Decoder

Decoder

·         A device which does the reverse operation of an encoder and is also a circuit that changes a code into a set of signals.

·         A combinational circuit that convert binary information from n input lines to a maximum of 2ⁿ unique output lines (binary-coded decimal decoder)

(the  2ⁿ output is said to be maximum because in case of the ‘n’ bit coded information has unused bit combination ,the output maybe less than 2ⁿ)

·         The example decoder circuit would be AND gate as

=the output of this is  “High” when all its inputs are “High”

=the output is called the “active High output”

·         Apart from that , instead of AND gate, the output of NAND gate will be    “Low “ when all the inputs are “High” and this known as “active Low output”

·         There are some types of decoders:

a.      1-to-2 decoder

b.      2-to-4 decoder

c.       3-to-8 decoder

d.      4-to-16 decoder

·         The most simplest decoder is the 1-to-2 decoder and its truth table is as below:

A	D1	D0
0	0	1
1	1	0

(A is the address and D is the dataline. D₀ is not A and D₁ is A… The circuit looks like…)

• More complex than the 1-to-2 decoder is  the 2-to-4 decoder and its truth table is as below:

The 2-to-4 decoder’s circuit is looked as below:

·         The 2-to-4 decoder can also created by combining two 1-to-2 decoder as below:

·          

• For other types of decoder, 3-to-8 decoder and 4-to-16 decoder, the truth table and the circuits are as below:

o   3-to-8 decoder

=3-to-8 decoder also can formed from two 2-to-4 decoder with enable signals…

• 4-to-16 decoder

 =4-to-16 decoder also can formed by combining two 3-to-8 decoder as   the diagram below:

From the above diagram , the enable inputs of both 3-to-8 decoder originate from a 4^th input (the selector between two 3-to-8 decoder)…

= allow  4^th input to enable the either the top decoder or bottom decoder ,which produces :

o   Output D₀ through D₇ for the 1^st  decoder

o   Output D₈ through D₁₅ for the 2^nd  decoder

=decoder-demultiplexor : decoder that contain enable inputs

Eg:

  4-to-16 decoder produced by adding a 4^th input that shared among both decoder, producing 16 outputs.

Row select in RAM (Random-Access  Memory )

o   Most RAM use a n-to-2ⁿ decoder to convert the selected address on the address bus to one of the row address select lines, thus;

ü  Decoder undoing the encoding so that the original information can be retrieved

Instruction Decoder

o   The part of the CPU that convert the bits stored in the instruction register

o   3-to-8 decoder in the instruction decoder is used to select 2 source registers of the register file in the simple CPU with 8 registers:

=to feed into the ALU as well as the destination register to accept the output of ALU.

Chong Cai Ning B031210080