Digital Decoder and its application in digital circuits

Unlike encoder which converts its 2^N input lines into N digits binary number according to input line which is on logic high, digital decoder decodes that encoded N digit binary number and turns an output line high out of its 2^N output lines corresponding to that binary number. It is also a combinational circuit and has ‘N’ number of line and 2^N number of outputs. It turns one of these output high corresponding to code present on its inputs.

The picture shown above is a block diagram of decoder with ‘N’ number of input and ‘M’ number of output. Relation between input and output is M = 2^N. ‘E’ is the enable pin.

2×4 digital decoder

It has 2 inputs and 4 outputs. It will turn one of that four output high according to what is on the input.

Truth table of 2×4 decoder

Now we can write the Boolean function using the truth table:

O₃ = E.I₁.I₀

O₂ = E.I₁.I₀’

O₁ = E.I₁’.I₀

O₀ = E.I₁’.I₀’

These functions are having product term so, we can use AND gates for implement it. Implementation of logic circuit for 2×4 decoder:

Binary to octal converter (3×8 digital decoder)

A 3 to 8 decoder has 3 inputs and 8 outputs. It is also called as binary to octal converter because it takes three bit binary input and gives output at any one pin out of 8 pins.

Truth table for binary to octal converter (3×8 decoder)

Boolean function:

7 = E.I₀.I₁.I₂

6 = E.I₀’.I₁.I₂

5 = E.I₀.I₁’.I₂

4 = E.I₀’.I₁’.I₂

3 = E.I₀.I₁.I₂’

2 = E.I₀’.I₁.I₂’

1 = E.I₀.I₁’.I₂’

0 = E.I₀’.I₁’.I₂’

Logic diagram implementation:

Hence, output of decoder is nothing but min term of its input.

3×8 decoder using 2×4 decoders

We can build a 3×8 decoder using two 2×4 decoders. We can find the number of lower order decoders that together can build a higher order of decoder using the formula given below.

So, we need 3 to 8 decoder which has 8 outputs by using 2 to 4 decoders which has 4 outputs. So, we need two 2 to 4 decoders.

As you can see that input of both decoders is common and enable pin of both decoder is complement of each other. If I₂ is ‘0’ then first decoder will be selected because not gate inverted the I₂ input and first four outputs will be enabled. If I₂ is ‘1’ then second decoder will be selected and next four outputs will be enabled.

4×16 decoder (binary to hexadecimal converter) using 3×8 decoders

As previously, we can implement 4 to 16 decoder by using either two 3 to 8 decoders or five 2×4 decoders. We will see both of that one by one but, first we will implement it using two 3 to 8 decoders. You can calculate the numbers of lower order decoders for implementing higher order of decoder using formula we previously discussed.

Number of 3 to 8 decoders for implement 4 to 16 = 16/8 = 2

Logic for this diagram is same as previous.

4×16 decoder (binary to hexadecimal converter) using 2×4 decoders

To implement 4 to 16 decoder using 2 to 4 decoder we need five of them. Here that formula is not applicable. But this time we don’t need NOT gate. Maybe this is the reason why we need five 2 to 4 decoders.

As per diagram you can see that a 2 to 4 decoder is used to select the other four decoders. Assume the case when I­₀ = ‘0’ , I₁ =’0’, I₂ = ‘0’ and I₃ is also zero then top most decoder will be selected. By changing the value of I₀ and I₁ we can select any first four output. Now when I₂ becomes ‘1’ then second decoder will be selected. Again by changing value of I₀ and I₁ we can select any next four output. Let’s understand this using truth table of 4 to 16 decoder.