Teaching digital logic fundamentals – theory, simulation and deployment – national instruments gas density at stp


Typically when students are learning digital logic fundamentals they are limited to theoretical understanding. Multisim allows students to put this theory into practice through schematic simulation and deployment to hardware. Multisim enables you to program your Digilent teaching boards directly from the Multisim environment, making the connection between theoretical concepts and hardware implementation more understandable.

• During execution toggle the switches and view the effect on the LED. The AND gate should respond according to the truth table below. When both switches are closed (high), the LED turns on. This allows the student to experiment with the switches to understand the AND gate operation.

• Multisim also includes interactive instruments that we can connect to our schematic, as we would benchtop instruments. In understanding digital logic the Logic Converter is a powerful tool. The Logic converter allows us to obtain a truth table and Boolean expression for our circuit. Place a Logic Converter onto the schematic.

• Click on the next conversion button. This analyses the truth table and generates the Boolean expression. For an AND gate, the Boolean expression should be AB. The logic converter also allows students to input truth tables and Boolean expressions and generate logic diagrams.

This tutorial will provide an example of how students can create a Programmable Logic Design (PLD) and deploy this to Digilent hardware. The tutorial has been written using the Digilent Nexys 3 Board, but is the same process for all Digilent cards.

• Create a new PLD schematic for your Digilent board. Details of this process can be found within the Getting Started with Digilent Boards in Multisim. During the creation choose to Uncheck all when selecting the IO terminals to include on the schematic.

• In this tutorial we are going to create an example that allows students to gain an understanding of the functionality of OR, AND and NOT logic gates. To add IO to the schematic, select either the input or output connector from the PLD toolbar. Click the input connector button.

• Once the schematic has been created, we are ready to deploy the logic schematic to the Digilent board so students can physically manipulate the switches and view the response on the LEDs. Full details on deploying to Digilent cards can be found in the Getting Started with Digilent Boards in Multisim. Select: Transfer » Export to PLD from the menu bar. Here you will see the three export options. We want to export to physical hardware so select Program the Connected PLD. Click Next.

• Choose the Xilinx tool to compile the Multisim PLD design to a programming file. If you installed the Xilinx ISE Tools to the default location, they should autopopulate. If not, click the Xilinx tool drop-down menu, select Manually select tool, and then browse to the file folder where the tools were installed. The Xilinx user constraint file contains directions that map the connectors in Multisim to the pins of the Xilinx FPGA, Part Number XC6SLX16.

• At this point, you must connect the Digilent board to your computer. To check if all requirements have been met and the device is properly connected to Multisim, click the Refresh button. If the board is detected, the Device status displays Detected – Date and Time, as shown below.

• To proceed, click Finish. This begins the 11-step process of programming the PLD. Multisim automatically invokes the Xilinx ISE Tools (Creates the Xilinx project, Checks syntax, Translates, Places & Routes, Generates the programming file, and so on).

Using traditional teaching tools, a hands-on approach to learning Boolean logic is not possible until students have undertaken more advanced courses teaching them hardware description languages (eg. VHDL). Multisim simulation, PLD schematics and Digilent support provide an overall solution to allow students to learn through experimentation.