CS 14 -- Lab 1

Welcome to the digital design lab! Today's task is to build a half-adder -- a simple adding circuit that will be presented during the lab time. You should begin small by constructing the pieces needed, and build your way to the entire half-adder.

Knowing what the toys are

Don't do anything yet! We're going to spend a few minutes going over the various chips, boards, power supplies, and other devices that you'll need to use for this and other labs. Here's a list of the devices you'll be using, and some key information about them:

• Power supplies: There are two kinds of power supplies in the lab. Each power supply has a positive (red) contact and a ground (black) contact. On some of the black power supplies, it's a bit hard to tell which is which, but each is marked unambiguously. If there's a red wire soldered on, its positive. If there's a black or blue wire soldered on, it's ground.
• Breadboards: These look like clipboards will lots of little holes in them. These are going to allow us to structure our circuits, as we'll have to plug all of the parts into them to make them work. Each breadboard has several posts at the top, where you can make a connection from the power supply. Red and black have the same meaning as above, for the power supplies themselves.
• Alligator clips: You use wires with alligator clips to connect between the power supply and the posts on the breadboards. It's essential to ensure that you are not getting positive and ground reversed. This can destroy chips and make them very, very hot. You should only should only connect your board to the power supply after you've built your circuit. Beware of short circuits.
• Power channels: There are several power channels on each board, marked by red or blue/black lines. The colors respectively mean positive and ground. There should be wires from the posts to two of the power channels. Each power channel is electrically connected inside the board, meaning that once a channel is connected to a post, power is available at each post in the channel. There should be wires connecting all of the red channels together and all of the blue/black channels together. If these wires are not already on your board, you can add them as you go. Again, don't get red and black reversed.
• Capacitors: There should be a capacitor on your board connecting the two power channels that are connected to the posts. The capacitor helps smooth out fluctuations in the current.
• Chips: We'll use three kinds of chips today: 7404 (NOT gates), 7408 (AND gates), and 7432 (OR gates). Data sheets describing the operation of these chips are attached. As the semester goes on, you'll be using lots of other chips. Data books on the shelf in the lab describe their operation. When possible, data sheets will also be posted on the class web pages, under the Documents section. Some important notes on the chips and their use:
  • Orientation: Each chip has a mark, either a small hole or a small cutout, to indicate the top end. The top pin on the left is pin 1. The pin numbers go down the left side and up the right side. Most of our chips have 14 pins, but some will have more.
  • Power and ground pins: Two pins on each chip must be connected to power channels. The pin marked Vcc on the data sheet should be connected to the red channel. The pin marked GND should be connected to the black/blue channel. Don't get them backwards, and don't install the chip upside down.
  • Installing chips: Chips must be laid on the board with the right end up and with the pins straddling the groove. Each row of five pins is electrically connected within the board. By placing the chip so it straddles the groove, you are ensuring that there are separate sets of holes for each pin of the chip. Be sure to press each chip in gently and evenly, without letting one end or the other twist up or down. This will help you avoid breaking pins.
  • Removing chips: Remove chips carefully and evenly, by using a chip extractor and pulling straight up. Do not try to remove the chips with your fingers, as you'll probably bend or break the pins in the process. Return each chip to the place you got it.
• Wires: There are bins of wires available for your use. When you make a connection on the board, try to do it neatly and try to avoid using wires that are too long for the purpose. Your work will be far easier to test and correct if the wires are short and well organized. When you need a wire of a certain length, you can either look for one or you can make one from a longer wire, using wire strippers to cut and remove insulation. You may even want to devise for yourself a wire-color scheme, such that each color implies something about the purpose to which the wire is being put.
• Switches: There are two kinds of switches, press buttons or long chip-like sets of switches. One side of each switch should be connected to ground and the other side to whereever you need it in the circuit. For most chips, an input from an open switch is equivalent to a positive input.
• LEDs: Light-emitting diodes are used to show the output of your circuits. You can connect an LED from the output of a gate to a 100-ohm resistor (these have a brown-black-brown stripe pattern on them). The other side of the resistor must go to ground. LEDs have a direction: the short leg of the LED should face the resistor. On some LEDs there is a flat edge to indicate the side facing the resistor.
• Logic probes: Logic probes are great debugging tools. If you connect them to power you can insert the probe tip into holes to determine if there is a positive voltage (a red light and a high-pitched sound), a ground connection (a green light and a low-pitched sound), or an open connection (no light or sound). One of the probes doesn't have sound. Probes don't always work in locations that are connected to LEDs.

We may not use all of these parts in the first lab, but you will need a number of them, and eventually you will use all of them.

What to build

Just as with programming, it is unwise to construct a complete solution without testing its parts along the way. With a large circuit, you want first to construct the smaller circuits that will later compose the large one. With each smaller circuit, you should debug it thoroughly before connecting it to other smaller circuits.

Today, we'll move one part at a time through constructing the half-adder:

1. Set up a single AND gate by using a 7408 chip. Set up an LED so that you can see the output of this gate. Try all four possible inputs to be sure that it's operating correctly. You've just constructed the circuit the generate the carry bit for your half-adder, where one input is your A value, and the other your B value.
2. Begin constructing an XOR circuit by using the NOT gates in the 7404 chip. You will want use two gates, one to produce NOT A, and the other to produce NOT B.
3. Use two more of the AND gates from the same 7408 chip as you used above. To one gate, connect A and NOT B, and to the other NOT A and B.
4. Finally, bring the outputs of the two AND gates used in the previous step, and connect them as inputs to an OR gate on your 7432 chip. The output of this particular gate is A XOR B, also known as the result bit.
5. Wire the outputs of these two circuits so that they appear adjacent to other another on an LED. Once you've done that, you have a half-adder!

Finishing up

Be sure to leave yourself a little time to complete the lab! Every week, there are some things you'll have to do whether or not you've completed the assignment.

1. Demonstrating your work: If you've finished the lab, then you need to show it to the professor or the TA. You won't get credit unless we've seen it work!
2. Saving your work for another day: If you need to save your work to finish another day, put your name on a label on the board, and place it in on of the lower drawers. Unlabelled work may be dismantled!
3. Cleaning up: When you're done, put away everything and clean up your area. If you do not clean up and put your board away (if needed), your work may be dismantled!