7 Segment Decoder

ENGR-171
Winter 2024
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Seven segment displays were once the default method for displaying numbers in base 10. These displays have 7 LED segments arranged in the shape of an 8. By lighting individual LED segments the numbers 0-9 can be created. The 7 segment display can also be used to display hex digits and most letters in the Latin alphabet. This Wikipedia Article provides a good description of the history and how the 7-segment display is used.

To display the proper information on a 7-segment display, the binary data must be decoded. The decoded data then enables the required segments of the display. There are many common single chip 7-segment decoders, the 74x4511 being a common one. In this design project you will design a binary to 7-seg decoder. Your design will use basic gates. Essentially, you are making a chip similar to the 74x4511. However, the 74x4511 uses BCD inputs. It can only display 0-9. Your decoder will display digits 0-F.

Skills

  • Convert project specifications into logic equations and the a digital circuit
  • Use logic equations, truth tables and boolean algebra to describe and simplify how a digital circuit functions.
  • Build and test the digital design using standard components

Specifications

Design a binary-to-7 segment decoder. The decoder should accept a 4-bit binary input and display the hex value of the input on a 7 segment display.

Tasks

  1. Design a decoder to meet the specifications given above
    1. Create a truth table for each display segment
    2. Use K-maps to simplify the logic equations for each segment
    3. Use the K-map equations for your decoder.
    4. Build the decoder using basic logic gates
      • LogicSim has a 7-segment display block for your output and simulation
  2. Simulate your decoder in LogicSim
    1. Use the truth table you created for the decoder
      • Your truth table should check the output of your decoder.
      • Use the 7-segment display for visual confirmation in your simulation.
    2. Visually verify that the display is correct for each binary input.
      • Documentation for this step should involve a couple of screen shots of your simulation. Make sure you write down that you observed all and include that info in your report
  3. Build and test one of the 7-segment outputs (a-g).
    1. Create a build schematic showing the pin numbers and actual gates used to build your decoder
      1. Test this schematic in LogicSim to make sure it still matches your truth table.
    2. Use Switches for the inputs
    3. You may use a 7-segment display or a single LED for your display.
    4. Refer to Circuit Build Tips for help building your circuit. This is the same information that was in the circuit analysis practice project
‼ Important
  • Your decoder in LogicSim should be a complete decoder that can drive a 7-segment display. Use the 7-segment display block in your simulation to show proper operation.
  • When you build your circuit you are only required to build the logic for one segment of the display. You can pick which segment. You are building a single output of your decoder, not all 7.

Report and Analysis

A report explaining your design is required for this project. The report should be a single file. It should be a narrative. You need to make sure it includes critical test data showing how your design meet the requirements.

Your report is being written for your classmates. It should include enough detail that they could easily duplicate your work. It should also convince them that your design is correct.

Describe the functionality of your circuit, show how you designed the circuit, and how you tested it. This needs to be described for both the simulated circuit and the one your built on the breadboard. You need to present data that shows it functions are required. Explain how the reader should look at this data so they know the circuit functions, ie don’t just provide the data, but explain what the table means.

This report is graded on a rubric. The rubric can also be found on Moodle.

Your report should be typed and not include any handwritten information. This means you will have to typeset critical equations

Data to include in report

The following data and explanations are required. Use data tables where appropriate.

  • Truth Tables and Logic equations that you develop
    • Make a truth table of expected results, simulated results, and measured results from your breadboard
      • This truth table makes it easy for everyone to see that all testing was done and all 3 versions of circuit match
  • All schematics
    • You will probably need multiple schematics
      • 1 schematic showing your logic based on your Logic Equations
      • 1 schematic showing how your built your circuit with actual parts. This schematic must have pin numbers on each gate.
      • It is ok to have more than 2 schematics
  • Results of simulations
    • This might just be screen shot of your simulation passing the test vector file
  • Results of testing the built circuit
    • Image of built circuit correctly working
      • You will need a couple of images. You don’t need an image for each possible input. Include 2 or 3 images.
      • Explain what these images show and why they are important
  • Your report should clearly show what your circuit performs the requested function. It should be clear how to build it with 74XX parts.
  • Provide data showing your completed all the required Tasks

Analysis / Discussion

Include an analysis of data in the report. The analysis needs to include an interpretation of the data and explanations of how the data demonstrations that you have meet specifications. Discuss how the data confirms or enlightens ideas related to engineering or digital circuits. The questions below should all be investigated and discussed. Remember that this discussion should be a narrative, not just a list of bullet point answers.

  • Does your design meet the given specifications?
    • How do you know this? How can you prove this?
    • Does what you built match what you simulated?
      • Prove this.

Use the data you collected to prove that your design matches the specifications.

Reflection on Learning

You also need to include a reflection on learning. For this reflection you can use the standard reflection questions in Grading Policy or you may reflection on the following questions:

  • What was the hardest part of the project?
  • What was the easiest part of the project?
  • What did you learn from this project?
  • What would you do differently next time?
  • What would you do the same next time?

Circuit Build Tips

List of useful 74xx components we have in the lab (we have more than this so feel free to do some research):

  • 00: 2-input Nand
  • 02: 2-input NOR
  • 04: Inverter
  • 08: 2-input AND
  • 32: 2-input OR
  • 86: 2-input XOR
  • We do not have XNOR or a 2-4 decoder.
  • The red and blue bars on your breadboard should be wired up as power and gnd. This makes it easier to make the many power and gnd connections we need.
  • To create the binary input, you will either need wires that you move or dip switches. They are called dip for dual inline plastic and are designed to fit on a standard breadboard. To create both logic levels the dip switches require pullup resistors. Schematic 2 shows the schematic of how to hook up a single dip switch so it provides a good input.
  • The chips and dip switches are spaced to bridge the gap in the middle of the bread board. The leds require resistors in series to limit the current. Schematic 1 shows how to hook up the LED circuit.
  • Schematic 3 is an example of how the switch is hooked up to an inverter. When the switch is closed a 0 is input to the inverter. The inverter outputs a 1. The high output turns the LED on. When the switch is open the inverter sees a 1 input and outputs a 0, turning off the LED. A logic 1 = VDD on the DMM and a logic 0 = 0V or GND on the DMM.
  • Label wires on your 2nd schematic with breadboard row coordinates.
  • On your schematic check off connections as you make them. This helps to keep track of what you still need to connect.
  • Hand draw the LED circuit on a print schematic to help with the build
Schematic showing LED and Resistor in series. Resistor is 390 Ohms. Notes indicate that resistor connects to digital output, and cathode of LED to GND
Schematic 1. How to safely connect a LED to a digital output
Schematic showing Switch and Resistor in series. Resistor is 1000 Ohms. Notes indicate that resistor connects to power and switch to GND. The connection between resistor and switch goes to the digital input.
Schematic 2. How to use switches to create digital inputs
Circuit that shows the Switch circuit driving an inverter and the inverter drives the led circuit
Schematic 3. Sample circuit showing how to connect switch and LED to a logic gate