Digital Fundamentals. Lab 5 Latches & Flip-Flops CETT Name: Date:

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1 Richland College School of Engineering & Technology Rev. 0 B. Donham Rev. 1 (7/2003) J. Horne Rev. 2 (1/2008) J. Bradbury Rev. 3 (7/2015) J. Bradbury Digital Fundamentals CETT 1425 Lab 5 Latches & Flip-Flops Name: Date: Objectives To construct and investigate the operation of a NAND and NOR latch To investigate the operation of a J-K Flip-Flop To investigate the operation of a D Flip-Flop Suggested Reading Chapter 5, Digital Systems, Principals and Applications; Tocci Equipment and Components 74L00 IC uadruple 2-Input Positive-NAND Gates 7402 IC uadruple 2-Input Positive-NOR Gates 74LS74 IC Dual D-Type Positive-Edge-Triggered Flip-Flops with Preset and Clear 74LS112 IC Dual J-K Negative-Edge-Triggered Flip-Flops with Preset and Clear 2 toggle switches Breadboard & wires VOM 0-10 Volt power supply Logic probe Circuit Simulator (MultiSIM or an equivalent) Page 1 of 8

2 Introduction The outputs of a sequential logic circuit are dependent upon the current and past input states, which requires a memory element. Examples of basic memory devices are Latches and Flip-Flops. NOR Latch The NOR latch has 2 active-high inputs, SET and CLEAR, that determine the state of the output (). This is an asynchronous device. The symbol and truth table for the NOR latch are shown below. S C 0 0 No Change 0 1 CLEAR, =0 1 0 SET, =1 1 1 Invalid S C NAND Latch The NAND latch has 2 active-low inputs, SET and CLEAR, that determine the state of the output (). This is an asynchronous device. The symbol and truth table for the NAND latch are shown below. S C 0 0 Invalid 0 1 SET, =1 1 0 CLEAR, =0 1 1 No Change S C J-K Flip-Flop The operation of the J-K flip-flop is similar to the S-C latches described above, except that it does not have an invalid state. The invalid state is replaced with a TOGGLE state, which causes the output () to change logic levels. The state of the flip-flop is determined by the value of the J and K inputs when the specified clock edge is detected. This flip-flop may also have asynchronous PRESET and CLEAR inputs. The asynchronous inputs can be used to change the state of the flip-flop regardless of the state of the synchronous inputs. The inputs can either be active-low or active-high depending upon the device type. D Flip-Flop The logic level of the input (D) will be transferred to the output () when the specified clock edge is detected. This device may also have asynchronous PRESET and CLEAR inputs. The asynchronous inputs can be used to change the state of the flip-flop regardless of the state of the synchronous inputs. The inputs can either be active-low or active-high depending upon the device type. Page 2 of 8

3 Procedure: 1. Draw the schematic for a NOR latch in the space below. Using a data sheet for a 7402 IC, write pin numbers on the schematic that will be used when constructing the circuit. 2. Using the schematic in procedure 1, construct the NOR latch circuit on a breadboard. Use a toggle switch for the inputs (S, C). Apply VCC and Ground to the proper pins on the IC. Place the S & C inputs to a logic 0 initially. 3. Turn on the power supply. Using a logic probe or VOM, measure and record the logic state 4. If is a logic 1, toggle the C input to a logic 1 and then back to a 0. If is a logic 0 then go to procedure Set the S input to a logic 1. Using a logic probe or VOM, measure and record the state What state is the flip-flop in? 6. Return the S input back to a logic 0. What effect did this have on the outputs? Explain your answer 7. Set the C input to a logic 1. Using a logic probe or VOM, measure and record the state Page 3 of 8

4 What state is the flip-flop in? 8. Turn off the power supply and disconnect the circuit. 9. Draw the schematic for a NAND latch in the space below. Using the data sheet for a 7400 IC, write pin numbers on the schematic that will be used when constructing the circuit. 10. Using the schematic in procedure 9, construct the NAND latch circuit on a breadboard. Use a toggle switch for each input (S, C). Apply VCC and Ground to the proper pins on the IC. Place the S & C inputs to a logic 1 initially. 11. Turn on the power supply. Using a logic probe or VOM, measure and record the logic state 12. If is a logic 1, toggle the C input to a logic 0 and then back to a 1. If is a logic 0 then go to procedure 13. Page 4 of 8

5 13. Set the S input to a logic 0. Using a logic probe or VOM, measure and record the state What state is the flip-flop in? 14. Return the S input back to a logic 1. Set the C input to a logic 0. Using a logic probe or VOM, measure and record the state What state is the flip-flop in? 15. Turn off the power supply and disconnect the circuit. 16. Refer to the data sheet for a 74LS112, edge triggered J-K flip-flop. What clock edge triggers the flip-flop? What logic level must be applied to the asynchronous inputs to SET or CLEAR the flipflop? What is the pin number for VCC? What is the pin number for Ground? How many J-K flip-flops are in this device? 17. Use MultiSIM or an equivalent circuit simulator to evaluate a J-K flip-flop. Select a 74LS112 flip-flop with active-low asynchronous inputs from the digital menu. Connect toggle switches to the J, K, CLOCK, and asynchronous inputs. Set the switches for the following initial conditions: J = 0 K = 0 CLOCK = 1 Asynchronous SET or PRESET = 1 Asynchronous CLEAR = 1 Note: For information about a component, highlight a device and select HELP under the help menu. Page 5 of 8

6 18. Measure and record the value of for the following input states. SET CLR J K CLK 1 0 X X X 0 1 X X X Set the asynchronous CLEAR input to 0. Toggle the CLOCK input from 1 to 0 and back to 1. Did change states on the clock edge? Explain why or why not. 20. Refer to the data sheet for a 74LS74, edge-triggered D flip-flop. Construct the following circuit in MultiSIM or an equivalent circuit simulator. Select the actual 74LS74 IC from the digital menu rather than a D flip-flop symbol. VCC D CLEAR Connect VCC and Ground to the appropriate pins on the IC. Connect a toggle switch to the asynchronous CLEAR line and set the switch to a logic 1. Connect VCC to the asynchronous PRESET or SET line. Connect the Function Generator to the CLOCK input. To get a 0 5 V square wave, select a 50% duty cycle, 1 khz square wave with an amplitude of 2.5V and an offset of 2.5V. (Connect the + output to the clock input and the common input to ground. The output should not be connected). Monitor the CLOCK input and the output using a Logic Analyzer. Print out a copy of your schematic and attach it to the lab. Page 6 of 8

7 21. Turn the simulator on and then turn off the simulator after a number of clock signals has occurred. Sketch a output waveform as shown on the logic analyzer. CLK 22. Measure and record the frequency of the waveform. Frequency = 23. What is the function of this simple D flip-flop circuit? 24. Turn the simulator back on. Toggle the asynchronous CLEAR input to a 0. What happens to the output? 25. Turn off the simulator. Page 7 of 8

8 Review uestions: Answer the following questions after the lab is completed. 1. A D flip-flop can be implemented from a J-K flip-flop and an inverter. Connect the J K inputs to either the D or D signal to cause circuit to behave like a D flip-flop. J D K 2. Based upon the results in procedure 21, sketch the waveforms for the following circuit. CLK 1 2 Page 8 of 8