Chapter 1 - Decoders
Circuit Implementation using decoders.

Lets move on to the QUIZ


From the previous section, we can see that minterms of an input are generated from decoders. This is of limited use on its own, so decoders are used with additional logic at the output stage. Let us say we have a 2 to 4 line (non inverted) decoder and need a function of the following Z = Sum(2,3) = A./B + A.B
We can use the minterms from the decoder with one single OR gate to make up the above function. This is simple. However there is nothing stopping us derive more than one function. So Z1 = Sum(1,0) Z2 = Sum(1,3) and Z3 = Sum(2,3). We can use 3 OR gates and we now have 3 different functions.

The resulting logic is shown as follows:

When we are trying to implement functions like this using inverted decoders, we can use NAND gates instead of OR gates. This can be proved by use of De Morgan's theorems:
The logic we currently have looks like this; (X+Y) = Z
Our input stage is then inverted because of the decoder, so if we still use OR gates our Z will be wrong; (X'+Y') != Z
However we can correct this by adding two NOT gates, however this is quite wasteful; (X''+Y'') = Z
Now using De Morgan we can re-write as; (X'.Y')' = Z as X'+Y' = (X.Y)'
This the exact circuit that we have, inverted inputs from the decoder and now the usage of NAND gates.

Minimization of output logic
The minimization process for multiple outputs from any device is simple. This is best shown using an example.

Example

Using a 3-8 decoder (non inverted), implement the following output functions.
Z0 = ∑ (2,3,5,6)
Z1 = ∑ (0,1,2,3)
Z2 = ∑ (0,1,5,6)
Z3 = ∑ (0,1,7)

To simplify the terms, we can rewrite them in this form to identify common terms:
Z0 = _ _ 2 3 _ 5 6 _
Z1 = 0 1 2 3 _ _ _ _
Z2 = 0 1 _ _ _ 5 6 _
Z3 = 0 1 _ _ _ _ _ 7
Note: We can pair (0,1), (2,3) and (5,6) as these are common across several needed outputs.

The solution shown below uses 7 x two input OR gates and 1 x 3-8 decoder to create the needed functions.

Lets move on to the QUIZ