AND, NAND n OR, NOR gates

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..
 

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. ;)
 

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
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.
 

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.
 
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