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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
Three-state buffers
  • Output = LOW, HIGH, or Hi-Z.
  • Can tie multiple outputs together, if at most one at a time is driven.
74x148 8 input priority encoder
74x148 8-input priority encoder
  • Active-low I/O
  • Enable Input
  • “Got Something”
  • Enable Output
cascading priority encoders
Cascading priority encoders
  • 32-inputpriority encoder
alternative formulation
Alternative formulation
  • WHEN is very natural for priority function
cmos transmission gates
CMOS transmission gates
  • 2-input multiplexer
other multiplexer varieties
Other multiplexer varieties
  • 2-input, 4-bit-wide
    • 74x157
  • 4-input, 2-bit-wide
    • 74x153
barrel shifter design example
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
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
4 16-bit 2-to-1 muxes

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

abel code for barrel shifter
ABEL code for barrel shifter

20 inputs

16 outputs

16 product terms

per output