Three state buffers
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Three-state buffers. Output = LOW, HIGH, or Hi-Z. Can tie multiple outputs together, if at most one at a time is driven. Different flavors. Manual Page Link. Timing considerations. Three-state drivers. Driver application. Three-state transceiver. Transceiver application.

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Three state buffers l.jpg
Three-state buffers

  • Output = LOW, HIGH, or Hi-Z.

  • Can tie multiple outputs together, if at most one at a time is driven.










Encoders vs decoders l.jpg

Decoder

Encoder

Encoders vs. Decoders






74x148 8 input priority encoder l.jpg
74x148 8-input priority encoder

  • Active-low I/O

  • Enable Input

  • “Got Something”

  • Enable Output


74x148 circuit l.jpg
74x148circuit



Cascading priority encoders l.jpg
Cascading priority encoders

  • 32-inputpriority encoder





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Alternative formulation

  • WHEN is very natural for priority function



74x151 8 input multiplexer l.jpg
74x1518-input multiplexer



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CMOS transmission gates

  • 2-input multiplexer


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Other multiplexer varieties

  • 2-input, 4-bit-wide

    • 74x157

  • 4-input, 2-bit-wide

    • 74x153




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Barrel shifter design example

  • n data inputs, n data outputs

  • Control inputs specify number of positions to rotate or shift data inputs

  • Example: n = 16

    • DIN[15:0], DOUT[15:0], S[3:0] (shift amount)

  • Many possible solutions, all based on multiplexers


16 16 to 1 muxes l.jpg
16 16-to-1 muxes

16-to-1 mux = 2 x 74x151 8-to-1 mux + NAND gate


4 16 bit 2 to 1 muxes l.jpg
4 16-bit 2-to-1 muxes

16-bit 2-to-1 mux = 4 x 74x157 4-bit 2-to-1 mux



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ABEL code for barrel shifter

20 inputs

16 outputs

16 product terms

per output


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