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CMOS gates


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  • 2 weeks later...

Gates is gates.
TTL runs at 5vdc ONLY.  Cmos can run at a wider range usually up to 15V and down to 3.
You can mix TTL and CMOS at 5V.

Most gates have 2 inputs and 1 output.  They come in packages that have 14 pins usually.  there are 2 power pins (Vcc and Gnd)
The reaming pins are divided up (normally) among 4 devices (3 wires each * 4 = 12)

These are called QUAD packages (Quad And,  Quad OR)  ect..

NOT gates only have 1 input and 1 output. In a 14 pin DIP there are 6 NOT gates. These are referred to as HEX parts (HEX NOT GATE)

Using TTL voltages as an example:

AND - if BOTH inputs are at 5V the output will also be 5V.  it is 0v otherwise.
OR  - if BOTH inputs are at 0V the output will also be 0V. it is 5v otherwise.
NOT - 5v IN give 0v out,  0v in gives 5v out.

These are often combined so often that to same chip space manufactures made combo chips also.

NAND - And gate with a NOT after it. Same as AND but the output is backwards.
NOR - OR gate with NOT after it.  Same as OR but the Output is backwards.

There is one odd ball (isn't there always) called XOR or EXOR.
This is just like an OR gate except that if BOTH inputs are high the output is low also.
In other words:
XOR - if both inputs are the same (5v or 0v)  the output is 0v

The method for figuring out how to connect the gates is called Boolean algebra.
It also uses a lot of "Truth Tables"

Truth Table:
    1) list every input you want to measure across the top.
    2) list every combination of on and off for all the inputs you provided.
    3) At the end of the table create one more column called OUTPUT (or Q)
    4) fill in that column with the desired results for each possible input.

    5) hard part!  Figure out what combination of gates will give you that result,
        Hint. Draw out the truth tables for ALL THE GATES and look for similarities.

Send your truth table and I can show you some examples.


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