1
\$\begingroup\$

I read that circuits are two type combinational and sequential. In combinational circuit the output depends on present state/input i.e the previous state do not effect the present output. but D flip flop have characteristic equation Q(t+1)= D which means here also output at any time follow the input only ... so can we say that D flip flop is a kind of combination circuit instead of sequential circuit...

\$\endgroup\$

3 Answers 3

3
\$\begingroup\$

You said, D flip flop have characteristic equation Q(t+1)= D which means here also output at any time follow the input only"

But that's not true -- the output of the D flip-flop changes state only when the CLK input goes high (rising edge trigger). At that time, Q = D. All other times, D can change and it has no effect on Q.

So the D flip-flop is sequential (has state), not combinatorial.

\$\endgroup\$
0
0
\$\begingroup\$

The line you quote is only one aspect of a transparent D latch (the way I use those names it is NOT a flip-flop, that is reserved for the simple thing with S and R inputs). Another line reads Q(t+1) = Q(t), which makes the transparent latch not belonging to the group of combinatorial (not combination) circuits.

\$\endgroup\$
0
\$\begingroup\$

The equation for a D flip flop is Q(n+1)=D.C+Q(n).C'.

This means that when Clock is 1, the data is written to the output, but when clock is zero, the data is in read mode and cannot be written. The Q(n) that you see in the above circuit accounts for the feedback. There can be NO flip flop without feedback. In fact even bistable multivibrators have regenerative feedback.

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.