ECE 171: Introduction to Digital Circuits

Fall 1999

Rev: 11.3.99

Design Project 1

Problem Description:

In digital computers, letters of the alphabet and the ten decimal numerals can be coded in the form of unique combinations of five or six bits. One of these codes is the 6-bit Flexowriter code, which is used for some punched paper tape. A code similar to this is given below in terms of octal equivalents.

Character Codes

A	64	G	77	M	24	S	41	Y	45	4	52
B	4	H	5	N	40	T	13	Z	73	5	66
C	44	I	33	O	71	U	51	0	43	6	46
D	16	J	6	P	0	V	17	1	63	7	22
E	7	K	32	Q	12	W	55	2	62	8	42
F	37	L	36	R	75	X	60	3	2	9	72

All character codes are in Octal. To convert the octal code to its six bit equivalent, convert each digit (octal) to its three digit binary equivalent. For example, Z = 73 = 111 011 = 111011. To convert each octal code to its decimal equivalent, multiply each digit by its decimal weight. For example, Z = 73 = 7x8 + 3x1 = 56 + 3 = 59.

Your job is to design a circuit which detects whether a code on six data lines represents a number, a letter, or an illegal code. The circuit should produce three output signals, A, N, and I, as shown below.

A	N	I	Interpretation
1	0	0	Alphabetic character sensed
0	1	0	Numeric character sensed
X	X	1	Illegal character sensed (X = don't care)

Objective

Design a circuit that implements the expression using only 2, 3, 4, and 8 input NAND gates and inverters. You are to use a maximum of 8 IC packages. While your grade on this project is not determined by the number of chips used, it is a factor in your grade on Project 2.

Hint: 3,4 and 8-input NAND gates only come on 14-pin IC's, but 2-input NAND gates are available on 20-pin IC's. Click here to see a complete listing from Texas Instruments.

Lab Book

You should document your thought process and attempts as you develop your design. This must include all false starts as well as successful paths. It must be written as you go, not after the fact as is usually done with other lab reports. It should be written in a manner, such that any engineer could read the lab book and understand your design. Thus, you must provide adequate explanations of what you are doing as you go. This should be handwritten, but readable.

The lab book should be bound (should have at least 20 pages) and you must number the pages of the entire book in the upper right corner. The first page should be a table of contents that makes it easy to find the following essential elements. You may use a pencil.

• Truth Table
  The truth table must be complete for your design.
• Karnaugh maps
  These must show how you produced the active low and active high minimal sum of products expressions for each of the three outputs and all factoring attempts.
• Logic Diagram
  Draw out your logic diagram by hand.
• Reduction Work
  Analyze the chip count for your final design. Show all work that you do to try reducing the number of IC's.
• Component List
  A list of the IC's used in your design and how many of each type.

Final Report

A formal report of the final design is to be written at the end of the project. It must be typed and include the following sections. Spell check this document and make sure that it does not contain any typos. These sections must be clearly labeled in your report.

• Introduction
  This section must describe the design problem (i.e. rephrase the purpose in your own words), describe the circuit inputs, and the circuit outputs.
• Final Expressions
  The final boolean algebra expressions used in the circuit design.
• Schematic (Logic Diagram)
  Drawn with Mentor's Design Architect.
• Analysis
  List the components and how many of each type your design uses.
• Conclusion
  Discuss any ideas you have on how the design might be improved in another iteration or any concerns you have about why the circuit might not work.
• Postscript
  Describe what you learned from this project, if anything. Was this project worthwhile? Your comments will be used to improve this project for future classes and will not affect your grade.
• Signed Statement
  To receive credit you must do this project yourself and sign the statement to that effect below.

I do hereby affirm that I designed this circuit by myself.

Signed __________________________________

Date _______________

Grading

Project 1 is worth 25 points of 50 total project points.