# Cost of circuits

Discussion in 'Homework Help' started by Shamieh, Feb 14, 2014.

1. ### Shamieh Thread Starter Member

Oct 24, 2013
36
1
Can someone explain to me how to evaluate the cost of a circuit? My teacher went over it briefly but I'm not quite sure.. Like what are the rules? Do you count inverters? I know that each gate is equal to 2 right? Each input is equivalent to 1? So, xyz would be 3 inputs so if i had 3 inputs and 2 and gates would that just be 3 + 4 = 7? Anyone have a link for a good example of how to do cost?

2. ### #12 Expert

Nov 30, 2010
16,665
7,310
Cost is usually considered in terms of money. Please explain more about which cost you are evaluating.

3. ### WBahn Moderator

Mar 31, 2012
18,085
4,917
What kind of circuit? TTL? CMOS? Discrete logic? ASIC?

If you are talking about CMOS, then one proxy for the "cost" that jives with what you are talking about is to count the number of transistors (or something that is directly proportional to the number of transistors) since this will, roughly, be proportional to the area of the circuit which is what most of your costs are going to be sensitive to.

This is often expressed in "inverter-equivalents". For basic CMOS, an inverter uses 2 transistors. NAND and NOR gates use two transistor per input, so the number of inputs is the number of inverter equivalents. AND and OR gates have an additional two transistors to invert the output, so you need to add one more. An XOR gate will depend on how it is implemented, but is commonly either 6 or 8 inverter equivalents.

And, yes, you definitely count inverters.

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4. ### Shamieh Thread Starter Member

Oct 24, 2013
36
1
I think discrete logic is the circuit I'm looking for...Although I don't know what any of those mean so I'm not sure. I'm trying to figure out what each of these are worth.. Am i correct in saying

NAND = 2
NOR = 2
NOT = 1 (What do I do if I have two inverters on one and gate? Is that just 1 + 1 or just 1?
INPUTS = 1(for EACH input)
AND = 2
OR = 2

5. ### WBahn Moderator

Mar 31, 2012
18,085
4,917
If you are using discrete logic, then this is a very poor judge of any kind of cost.