# Understanding Flip Flops/Registers

#### Mercfh

Joined Apr 24, 2012
11
So im wanting to build a Flip Flop Circuit using Transistors, Maybe using something like this:
http://www.pcbheaven.com/scripts/imagepresent.php?filename=/wikipages/images/transistorcircuits_1235488532.jpg

(here's the video demo'ing it)

So I get the construction but then I see a simple SR Flip Flop here:

And I understand the truth table for it for Q......but what exactly does Q(with the bar on top of it) go to? Does it go to ground?

Basically im wanting to make a 1 bit register out of Transistors and Im trying to understand the logic. and then go on to make something like a 4 bit register, and Im getting a bit confused. Does anyone have any advice....schematics. Im just trying to understand these low level components and "make" them on my breadboard so I can "get them" better.

My BIG goal for now is to get a Multi-Bit Register to "store" values. I mean an ALU is a sort of type of register correct? And maybe use a LED or something of the sort to be able to see what was stored. (THis is all for learning purposes).

Any books recommended would be helpful. Im finding Elements of Computing Systems kinda confusing in some parts. (Im on the sequential logic section right now)

#### tshuck

Joined Oct 18, 2012
3,535
And I understand the truth table for it for Q......but what exactly does Q(with the bar on top of it) go to? Does it go to ground?
No, this is an output.. if you aren't using it, don't connect it to anything... This is the complement of Q, that's all....

By the way, Q(with the bar on top of it) is usually written Q'(read - "Q NOT")

My BIG goal for now is to get a Multi-Bit Register to "store" values. I mean an ALU is a sort of type of register correct? And maybe use a LED or something of the sort to be able to see what was stored. (THis is all for learning purposes).
Nope.. an ALU is an arithmetic logic unit, computes data given other data. A flip slop simply stores information... perhaps to be used by an ALU... Your LED idea, however is a good one, that is how I started learning flipflops...

Any books recommended would be helpful. Im finding Elements of Computing Systems kinda confusing in some parts. (Im on the sequential logic section right now)
I will always recommend the Sedra/Smith Microelectronic Circuits book it describes the logical building blocks and describes the CMOS process, you aren't quite there yet, but soon enough!....you'll find that on the shelves of many of the people here

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#### WBahn

Joined Mar 31, 2012
25,250
Rounding out what tshuck has already said:

You will see Q', /Q, \Q frequently in situation when you can't represent the overbar. These will also frequently be spoken as "Q-bar" as well as "Q-not".

To add to the confusion, Q' is also frequently use to mean "the next value of Q", particularly in feedback sequential circuits.

The ALU is usually a combinatorial circuit and has no registers in it. For multicycle processors there are registers coupled to it to retain partial results and whether these are considered part of the ALU is a matter of semantics.

John Wakerly's book, Digital Design: Principles and Practices is an excellent book that covers the basic logic families very nicely and proceeds from there.