A feedback shift register is a shift register whose serial i

A feedback shift register is a shift register whose serial input is connected to some function of selected register outputs. In the following feedback shift register, the serial input is the exclusive-OR of outputs QC and QD. Write the sequence of states of the register, starting from state 1000.

Solution

ANSWER:

Initially, the flip-flops A,B,C,D are at logic levels 1,0,0,0 respectively.

In this case, serial input to the shift register is 0 xor 0 = 0. So, the next state would be 0100. Then the next state would be 0010.

Now, the serial input is 0 xor 1 = 1. Hence, next state would be 1001. The transitions would similarly go on.

I have compiled a table below to make sure that you understand. Please have a look at it.

The sequence keeps on repeating until the clock is alive.

If you have any doubts, let me know. I will clarify them.