The Master-Slave Flip-Flop

One way of overcoming the problem with oscillation that occurs with a JK Flip-when J=K=1 is to use a so-called master-slave flip-flop which is illustrated below.

The master-slave flip-flop is essentially two back-to-back JKFFs, note however, that feedback from this device is fed back both to the master FF and the slave FF.

 

Any input to the master-slave flip-flop at J and K is first seen by the master FF part of the circuit while CLK is High (=1). This behaviour effectively "locks" the input into the master FF. An important feature here is that the complement of the CLK pulse is fed to the slave FF. Therefore the outputs from the master FF are only "seen" by the slave FF when CLK is Low (=0). Therefore on the High-to-Low CLK transition the outputs of the master are fed through the slave FF. This means that the at most one change of state can occur when J=K=1 and so oscillation between the states Q=0 and Q=1 at successive CLK pulses does not occur.