Encoder and Decoder: Application of Combinational Logic Circuits, Cheat Sheet of Computer science

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ENCODER and

DECODER

Application of Combinational Logic

circuit

Encoding and Decoding :

• When connecting a logic circuit with the outside world,

incoming information from a keyboard or other input device

must be changed (encoded) into an appropriate binary form.

• Also before binary data produced by the digital system can

be used by an output device, such as a display, it must be

decoded into a form that can be used by the display.

• Encoders and decoders used for such jobs are also

combinational logic circuits

Priority Encoders

• Binary Encoders generally have a number of inputs that must be mutually exclusive, i.e. only one of the inputs can be active at any one time.
• The encoder then produces a binary code on the output pins, which changes in response to the input that has been activated.
• Example^ decimal to BCD (or 10-to-4 line) encoder would be expected to have ten input pins, but in fact the 74HC147 has only 9.
• The tenth condition (zero) is assumed to be present because when none of the 1 to 9 input pins is active, this must indicate zero.
• The input pins may be used to connect to switches on a decimal keypad, and the encoder would output a 4-bit BCD code, (0000 to 1001) depending on which key has been pressed, or simply to identify which one of ten input lines in a circuit is active, by outputting an appropriate number in four bit BCD code.
• if more than one input is made active at the same time, the output will select only the most significant active input. For example, if 6 and 7 are pressed together the BCD output will indicate 7.

IC 74HC148 8-to-3-Line Encoder

• The 74HC148 also uses priority encoding and features eight active low

inputs and a three-bit active low binary (Octal) output.

• The IC is enabled by an active low Enable In ( EI ), and an active low

Enable Out (EO) is provided so that several ICs can be connected in

cascade, allowing the encoding of more inputs, for example a 16-to-6- line

encoder using two 8-to-3 encoders.

• the IC is only active when EI is low, and also that for each input selected

by a low logic level (L), all lower value inputs indicate ‘Don’t Care’, typical

of priority encoding.

IC 74HC148 8-to-3-Line Encoder

• The Group Select ( GS ) output is used together with EO for

connecting additional 74HC148 ICs in cascade.

• The EI input is normally used on the most significant IC and

whenever an input on this IC is selected, the EO output goes

high (disabling any less significant ICs), and the Group Select

( GS ) output goes low indicating that the group of outputs of

this IC are active.

4 input priority Encoder

Decoder

• A circuit that coverts binary information from n input lines to a maximum of

2^n unique output lines

• May have fewer than 2^n outputs
• A n-to-m-line decoder (m ≤ 2^n):^ Generate the m minterms of n input

variables

• For each possible input combination, there is only one output that is equal to 1
• The output whose value is equal to 1 represents the minterm equivalent of the

binary number presently available in the input lines

Encoder Vs Decoder

Binary Encoders