# AND, NAND n OR, NOR gates

#### Triaca

Joined Dec 15, 2012
7
Among AND and NAND , OR and NOR gates, practically which would be preferable
?

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#### kubeek

Joined Sep 20, 2005
5,724
How about the one that fits the circuit the best?
You can build any circuit from either xors, nors or nands, or from AND and NOT gates, or from OR and NOT gates.
Nands are sometimes used as basic building blocks, since they only use 4 transistors in cmos circuit. Nor uses 4 transistors as well.
Without knowing what you´re building it is hard to tell. You could be talking about ASIC design or building from discrete chips..

#### Triaca

Joined Dec 15, 2012
7
em questioning about the time delays.

#### kubeek

Joined Sep 20, 2005
5,724
em questioning about the time delays.
Sorry what? Could you please use this cool new concept called sentence and ask something that makes more sense?

#### ScottWang

Joined Aug 23, 2012
6,913
You should post what you want to do?
What's the input and what's the output?
I don't think the members at here are the mind readers.

#### JMW

Joined Nov 21, 2011
127
Welcome to the forum. In the olden days "N" type gates were preferred. The reason being is they would "sink" or pull down a voltage. If a short occurred no damage. Or's and And's "sourced" a voltage. If you shorted the output you toasted the IC.

#### crutschow

Joined Mar 14, 2008
25,132
Welcome to the forum. In the olden days "N" type gates were preferred. The reason being is they would "sink" or pull down a voltage. If a short occurred no damage. Or's and And's "sourced" a voltage. If you shorted the output you toasted the IC.
That's not true of any modern IC logic gates, of course.

#### Austin Clark

Joined Dec 28, 2011
412
em questioning about the time delays.
If you just wanted to introduce delay in the signal, a BUFFER would be perfect. An EVEN number of NOT gates in series would work as well.

#### crutschow

Joined Mar 14, 2008
25,132
If you want minimum delay then you need to look at the data sheets for a particular logic family. Generally AND and OR gates are slower than NAND and NOR gates because they have a stage added to invert the signal from the basic input NAND or NOR function.

#### WBahn

Joined Mar 31, 2012
25,927
You can build any circuit from either xors, ....

How do you make an OR from nothing but XORs?

#### WBahn

Joined Mar 31, 2012
25,927
Among AND and NAND , OR and NOR gates, practically which would be preferable
?
As others have indicated, NAND and NOR gates, as a rule, are faster and require less silicon area because they only use 4 transistors instead of the 6 required for an AND or OR (talking CMOS here). Between a NAND and a NOR, the NAND has an advantage because the NFETs are in series and the PFETs are in parallel. NFETs, for the same size, have better drive strength so you can use, roughly, the same size (i.e., minimum geometry) transistors and get a balanced circuit. With a NOR, you generally need to increase the size of the PFETs considerably to achieve this.

#### kubeek

Joined Sep 20, 2005
5,724
How do you make an OR from nothing but XORs?
You're right, XORs are not functionalluy complete like NANDs and NORs are.

#### WBahn

Joined Mar 31, 2012
25,927
Yep. Somewhat counterintuitive that they aren't. But when you flip inputs and outputs on and XOR you either get an XOR or an XNOR back.

Another gate that is functionally complete is a 2:1 MUX.