KB07: Build the Test Circuit Component and Jumper Layout
Part A: Parts and Tools
This lab involves designing, building, and testing circuits using design concepts from the Digital Logic course EE-2440. You should have a lab kit box containing parts and equipment for the duration of the semester. The equipment includes:
- Breadboard (also referred to as a protoboard) where you will build your circuits. In the lab room, the breadboard should come with a test circuit and already connected. If you are taking an online lab course, you must build the test circuit on the breadboard first.
- Several cables (for use with power supply and oscilloscope).
- Wire cutter/stripper tool for cutting wire and stripping the ends for insertion in the breadboard.
- IC (integrated circuit) chips for building circuits you design in the various experiments.
- Logic probe: this is the main tool you will use in the lab for troubleshooting your designs.
If you are taking the on-campus lab, you must have the following additional items:
- USB flash drive: you'll need to store waveform data captured on the oscilloscope screen and transfer it to a PC.
- Lab Journal: you'll need to purchase a composition book to record your prelab work (design work including truth tables, K-maps, state tables, state machines, logic diagrams, and wiring diagrams), your lab experimental results (including your observations, results – waveforms, truth tables, and state tables/machines), answers to any questions posed in the lab manual, and any note that you take related to the experiments.
Part B: Circuit Diagrams
Your circuit diagrams should serve two functions:
- Logic Diagram — to show the flow of logic in your design.
- Wiring Diagram — to guide you in making circuit connections.
It is recommended that you draw your circuit diagrams directly in your lab journal. However, when you submit a laboratory report, you'll need schematic capture software to clearly and neatly draw circuit diagrams. You will be responsible for submitting typed laboratory reports for experiments 3, 5, 6, 7, and 10.
For your laboratory reports, use one of the "schematic capture" programs to draw the schematic diagram:
- ExpressSCH (a part of ExpressPCB) to draw your circuit diagrams: https://www.expresspcb.com/
- KiCad EDA (a cross-platform and open-source Electronics Design Automation suite): https://KiCad.org
Download and install the schematic software early in the semester — you'll need it starting with Lab 2.
With ExpressSCH, the following requirements will be easy to meet:
- To make circuit logic as clear as possible, gate and flip-flop symbols should be shown separately — not together inside a chip rectangle as they are shown at the end of this manual. That
way, they can be placed individually in a diagram wherever it is convenient. Only when a chip acts as a single unit (counter, MUX, etc.), not a collection of identical elements, is represented as a rectangle.
The unit is shown as a rectangle (counter, MUX), pins are not shown in their true positions around the rectangle. Instead, they are arranged to make the logic flow easy to follow, with logic inputs at the left and outputs at the right. Control inputs (e.g., clock, reset, enable) can appear at the left or the top or bottom of the rectangle.
- Active-low inputs and outputs are shown with small inverting circles. Alternatively, a bar above a variable's name or a forward slash before it (i.e.,/X) indicates inverted logic. These symbols are very helpful when following logic flow or troubleshooting a design.
- In all rectangles, pin names (e.g., D, Q for flip-flops, Cin for adders, Clk for counters, etc.) are shown inside, pin number outside. Chip part numbers are placed next to the device symbols (e.g., 7432 next to an OR-gate symbol).
- Each rectangle and gate symbol should contain a "U" (Unit) number. In the case of gates and flipflops, these unit numbers identify which chip they are part of. Suppose your design involves six 2-input NAND gates, four gates to a chip. You will need two 7400 chips. Suppose you number these U2 and U3. Then four gate symbols might be designated U2, the other two U3. A connecting wire from pin 3 of U2 to pin 9 of U3 could then be described as "U2-3 to U3-9". A number system such as this will do much to speed up the troubleshooting process. One partner can call out each connection from the diagram, while the other partner checks the circuit to see if the connection has been correctly made.
- Your circuit will be laid out as a line of chips (with spaces between so you can lever a chip out if it needs replacing). All chips will be facing left, as shown in the diagram on the next page. In your diagram, label the left-most chip U1. Then the next chip from the left would be U2, etc. This will make it easy to associate chip components (like gates) on the diagram with the physical chip on the board.
Different gates or flip-flops within a chip can be assigned a different label, e.g., A, B, C., etc. Suppose the first chip on the left is a 7474, which you label unit 1 (U1) on the diagram. Now, 7474's have two D flip-flops. Thus, U2A could refer to one flip-flop, and U2B could refer to the other flip-flop. However, the flipflops can also be told apart because they each have different pin numbers, so the labels A, B, etc., aren't really necessary. Make sure when drawing a diagram that no two gates or flipflops show the same pin numbers unless they belong to different chips. Later, when connecting your circuit, make sure you use the same pin numbers as shown in your diagram.
Part C.1: Test Circuit (For On-Campus Lab Course)
Figure 7.1 is a layout diagram of the circuit to be used in testing your experimental designs. If you are taking an on-campus lab course, each group should have a breadboard that comes with a connected test circuit in the locker. Please check the connections on the breadboard, make sure all the connections are correct.
If you are taking an at-home lab course, you have to build Clock and Digital Outputs circuits on your breadboard.
The test circuit consists of the following sections (starting at the left side of the layout diagram in Figure 7.1):
- Pulser Debouncer Circuit
- LED Bargraph Display
- Digital Outputs
Figure 7.2: Pulser Debouncer Circuit Diagram
There are two pushbutton switches, each having normally-open and normally closed contacts. The outputs of each switch are sent through a NAND debouncing circuit, and the NAND outputs are identified as "normally low" and "normally high".
In this case, "normally" means when the button is not being pressed. When the button is pressed, the normally-low output goes high, and the normally-high output goes low. When the button is released, the outputs return to their normal levels.
The photo at the right of the two pulsers (push-button)\ switches) and the 7400 NAND chip that denounces their outputs. These outputs (also shown in Figure 7.2) are circled: the two holes at rows 14 and 15 are denounced output of the top pulser; those at rows 16 and 17 are of the lower pulser. (Each pair consists of normally high and normally low output.)
When a pulser is pressed, its metal contacts come together and, because of their springiness, their voltages initially "bounce", as shown: or
Because of this behavior, the direct output of the pulsers can not be used to clock a counter or flip flop chip since one press of a pulser results in many pulses (more than shown here). So the outputs are brought to a 4-NAND 7400 chip. The gates are connected as two latches. As soon as a pulser output changes level, the latch output flips accordingly and the bounces that follow are ignored:
Part C.2: Test Circuit (For At-Home Lab Course)
If you are taking this lab as an online lab course, you have to build a clock generator circuit on the breadboard.