Microsoft Word - Digital Logic Design v_4_6a

3.3. Designing Logic Circuits

The process of combinational logic design is best described in six steps:

1) Draw the system diagram and identify input and output variables.

2) Write out the truth table for the function.

3) Use K-maps or CAD to minimize the function and write the algebraic function.

4) Identify the logic gates required and draw the schematic to implement the terms of the

algebraic function. The schematic should include:

• Designer and project name.


• Component identification and pin numbers.


• List of all the pins that are connected to Vcc and Ground, and are not use.
  All Connecting lines must be either vertical or horizontal (No diagonal or curved lines).
  Additionally, a dot is used to mark an actual connection when two lines cross over each
  other.

Note: Hardware Description languages such as VHDL or Verilog HDL or State CAD by Visual

Software Solutions, can also be used to document the design.

5) Implement (pay attention to layout and ease of support/use).

6) Test (each design must have a test plan).

Remember that the design process is an iterative process; it is important to use the learning from

later steps of the design process to improve earlier work. The best designs typically have many

design iterations before the final design has been completed.

 Example

Design a 4-key digital lock that can be opened only when every other keys are pressed.

Project Name

Designer Name

Date

ID Component Desc. No Conn. Vcc Gnd

U1 U 1 , 74LS32 4 - 6, 8- 13 14 7

Y X^

U1 Z

3

1

2

Vcc

Gnd

1kΩ 1kΩ^