Digital Logic (Propagation Delay)

Question

Why Propagation Delay for generating a square wave takes 2Nt_pd ?(where N is no. of FlipFlops)

Comments

Assume i have N not gates in series,where N is odd.

a--b--c---d--e--

Now these are 5 not gates abcde .Each gate has delay of TPD.Now this is one complete circuit.We apply input at a and read output at e.

If i apply 0 to a ,then after propagation delay of all gates i will get 1 as output at e.If number of gates are even then i will get 0 and it wont be square waveform.

Now after N*TPD my signal goes from o to 1 but now to form square waveform the signal has to go from 1 to 0.Means we need to invert the output so we will give the current output as a feedback input to the circuit.e will be connected to a input.

Now a gets 1 at the input. After N*TPD i will get 0 at the output.

My signal goes 0-1-0 and it forms square waveform and it took N*TPD+N*TPD=2*N*TPD