16. The flip-flop T

The bistable T (toggle) is a sequential circuit that allows storing a bit of information.

The T flip-flop has two inputs, the T or toggle input, which serves as a data input and the clock input (Clock or CLK) that validates the input data. This flip-flop changes state every time it receives a clock pulse, as long as its T input is active (logic 1).

The T flip-flop has two outputs like the rest of the flip-flops. The complementary outputs Q and /Q that always have opposite values.

The T flip-flop is very useful for building:
  • Binary counters: circuits that count, in binary code, the input pulses they receive. Each flip-flop added in series represents an additional bit in the binary count.
  • Frequency dividers: circuits that divide the input frequency into lower frequencies. This is useful, for example, in digital watches, which divide the frequency of a quartz crystal to generate one-second pulses for the motor that moves the hands.

Simulation

In the following simulation we can observe a bistable T:

Truth table

Changes in the output of the T flip-flop only occur when the clock input (CLK) changes from a low level (logic 0) to a high level (logic 1), that is, on the rising edges of CLK.

The truth table of the flip-flop T is the following:

T CLK Q /Q
0 _↑¯ Q /Q
1 _↑¯ /Q Q

If the T input is low (logical 0), when a rising edge arrives at CLK, the outputs remain in the same state they were in before.

If the T input is at a high level (logical 1), when a rising edge arrives at CLK, the outputs change level, exchanging the high (logical 1) and low (logical 0) states with each other.

The behavior of the T flip-flop is the same as that of a JK flip-flop, in which the two inputs J and K have been united into a single input called T.

Exercises

  1. What is a T flip-flop, why is it called that and what is it for?

  2. What similarities and differences does the T flip-flop have with respect to the JK flip-flop?

  3. How many inputs does a T flip-flop have and what function does each one perform?

  4. How many outputs does a T flip-flop have and what function does each one fulfill?

  5. Draw the circuit of a T flip-flop with the names of its inputs and outputs.

  6. Draw the truth table of a T flip-flop and briefly explain its normal operation.

    Does the T flip-flop have any similarity to the JK flip-flop?

  7. Simulate all the states of the T flip-flop with the simulator on this page.