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Flip-Flops
The toggle, or T, flip-flop is a two-input flip-flop. The inputs are the toggle (T)input and a clock (CLK) input. If the toggle input is HIGH, the T flip-flop changesstate (toggles) when the clock signal is applied. If the toggle input is LOW,the T flip-flop holds the previous state.
T flip-flop symbol.
The standard symbol for a T flip-flop is illustrated in the figure above.The clock input may be preceded by an inverter. An inverter indicates a flip-flop will toggleon a HIGH-to-LOW transition of the clock pulse. The absence of an inverter indicates theflip-flop will toggle on a LOW-to-HIGH transition of the pulse.
Now, follow the explanation of the circuit using the truth table and thetiming diagram shown in the figure above.The timing diagram shows the inputs and the resulting outputs. We willassume an initial condition (t0) of Q being LOW andQ being HIGH.At t1, when the clock changes from a LOW to a HIGH, the outputsremain the same as the T input is LOW. The T input goes HIGH att2.At t3, the clock changes from a LOW to a HIGH and thedevice changes state; Q goes HIGH and Qgoes LOW. The outputs remain the same at t4 since the deviceis switched only by a LOW-to-HIGH transition. At t5, when theclock goes HIGH, Q goes LOW and Qgoes HIGH; they remain that way until t7.
Between t3 and t7, two complete cycles ofCLK occur. During the same time period, only one cycle is observed for Q orQ. Since the output frequencyis one-half the clock (input) frequency, this device can be used to divide theinput frequency by 2.
The most commonly used T flip-flops are J-K flip-flops wired to perform a togglefunction. This use will be demonstrated later in this section.
