Digital Design Example

This will be a continuation from yesterdays post on the foundation of Digital logic and Design. As stated before this is not going to be an in depth explanation of all things digital logic. Givone's Book does a great job of explaining (something ENEE244 does not do) all things digital. However I will in this part go about the process of designing a simple 7 Segment LED display in order to show the power and simplicity of digital design. This design has pre-assembled IC Chips you can purchase to accomplish this, but it is more fun to do it yourself!

7 Segment LED Display

This project will utilize some of the fundamental design principles but will not cover everything. there are many topics covered in the course ranging from K-maps and how to simplify logic, but this will give a upfront idea of what you can do with this knowledge (in a very simple project).

The basic outline of this circuit is:

Where the 4 lines on the left are the inputs (a,b,c,d) and they are sent through a network decoder, accepting the inputs in 8421 format. So 1111 = 8+4+2+1, where 1000= 8+0+0+0, etc.

The decoder breaks this down into 7 outputs (a through g) which each corelate to one segment of the display.

You can have it set up anyway obviously, but if it were set up as above, where if a is high, that segment is on then the truth table for the outputs would look like.

W,X,Y,Z correspond to the for inputs (and correlate to the decimal number desired to be displayed). There are other possible conditions for inputs such as 1010, 1011,1100,1101,1110,1111 but these are 'Dont Care' conditions due to the fact we only want to display decimal numbers. These DC's are used in the K-map simplification of the circuit and is thoroughly discussed in the textbook.

I wont cover the k-map process here but the end result is a digital logic equation for each segment (a through g).

• a = w + y + xz + x'z'
• b = x' + y'z' + yz
• c = x + y' + z
• d = w + yz' + x'y + x'z' + xy'z
• e = x'z' + yz'
• f = w + y'z' + xy' + xz'
• g = w + xy' + x'y + yz'

What you would do with these equations is then create a "chip" from logic gates either by creating the gates from transistors (would be a pretty busy breadboard) or using pre-constructed logic gate chips.

However, these days there are IC chips that accomplish this as well as including some nice troubleshooting/data saving features. The 4511 chip comes to mind.

It may seem like a lot of work for a simple display, but once you go through the process it becomes second nature and then anything you wish to implement becomes possible

If there are any questions/suggestions please leave a comment.

-Brian