The Decoder

The decoder is a combinational circuit which is used to identify the specified input value by setting the output of the bit on the corresponding index to the input. The decoder is equivalent to a 1-bit multiplexer and the constant data input of 1.

For example, for a 4-bit decoder, the port table is shown below (the _ character can be used in Verilog to separate bits or numbers belonging to a base in writing a constant, for ease of reading; 16'b0100_0011_0100_0111)

INPUT	OUTPUT
0000	0000_0000_0000_0001
0001	0000_0000_0000_0010
0010	0000_0000_0000_0100
0011	0000_0000_0000_1000
0100	0000_0000_0001_0000
0101	0000_0000_0010_0000
0110	0000_0000_0100_0000
0111	0000_0000_1000_0000
1000	0000_0001_0000_0000
1001	0000_0010_0000_0000
1010	0000_0100_0000_0000
1011	0000_1000_0000_0000
1100	0001_0000_0000_0000
1101	0010_0000_0000_0000
1110	0100_0000_0000_0000
1111	1000_0000_0000_0000

Interface

The interface of a decoder is made up of two signals:

• input of n bits;
• output of 2 ⁿ bits.

Implementation

There are several possible implementations for the decoder:

• using blocks generate for;
• using blocks always combinational and case;
• using a block assign and the left shift operator.

Although the last variant is not optimal from the point of view of the synthesized circuit, we will use this method because of the ease and short dimension of the description.