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Another Look at the Dual Edge Flip Flop

September 22, 2008

After writing the solution to one of the puzzles and after contemplating about our dear friend the dual edge flip flop, I noticed something very interesting.

If you look carefully at the implementation of the flip flop which is made out of MUXes, you will see that it is very easy to make a posedge or negedge flip flop by just exchanging the MUX feedback connection.
I wondered if it would be possible to construct a dual edge flip flop with MUXes.
Turns out it is quite possible and requires only one more MUX!

I find the above circuit to be pretty neat because of its symmetry.

Anyways, I wondered if I was the first one to think of this trick; Turns out that… well NO. A short search on the web showed me that someone already invented and wrote a paper about this circuit, check it out here.

I am not aware of any library utilizing this design for a standard cell (if you have different information please comment or send me an email). What is this good for? I guess you could use this neat trick in an ECO, since a lot of times MUX structures are readily available.

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