ENGR/ECE%20212%2021%20February,%202003

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ENGR/ECE 21221 February, 2003

• All slides except those with blue backgrounds
• are taken from John Wakerlys
• Stanford EE 121, Digital Design Laboratory

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Decoders

• General decoder structure
• Typically n inputs, 2n outputs
• 2-to-4, 3-to-8, 4-to-16, etc.
• with a few enable inputs

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Binary 2-to-4 decoder
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MSI 2-to-4 decoder

• Input buffering (less load)
• NAND gates (faster)

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Decoder Symbol
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3-to-8 decoder
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74x138 3-to-8-decoder symbol
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Decoder cascading
4-to-16 decoder
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More cascading
5-to-32 decoder
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ABEL / PAL Version of 74x138

• module Z74X138
• title '74x138 Decoder PLD J. Wakerly, Stanford
  University
• "Z74X138 device 'P16L8'
• " Input pins
• A, B, C, !G2A, !G2B, G1 pin 1, 2, 3, 4, 5,
  6
• " Output pins
• !Y0, !Y1, !Y2, !Y3 pin 19, 18, 17, 16
  istype 'com'
• !Y4, !Y5, !Y6, !Y7 pin 15, 14, 13, 12
  istype 'com'
• " Constant expression
• ENB G1 G2A G2B

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ABEL decoder program (continued

• equations
• Y0 ENB !C !B !A
• Y1 ENB !C !B A
• Y2 ENB !C B !A
• Y3 ENB !C B A
• Y4 ENB C !B !A
• Y5 ENB C !B A
• Y6 ENB C B !A
• Y7 ENB C B A
• end Z74X138

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Decoder applications

• Microprocessor memory systems
• selecting different banks of memory
• Microprocessor input/output systems
• selecting different devices
• Microprocessor instruction decoding
• enabling different functional units
• Memory chips
• enabling different rows of memory depending on
  address
• Lots of other applications

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Some decoder datasheets

• 74x138
• 3-line to 8-line decoders/demultiplexers
• 74x139
• Dual 2-line to 4 line decoders/demultiplexers
• 74x154
• 4-to-16 line decoders/demultiplexers

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Encoders vs. Decoders
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Binary encoders
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Need priority in most applications
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8-input priority encoder
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74x148 8-input priority encoder

• Active-low I/O
• Enable Input
• Got Something
• Enable Output

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74x148circuit
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Cascading priority encoders

• 32-inputpriority encoder

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Some encoder datasheets

• 74x148
• 8-line to 3-line priority encoders
• 74x147
• 10-line to 4-line priority encoders

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Multiplexers
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74x1518-input multiplexer
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Some multiplexer datasheets

• 74x151
• 8-Input Multiplexer
• 74x153
• Dual 4-Input Multiplexer
• 74x157, 74x158
• Quad 2-Input Multiplexer