Universal logic modules vs NAND-like modules

Wow, formal description..... I'd call it an IF gate, I always have problems with formal names, I still imagine XOR as a Diff gate, since it shows if there is a difference.

sorry, but that wont be any help

martin

After the first death, there is no other. (Dylan Thomas)

The type of device you describe is common in FPGA architectures (indeed, it looks almost exactly like the a Logic Block, as found in Altera and some Xilinx devices).

Such a logic block (with a function F, usually from a lookup table) plus the ONE, ZERO and NOT, is a subset of what is used on those devices. (They also have some straight AND/NAND gating as well).

For want of a better term, I would call it a universal logic *function block*, as opposed to a universal logic *gate* (which a NAND is, by definition, as well as NOR).

Cheers

PeteS

I'm not sure which IF gate you are referring to, but the one defined by IF A=1, THEN B, ELSE C, which is also by definition a ULM, can not by itself describe a NAND gate as it is unable to create a NOT gate. But there are other functions that behave exactly like NAND or NOR gates in the sense that, by themselves, they can also describe any other function. I think there is something special about them and I would like to distinguish them from the ordinary ULMs.

Candida

This is a good suggestion. Thank you for the input.

Candida

The NAND and NOR gates are universal logic building blocks, but not by definition. One can easily construct AND, OR, and NOT gates from NANDs and NORs. From AND, ORs,and NOTs, one can build anything logical. That is what makes them universal. Their inbuilt NOT property cannot be duplicated from only ANDs and ORs. Ratch

I really can't figure out what you're looking for, but fixed functions are often referred to as "unit logic". Programmable logic (AND/OR/ZERO/NOT) is a ROM, or perhaps even a multiplexor.

Semantics perhaps :)

My point is that given sufficient NAND gates, one may construct any logic function (ditto for NOR), leading to my comment that a NAND is, by definition (mathematical, that is) a universal gate. I can see the issue with calling it a gate (it's really two), but in common parlance, at least, we call it a gate. Mathematically, I would agree it's a function.

Cheers

PeteS

I think you have that backwards. In the real (physical) world, NAND's and NOR's are native (minimum device count). It takes an extra inverter to get to AND's and OR's.

Again, notational, not physical.

...Jim Thompson

This whole discussion is semantics. As you said, NAND/NOR/XNOR functions are really AND/OR/XOR followed by NOT. Also, there are AND/OR/XOR constructs with NOT(s) at the inputs, but there is no common label for these. For example, Xilinx calls a 2-input AND gate with one NOT input an AND2B1 (B is for bubble, I presume).

Fairchild (and others) make "universal configurable 2-input logic gates" that allow any 2-input function to be implemented. In this datasheet the same example I used above is called "2-input AND with inverted input". See:

We don't have convenient labels to describe all possible 2-input functions, let alone n-input functions. It is probably best to use schematic or equation representation to be clear. Failing that, maybe use ARB to designate an arbitrary function, but the function still needs to be described somewhere. When I use one of the Fairchild universal gates in a design it is drawn as a box, and I put the function (equation) on the schematic so that the logic can be followed.

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Greg Neff VP Engineering

AND/OR are really NAND/NOR followed by a NOT. ;-)

Sure there is. They're called NOR/NAND/XOR. ;-)

I call it O Fairchild (and others) make "universal configurable 2-input logic

Wow. That seems like a waste. I guess if one only needs one gate and has no power to spare it is useful, but I've never been in that situation.

(snip)

These gates have the very useful property of handling overvoltage inputs. These (and similar) devices are handy when you need both voltage translation from a higher voltage domain to a lower voltage domain, and you need some simple logic.

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Greg Neff VP Engineering

They don't look like they're very robust to me. The Recommended input voltage is 0 to 3.6V with an absolute maximum of -.5V to 4.6V. I've found quick-switches to be useful for translating from 5V to 3.3V logic. I guess some find these things useful though (they seem to sell them).

The nc7sp57 and nc7sp58 gates are spec'd down to 0.9V, so imagine you're in 2005 and working in a 1.2V logic environment (14ns prop delay). A 3.6V input-overvoltage tolerance looks rather high... more than high enough to deal with your 3.3V high-voltage supply!

If you want a similar capability (but only spec'd down to 1.65V), with a 5.5V max supply rating, use the nc7sz57 and nc7sz58 parts.