 LEDs to construct it. I suggest using DIP switches in the "pull-down" configuration to control the values of A and B. Remember to use 4.7K ohm resistors for your switches.

Connect inputs A and B to NAND gate 1.
Connect the output C of NAND gate 1 and input A to NAND gate 2. Connect the output C and input B to NAND gate 3. Connect the output C of NAND gate 1 to both inputs of NAND gate 4. Designate the output of NAND gate 4 as Y.
Connect the output of NAND gate 2 and the output of NAND gate 3 to NAND gate 5. Designate the output of NAND gate 5 as X. See Figure 2-3 below.
Connect two LEDs as before to both outputs Y and X of the circuit. Place the LED for output Y to the left of the LED for output X.
Apply power and test the circuit.

Determine the truth table from observation. Place the column for Y to the left of the column for X. In other words, treat YX as a two-bit number.
Using your observations as a guide, describe what simple operation this circuit implements. (Hint: Place the LED for output y to the left of the LED for output x.)
Demonstrate your circuit to the instructor (or lab assistant).