Lab 5: Flip-Flops

Introduction

A flip-flop is a digital device whose output is determined by sequentially loaded inputs which are implemented regularly at a clock transition. The use of the clock input is what differentiates a flip-flop from a latch. These devices are extremely usefu l in creating sequential circuits such as shift register, counters and clocks. There is a wide variety of commercially available flip-flops, but the J-K and D type flip-flops are the most common. In this lab you will investigate the various properties of the J-K and D type flip-flop.

Equipment needed

Procedure

Part 1 -- D-type flip-flops and registers

  1. Wire the circuit shown in figure 1. This circuit represents a D-type flip-flop created from a 7400 IC. Don't forget to connect pin 14 to +5 volts and pin 7 to GND.

  2. Create a truth table this circuit by noting the output states for each set of inputs.

  3. Replace the data switch on the D input with a logic clock signal. Set the clock frequency to 1 Hz and connect the oscilloscope to the frequency generator to verify and monitor the correct frequency.

  4. Set the T input to binary 1 and observe the flip-flop output for a brief period. note the relationship between the clock state and the Q output. Set the T input to binary 0 and again observe the outputs.

    What are the differences between this Nand D-type flip-flop and the Set-Reset latch you analyzed in the previous lab?

  5. Wire the circuit shown in figure 2. The 74LS75 contains four TTL D-type flip-flops similar in operation to the Nand D-type flip-flop shown in figure 1. Figure 3 shows the internal structure and pin connections for this device. The data switches wil l be used as a switch register. Logic switch A will be used as the LOAD of strobe signal which transfers input data into the register.

    Figure 2

  6. Apply power to the circuit and record the number in the register. (Assume L4 output is the LSB)

  7. Set all the data switches to binary 0. Then momentarily apply a binary 1 at the A logic switch. Record the binary number in the register.

  8. Set all the data switches to binary 1. Momentarily apply a binary 1 at the A logic switch. Record the register contents.

  9. Load the sixteen binary numbers 0000 through 1111 into the register one at a time by setting the data switches and then actuating the A logic switch. Record the output each time the A logic switch is actuated. (create a truth table)

    Can the input data ever be different from the register contents? What about when the LOAD input is low? Why is this called sequential logic?

Figure 3

Part 2 -- The J-K flip-flop

  1. Connect the circuit shown in figure 4. Use logic switch A as the clock input. Figure 5 shows the pin connections for the 74LS76 dual J-K flip-flop.

    Figure 4

    Figure 5

  2. First check the asynchronous operation of the J-K flip-flop. Set J=K=1 and create a truth table using S and C as inputs.

  3. Repeat step 2 with J=K=0.

    Do the J-K inputs effect the asynchronous operation?

  4. Construct the circuit shown in figure 6. The circuit will be driven by a 1 Hz clock at the T input. The A and B logic switches will control the circuit. You observe the outputs states on the LED indicators at L1, L3 and L4.

    Figure 6

  5. Use the oscilloscope to observe the relationship between the input (L1) and the outputs (L3) and (L4) waveforms. You can do this by counting the number of input and output pulses. Sketch a timing diagram illustrating this relationship.

  6. While the circuit is operating, actuate the B logic switch. What effect does this have on the circuit? Try this several times.

  7. Actuate the A logic switch while the circuit is operating. Note the effects on the output. Try this several times.

  8. Set the CLK frequency to 100 Hz and 1 KHz and observe CLK, FF1 and FF2 noting their frequency relationship.

    Explain the operation of these flip-flops in accordance with your experimental findings.