CS 232: Project #1

### Project 1: Combinational Circuits

Purpose: to give you experience with basic digital design components.

1. Create a circuit with 4 inputs, treated as a 4-bit positive binary integer. The circuit should output a 1 if the input is a prime number and 0 otherwise. Note, the numbers 0 and 1 are not prime numbers.

Simluate your circuit by testing all possible combinations of inputs, similar to the way we tested the circuit in the lab exercise. Take a screen shot of both your final circuit design and your simulation. Include the screen shot in your report. Note, demonstrating that your circuit works is a required part of the project writeup. Be sure to test your circuit and document that testing in your writeup.

2. Create a circuit that has two components. One component should be a counter, which is available as a module. The important inputs to the counter are a clock and a reset button.

The second part of the circuit should be a combinational circuit that controls the six lights of a 4-way traffic light. There are NS-red, NS-green, and NS-yellow, EW-red, EW-green, and EW-yellow. They should follow the timings below, given the 4-bit input from the counter.

Input ValuesLight setting
0N/S Red, E/W Red
1-5N/S Green, E/W Red
6-7N/S Yellow, E/W Red
8N/S Red, E/W Red
9-13N/S Red, E/W Green
14-15N/S Red, E/W Yellow

Test your circuit by driving the counter with a clock. The easiest way to test it is using the board and having a push-button as the clock input. Demonstrate your six output signals for all possible inputs.

If you want to test using ghdl, the altera counter seems to be having issues. You can download this VHDL counter instead. With your traffic project open, open the counter.vhd file. Then select Project:Add current file to project. Then select File:create/update:make symbol files from current file. After that, replace your lpm_counter with the new counter, reconnect the inputs, and try again. Note that the counter I'm providing has an enable input, which should always be 1 (you can use the symbol vcc to create a 1) and a reset signal, which should start at 1 to reset the counter and then go to 0 a short time later to let the counter run. Make the reset signal be an input to your traffic circuit.

Here is also an example of a testbench for the counter circuit. It shows you how to make a clock of arbitrary length (which might be easier for driving your traffic circuit). You can modify it to drive the traffic light circuit instead of the counter.

Take a screen capture of your final circuit and your simulation. Include them in your project report.

### Extensions

• Demonstrate that one or more of your outputs are generated by a minimal logic circuit. You can use a Karnaugh-map to do this.
• Extend the prime numbers circuit to 5 or even 6 bits.
• Do something interesting with the traffic light circuit, like add a walk button that makes the light leave the opposite green early (this is challenging).
• Figure out how to use an FPGA board to test your circuits. For example, show the traffic light outputs on the board LEDs.