# Flip-flop feedback timing problem

So this is my first question here.

I am reading this book, "Digital Electronics & Computer Design - By M.M. Mano". While I was reading the Sequential logic and flip-flops, I found this:

The feedback path between combinational circuit and memory elements(flip-flops) in a sequential circuit can produce instability if the outputs of memory elements are changing while the outputs of the combinational that go to flip-flop inputs are being sampled by the clock pulse.

I didn't understand this statement. So I googled and found a similar statement at circuitstoday.

If a clock pulse is given to the input of the flip flop at the same time when the output of the flip flop is changing, it may cause instability to the circuit.

I tried to get my head around this but didn't come up with any idea that gets me peace. I'm confused especially with this "the outputs of memory elements are changing or when the output of the flip flop is changing. What does it mean? Because the output of a flip-flop will only change if there's a change applied on input signal. It will not change by itself.

• i.e. if previously there's an input applied to flip-flops, then we have to wait at least for the time equal to propagation delay of the circuit to give new input? Like if we have a combinational circuit, which takes, a, b & c as inputs where c is feedback from memory element and outputs x & y, where y is the input to flip-flops. Jul 26, 2020 at 12:02
• Then in the first clock pulse, when y goes to flip-flop, there should be enough time(or we have to wait for the time equal to) to propagate signal y through the flip flop and through its feedback path, back to the combinational circuit? Jul 26, 2020 at 12:09
• If this is the matter then there's another confusion. i.e. the combinational circuit will not work until all the three a, b & c inputs are available. So no matter if user supplies a & b to the input terminal of the combination circuit, it'll only work when the c will be present. Jul 26, 2020 at 12:14