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6) Why Limited number of Inputs to CMOS gates (e.g. NAND or NOR gates) usually Four ?

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Haripal Kochhar

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6) Why is the number of gate inputs to CMOS gates (e.g. NAND or NOR gates) usually limited to four?

Is it to limit the height of the stack, or Logical effort. Little confused Please make it clear.
 

Number of gates used depends on circuit which you r implementing , such as inverter will have basic 2 cmos gates , whereas NOR , NAND will have minimum of four cmos gates . But , you can divide each NMOS gates in NAND depending on your need , after diving you will get 2 PMOS + 4 NMOS = 6 CMOS gates . Same in case of NOR , where you can divide PMOS gates , reaulting in 4PMOS + 2 NMOS = 6 cmos gates .

This is generally related to height of stack . logical functionality remains same.
 

jeet_asic said:
Number of gates used depends on circuit which you r implementing , such as inverter will have basic 2 cmos gates , whereas NOR , NAND will have minimum of four cmos gates . But , you can divide each NMOS gates in NAND depending on your need , after diving you will get 2 PMOS + 4 NMOS = 6 CMOS gates . Same in case of NOR , where you can divide PMOS gates , reaulting in 4PMOS + 2 NMOS = 6 cmos gates .

This is generally related to height of stack . logical functionality remains same.
Click to expand...





I agree with you but i am not talking about the number of CMOS gate, but i am talking about the Number of inputs of CMOS gates.. why we dn't use 4 or 5 input NAND and NOR.
 

me too curious about this....Eight input NAND gate is commercially available. but, not familiar coz, everyone trying to implement in combinations. Am i right???
 

sorry , my bad. I looked question other way.
Haripal Kochhar said:
I agree with you but i am not talking about the number of CMOS gate, but i am talking about the Number of inputs of CMOS gates.. why we dn't use 4 or 5 input NAND and NOR.
Click to expand...
 

If you made a 12 input NAND gate, how many would you sell? You could make anything out of 2 input NAND gates but that would be a bit tedious. So, you can get 3 and 4 input ones. Larger and larger ones would be progressively less useful and sell in small numbers. You can always make larger gates from the smaller ones if needed.

Keith.
 

Tall stacks have weak and varyingly asymmetric drive (8 series
vs 1-8 parallel). In extreme environments we would limit stacks
to 3 or 4 high because taller ones also were more variable w/
environmental factors.
 

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