Title:
Basic Logic Circuits
Objectives:
- Use the digital design process to build a basic logic circuit
- Become familiar with standard TTL SSI packages (chips)
Theory:
The digital design process consists of 1) specification, 2) design, 3) implement,
and 4) test. The design step can be broken into the following steps: a) define inputs
and outputs, b) create truth-tables, c) derive simplified logic expressions, d) create
gate-level design, and e) simulate gate-level design.
The standard TTL chips that will be used are Dual In-Line Packages (DIPs). They all
have 14 pins and are numbered 1 through 14 going counter clock-wise around the chip
from the top. The top of the chip is denoted by the notch in the one end of the chip.
Pin 7 and pin 14 provide power to the chip. Pin 7 must always be connected to ground
(0V) and pin 14 must always be connected to VDD (+5V). Pages 751 and 752 of the text
show the gate layout inside the chip.
Parts:
- Power Supply
- Digital Multimeter
- Proto Board
- Light Emitting Diode
- 7404 (NOT gates) chip
- 7408 (AND gates) chip
- 7432 (OR gates) chip
Procedure:
- Write Specification. "Design a circuit that has two inputs, x and y, and one output, f so that f is 1 when x and y are different, and f is 0 when x and y are the same."
- Define inputs and outputs. Draw a block diagram showing the inputs and outputs for
the circuit.
- Create truth-table. Generate a truth-table that shows the output for all possible
input combinations.
- Derive simplified logic expression. Write a canonical SOP expression that represents
the truth-table. If possible, simplify the expression using boolean algebra.
- Create gate-level design. Draw a logic circuit (schematic) using NOT, AND, and OR
gates that implements the simplified logic expression.
- Implement. Build the logic circuit using the standard TTL SSI chips. Connect pin 14
of all necessary chips to +5V and connect pin 7 of all necessary chips to 0V. Then
make all other interconnections. Connect the LED to represent the output f. The anode
of the diode should be connected to the output pin for f and the cathode should be
connected to 0V. When the ouput of f is 1 (a high voltage), the LED will turn on.
When it is a 0 (a low voltage), the LED will be off. Draw in your lab book the circuit
you build. Use the drawing on page 752 as an example of what to draw.
- Test. Test your logic circuit to see that it works correctly by trying all input combinations
and verifying that the appropriate output is produced. Record your actual results.
Conclusions:
- Discuss how well the objectives were met.
- Discuss what you learned.
