You're responsible for a portion of the logic in an electronic vending machine. Specifically, your subsystem determines the change to be returned. All items cost 75¢ and the machine accepts dollar bills, so your system must deliver 25¢ in change using the fewest number of coins. Your inputs are a 2--bit counter D1D0 indicating the number of dimes available to dispense and a 3--bit counter N2N1N0 indicating the number of nickels available to dispense. Your outputs should indicate the number of dimes and nickels to dispense. An additional output, NC (No Change) should be 1 if you're unable to give change with the coins available, 0 otherwise. You do not need to update the counters. Their values are provided as inputs to your module.
Be sure to show truth tables, K--maps, and reduced equations. Make effective use of don't cares. Describe your circuit using a Verilog behavioral dataflow description and simulate it by writing a testbench. Assume the device has a propagation delay of 6 time units. Your solution should employ sum of products form.
You can compile and simulate your Verilog program using the Verilogger software system or any other Verilog environment. Save and print the simulator timing diagram so that you can include it in your final report.
After you get your behavioral dataflow model working and verified, create a Verilog structural description for the same design and verify it. You may use any of the actual logic gates in the 74HCT family. Be sure to model your design using actual 74HCT family propagation delay values for the devices you chose. This means specifically that your Verilog program must assign tPLH and tPHL values to every gate. You can get these from the 74HCT data sheets, which can be found on the Texas Instruments web site (www.ti.com) - click on "Logic" under Find Products. Some are also available via links on the course page. You do not need to use SOP form for the structural description, but if the logic you implement for the structural description differs from the equations you obtained for the dataflow model, you need to show how you derived them.
Note that this means that your total propagation delay will be determined by actual devices and will differ from the dataflow design you did.
You can use the same testbench that you used to verify your dataflow description (though you may need to change the timing).
This final report must include
1. A brief problem description
a. The problem your circuit solves
b. Specific requirements
2. The project deliverables (exactly what you are generating and turning in)
3. Approach/methodology (the steps you will take to solve the problem)
4. Your design work ("black box" diagram, truth table, K--map, reduced equations)
5. Verilog source code listings (for both designs and the testbench)
6. The timing diagram showing how your design performed
7. A schematic with reference designators and bill of materials
8. A statement indicating the worst--case propagation delay of your circuit (indicating the input and output and delay).
Your report must be typed, stapled and submitted on 8.5 x 11 inch paper. Do not use any kind of report cover. K--maps can be hand--drawn.
While your schematic may be hand drawn, you will receive 5 points extra credit if you use a schematics capture package to create your schematic. See details on the course web site Resources page for details on schematics capture packages.