Decoders, Encoders,

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Decoders, Encoders, Multiplexers

• ELEC 311
• Digital Logic and Circuits
• Dr. Ron Hayne
• Images Courtesy of John Wakerly and Prentice-Hall

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Decoders

• Converts coded inputs into coded outputs
• One-to-one mapping
• Enable
• Must be asserted to perform mapping
• Example Codes
• n-bit binary
• BCD
• 1-out-of-m

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Binary Decoder (n-to-2n)

• Activates exactly one of 2n outputs based on an
  n-bit input value

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2-to-4 Decoder
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74x138 3-to-8 Decoder
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74x138 3-to-8 Decoder
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74x138 3-to-8 Decoder
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Decoders in VHDL
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Cascading Decoders
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Encoders

• Output code has fewer bits than input code
• Example
• 2n-to-n

• Only works properly if inputs asserted one at a
  time

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Priority Encoders
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Priority Encoders

• H7 I7
• H6 I6 I7'
• H0 I0 I1' I6' I7'
• A2 H4 H5 H6 H7
• A1 H2 H3 H6 H7
• A0 H1 H3 H5 H7
• IDLE (I0 I1 I6 I7)'
• I0' I1' I6' I7'

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Multiplexers

• Digital Switch
• Selects 1 of n sources
• Based on s control lines
• n 2s
• Sources b bits wide

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74x151 8-input, 1-bit Mux
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74x157 2-input, 4-bit Mux
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74x157 2-input, 4-bit Mux
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Multiplexers in VHDL
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Demultiplexers

• Digital Switch
• Selects 1 of n destinations
• Based on s control lines
• n 2s
• Destinations b bits wide

• Decoder with enable used as demux
• Enable input connected to data line

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Three-State Buffers
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Three-State Buffers
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Summary

• Decoders
• (Priority) Encoders
• Multiplexers
• Demultiplexers
• Three-State Buffers