CS-343 Assignment 3

Due Date and Submission

This assignment is due by midnight, March 1. Submit it by sending email to me at the address, vickery@babbage.cs.qc.edu. Be sure to put "CS-343 Assignment 3" in the subject of your email and to put your name/ID in the message body.

For this assignment, you are going to submit two CircuitMaker circuits. These .ckt files are actually text files that describe the circuit you designed and simulated. You can safely send them as attachments, or even insert them directly into the body of your email message.

The Assignment

1. Design and simulate a 4-bit parallel adder/subtracter like the one we did in class. Include the logic for generating all four condition code bits (C, V, N, and Z). Use Hex Keys for the A and B input values, and a Hex Display to show the sum/difference. Use a data switch to set the add/subtract control input, and use four logic indicators for the condition code outputs. Verify that your circuit correctly adds and subtracts two's complement numbers and generates the correct condition code values.
2. Design and simulate a four-bit timer. There are two buttons on the timer, once called "Set" and the other one called "Start-Stop (S-S)." The timer also has a mechanism for supplying a four-bit value that will determine how long the timer runs, a seven-segment display to show the current value in the timer, and a "Finished" light to indicate when the timer has finished running.

   The timer operates as follows: The user enters the time interval using a hex key. When the user clicks the Set button, the value of the hex key is loaded into a four-bit register. When the user then clicks the Start-Stop (S-S) button, the register counts backwards until it gets to zero. If the user clicks the S-S button while the counter is running, it pauses until the S-S button is clicked again. When the counter gets to zero, the Finished light turns on until the Set button is clicked again.

   There are several ways the timer could be designed. You are to do the design as a set of four state machines that interact with each other. One state machine acts as the timer's controller, and the other three implement the datapath for the timer.

   The controller state machine responds to clicks of the Set and S-S buttons and to a feedback output of the datapath named "Zero" to move among four states. The state names are Start, Run, Pause, and Done. Clicking the Set button at any time puts the timer in the Start state. In the Start state, clicking the S-S button takes the timer into the Run state. Clicking the S-S button in the Run state puts the timer into the Paused state, and Clicking the S-S button in the Paused state puts the timer back into the Run state. The timer goes into the Finished state when the counter goes to zero in the Run state.

   The controller's job is to manage the operation of the datapath of the timer, which is to be implemented by three different state machines that operate on the four-bit counter. One state machine is a "do-nothing" machine that simply preserves the current state of the counter. A second state machine takes the counter into a state that matches the values of the Hex key from any other state. The third state machine causes the counter to count backwards from whatever state it is in to the numerically next smaller numbered state. The three state machines are to be implemented separately and their "next state" signals are to be connected to the counter's flip-flop's D inputs through 4x1 multiplexers. All four multiplexers are to be controlled by two bits named "Load" and "Count," which are the outputs of the controller state machine.

   There is to be an output of the datapath named "Zero" that is true when the counter value is zero. The controller state machine is to use Zero as a third input (in addition to S-S and Set) to tell when to go into the Finished state.

   Although your assignment is to submit a CircuitMaker simulation of the timer, a complete solution requires you to show the state diagrams and state tables for the entire design. However, you do not have to submit these parts of the assignment.