Description

Digital Circuits: Logic to Gates

Lab dates and times:

Location: EA Z04 (in the EA building, straight ahead past the elevators) Groups: Each student will do the lab individually. Group size = 1

Preliminary Work (20 points)
(You should do this part before coming to lab).
Physical gates are built out of transistors, and require physical signals that use correct voltage levels for inputs and produce physical signals with correct voltage levels for outputs. To work correctly, the transistor circuits that comprise a gate must have connections to a voltage supply and to ground. For example, in the case of 74-series logic circuits used in this lab, the supply voltage (Vcc) must be 5 volts. In these integrated circuit packages, several gates are contained. Search Google specifying that gate number and its function (e.g. “7486 XOR gate”) for pin connection diagrams, such as the pin diagram shown in Figure 1. You must have the pin diagrams for each gate you want to use, in order to do the following tasks. You can find pinout of rest of 74-series gates here: http://www.qsl.net/on7pc/datasheet/ttl7400/7400family.pdf. Other gates you need today are: 7408 quad 2-input AND, and 7432 quad 2-input OR.

Fig 1: 7486 Quad 2-input XOR gates

First, read the document posted in Moodle for CS223 labs: “Circuit Schematic versus Logic Diagram”. Then, using the logic diagram in Fig. 2 (below) as a starting point, draw a circuit schematic of the digital circuit you will build. This should include pin numbers marked on the inputs and outputs of all the gates, part numbers (IC’s code) of the IC package marked on each gate, plus power and ground connections marked on the side of the drawing. After that, draw another schematic for full-adder circuit (Figure 3).

Fig 2: Half adder

Fig 3: Full adder
Recommendations
In CS223 labs, you build circuits by ICs and later by FPGA. It is better to obey some simple rules to avoid damaging electronic parts or confusing yourself with debugging your circuit.
 Avoid touching IC or FPGA pins directly by your hand. Static electricity of your body can damage them permanently.
 The white board which you setup your circuit on it, is called “breadboard”. Search in internet and find out how its pins are connected internally.
 Postpone connecting power pins (Vcc and ground) to last step. Check circuit connections and if everything seems ok then connect power pins.
 For easier debugging of circuits, always follow a wire color convention. For example, always use black or white wire for ground and red wire for Vcc.
 If LED’s light is weak, or if the IC’s package is very hot (you can touch plastic part) you have a problem in the power pin connections (short circuit, connecting Vcc wire to ground pin,…).

Part 1: Practice with Logic Gates (35 points)
2) Using your circuit schematic, build the circuit step-by-step, following the
Digital Circuit Suggestions given in Moodle and recommendations in page-2. Connect the inputs to switches on the logic board. Connect the outputs of your logic circuit to LEDs on the logic board. Don’t forget to connect +VCC power and GND ground to the VCC and GND pins on both IC packages.
3) Make a test probe by connecting another LED on the logic board to one end of a long wire, whose other end will be used to touch circuit points and “see” the logic values. A full voltage level ~5 V will cause the LED to shine brightly; a 0 V level will not light up the LED. A low light output from the LED means that the voltage being sensed is in between logic 0 and logic 1, meaning something is WRONG with your circuit.
4) Now draw the truth table for the 2-input 2-output logic circuit that you have made, and fill in the left-hand (input) side in standard binary counting order. For each row, apply the input combinations by adjusting the switches, and measure the output. Use this information to complete the truth table, filling in the right-hand (output) side.

Part 2: Understanding and Building a Full Adder (45 points)
2) Using your circuit schematic, build the circuit, following the Digital Circuit Suggestions given in Moodle. Connect the inputs to switches on the logic board. Connect the outputs of your logic circuit to LEDs on the logic board. Don’t forget to connect +VCC power and GND ground to the VCC and GND pins on all the IC packages.

Clean Up
1) Clean up your lab station, and return all the parts, wires, the Beti trainer board, etc. Leave your lab workstation for others the way you would like to find it.
2) CONGRATULATIONS! You are finished with Lab #1 and are one step closer to becoming a computer engineer.

LAB POLICIES
1. You must wear your face mask at all time.
2. There are three computers in each row in the lab. Don’t use middle computers, unless you are allowed by lab coordinator.
3. You borrow a lab-board containing the development board, connectors, etc. in the beginning. The lab coordinator takes your signature. When you are done, return it to his/her, otherwise you will be responsible and lose points.
4. Each lab-board has a number. You must always use the same board throughout the semester.
5. No cell phone usage during lab. Tell friends not to call during the lab hours–you are busy learning how digital circuits work !
6. Internet usage is permitted only to lab-related technical sites. No Facebook, Twitter, email, news, video games, etc–you are busy learning how digital circuits work !
7. If you come to lab later than 20 minutes, you will lose that session completely.
8. When you are done, DO NOT return IC parts into the IC boxes where you’ve taken them first. Just put them inside your Lab-board box. Lab coordinator will check and return them later.