Introduction to Truth Tables, Logic Expressions and Circuit Simulation in Quartus II

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Cpr E 281 Lab10
Electrical and Computer Engineering

Iowa State University

Designing Registers and Counters

1.0 Objectives

In this Lab you will design shift registers and counters, and observe their functionality.

2.0 Shift Register

A shift register is a specialized register that takes an input serially and shifts it from one bit position to the next bit position. A full explanation of shift registers is available in Chapter 5.8 of the textbook. Figure 5.17a shows a 4-bit shift register built using D flip-flops.
For this step in the lab you will design a 4-bit shift register using D flip-flops. You may use Figure 5.17a as a reference. You will use the D flip-flop of the primitives library to build a shift register. Open a new .bdf file and name it lab10step1. Insert a symbol for the D flip-flop (dff). Set the preset PRN and the clear CLRN inputs of this flip-flop to high. One way to do this is to wire both ports to a vcc symbol.
Use the DE2-115 board to verify your shift register is operating correctly. Create an output pin for the output of each D flip-flop, label the first Q1, second Q2, third Q3 and the last Q4. Use toggle switches for the In and Clk pins, and use green LEDs to represent the outputs. Fill in the sequence table on the answer sheet and demonstrate your circuit to the TA.
Note: Mechanical switches have a bad electrical property called bouncing. As the two metal conductors in the switch get close to each other an electrical charge can jump across the gap between the conductors. This can happen several times before the two conductors make complete physical contact. This may cause a digital circuit to react as if the switch was opened and closed multiple times. An engineer must remove this property (debounce the switch) in order to achieve the desired action of the switch. This can be done using hardware in the circuit, or by adding a delay to switch reads in software. For this lab you just need to be aware of the problem and make sure that with every manual clock pulse you are not seeing more than one action from your circuit.

3.1 Synchronous Up-Counters

Counters are either synchronous (common clock signal) or asynchronous. A discussion of synchronous counters may be found in Chapter 5.9.2 of your textbook. Figure 5.23 describes how a 4-bit counter can be built using D flip-flops.
Open a new .bdf file and name it lab10step2a. Use the same D flip-flops you used in the last step, and additional gates, to build a synchronous 4-bit up-counter. Use the Figure 5.23 in your text as a reference.
Use the DE2-115 board to verify your up-counter is operating correctly. Use toggle switches for the Enable and Clk pins, and use green LEDs to represent the outputs. Once you are confident your circuit is functioning properly, demonstrate your circuit to the TA.

Open a new .bdf file and name it lab10step2b. Use T flip-flops, and additional gates, to build a synchronous 4-bit up-counter. Use Figure 5.21a in your text as a reference. You can get T flip-flops (tff) from the primitives library the same way you got the D flip-flops. Set the preset and the clear inputs of this flip-flop to high.

Use the DE2-115 board to verify your up-counter is operating correctly. Use a toggle switch for the Clk pin, (the first T flip-flop input should be tied to vcc), and use green LEDs to represent the outputs. Once you are confident your circuit is functioning properly, demonstrate your circuit to the TA.

3.2 Asynchronous Counters

In this step lab10step3a, you will build a 4-bit asynchronous up-counter. Figure 5.19a in your textbook shows a 3-bit asynchronous up-counter using T flip-flops. Extend this circuit to build a 4-bit asynchronous up-counter. You can use an inverter on the Q output to get the ~Q from the flip-flops.
Use the DE2-115 board to verify your up-counter is operating correctly. Use a toggle switch for the Clk pin, (the first T flip-flop input should be tied to vcc), and use a seven segment display to represent the outputs. Once you are confident your circuit is functioning properly, demonstrate your circuit to the TA.
In this step lab10step3b, you will build a 4-bit asynchronous down-counter using T flip-flops. Figure 5.20a in your textbook shows a 3-bit asynchronous down-counter using T flip-flops. Extend this circuit to build a 4-bit asynchronous down-counter.
Use the DE2-115 board to verify your down-counter is operating correctly. Use a toggle switch for the Clk pin, (the first T flip-flop input should be tied to vcc), and use a seven segment display to represent the outputs. Once you are confident your circuit is functioning properly, demonstrate your circuit to the TA.

4.0 Complete

You are done with this lab. Close all lab files, exit Quartus Prime, log off the computer, power down the DE2-115 board, and hand in your answers. Don’t forget to write down your name and lab section number.

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