T Flip-flops
A common building block used in sequential logic circuits that counts pulses on a signal line is the T flip-flop.
The T flip-flop has only one excitation input signal T, as shown on the logic symbol for the device pictured in Fig. 6.33a.
The function of this device is to change (toggle) its state upon each negative-going transition of its excitation input signal, as shown in the excitation table and state diagram in Figs. 6.34a and b.
Therefore, the characteristic equation of the edge-triggered T flip-flop is simply: Q* = Q