The Toggle (T) Flip-Flop

        A D flip-flop can be converted to a toggle (T) flip-flop by connecting the output to the input, inverting the output as it is fed back.  That's shown in the simulation below.  Again, you can put a clock pulse into the circuit.

Simulation - T (Toggle)  Flip-Flop

        In this simulation, the D flip-flop is transformed into a toggle flip-flop using inverted feedback around the D flip-flop.

Question

Q1   If you go through four clock cycles (i.e. the clock goes from zero to one and back to zero - that's one cycle), how many cycles does the output of the toggle flip-flop go through?

        The toggle flip-flop is a frequency divider when you use it as it was used in the simulation above.  If the clock runs 100 times a second, the output of the toggle will be a pulse that runs 50 times a second, so the frequency of the toggle output is half of the frequency of the toggle input.

        Now, consider a "What if?" question.  Here's the question.  What if you take two toggle flip-flops and put the output of one into the input of the other.  The next simulation will let you explore that.

Simulation

        Here is the simulation for two T (Toggle) flip-flops with the output of the first one being the input to the second one.

Questions

        Using the simulation above with two T flip-flops, determine the answers to the following questions.  In answering these question, interpret the LED indicators on the flip-flops as a two-bit digital number.  The most significant bit (MSB) is the one on the right.  The LSB is the one on the left.  Click here to get the simulator in a separate window.

Q2   When the simulation loads, what number is pre-loaded into the system?

Q3   After one clock cycle (clock becomes one, then becomes zero again, i.e. two mouse clicks), what number is present?

        Flip-flops are interesting circuits, and it is important that you understand them.  The best way to get to really know them is to use them in the lab.  You may have done that before you took this lesson depending on your instructor.  However, the lab is worthwhile whenever you do it.  There is a link below in the link to Logic Laboratories.