Encoders , Decoders and Multiplexers

ENCODERS AND DECODERS

• Binary signals along a wire can represent a single state, or form part of a binary code
• Sometimes we want to convert between these representations:
• We may want to encode independent input wires as a single output binary code OR
• We may want to decode a multi-bit binary value to the single output line it represents

ENCODERS

• Encoders are used to code binary data
• Converts an active input signal into a coded output signal
• If an encoder has n lnput lines and m output lines then:
• n <= 2m

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ENCODER EXAMPLE: 4-TO-2 PRIORITY ENCODER

• Imagine 4 input lines each indicating a priority level
• We encode the priority as a 2 bit binary number according to the truth table:

• Imagine 4 input lines each indicating a priority level
• We encode the priority as a 2 bit binary number according to the circuit:

• Key feature:
• If multiple inputs are raised High, it will always encode the high priority!
• Lets try it

ENCODER EXAMPLE: 8-TO-3 PRIORITY ENCODER

• What about an 8-to-3 encoder?
• Try and have a go!
• Hint: use 3 x 4 input OR gates

COMMON ENCODER APPLICATIONS

• Interrupts we talked about last week!
• Keyboards: Imagine microcomputer reads from keyboard only one button at a time. buttons of a QWERTY keyboard into a binary code
• Eg ASCII code in 7 bits (= output wires)

DECODERS

• Decoders are the opposite of encoders:
• used to decode binary information
• Multiple input lines represent a binary code
• This code is used to select a single output line to raise High
• If a decoder has n input lines, it can have a maximum 2n output lines

DECODER EXAMPLE: 2-TO-4 DECODER

Have a go at designing a circuit to map a 2 bit binary number to select 1 of 4 output lines

DECODER APPLICATIONS

• Memory Address Decoder
• Memory commonly stored across multiple chips but transferred over a common bus
• Each memory chip has a Chip Select (CS) input
• Memory address is decoded to select the desired chip

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• Displays (eg. 7-segment display)
• 3-8 decoder outputs can feed directly into 7-segment display
• Similar concept for LCD displays

MULTIPLEXERS

• Mulitplexers (many-to-one) are similar to encoders
• Used for routing digital data from many to one line.
• Selects a single input line from several possible lines and sends the data to the output line.

MULTIPLEXER USE-CASES

• Telephony
• Communication systems
• Memory
• Parallel to serial conversion

DE-MULTIPLEXERS

• De-Mulitplexers (one-to-many) are similar to decoders
• Used to transimit digital data from one to many lines.
• Selects an output line from several possible lines and sends the input data to that line.

DE-MULTIPLEXER USE-CASES

• Communication systems
• Connecting ALU with registers
• Serial to parallel conversion

SUMMARY

• data manipulation and transfer

• Encoders convert an active input signal into a coded output signal
• Decoders selects a single output line based on a coded output
• Multiplexers (many-to-one) choose which line to channel data from
• De-Multiplexers (one-to-many) choose which output line to channel data to