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Three-state buffers

Three-state buffers

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Three-state buffers

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  1. Three-state buffers • Output = LOW, HIGH, or Hi-Z. • Can tie multiple outputs together, if at most one at a time is driven.

  2. Different flavors

  3. Manual Page Link

  4. Timing considerations

  5. Three-state drivers

  6. Driver application

  7. Three-state transceiver

  8. Transceiver application

  9. Three-state enables in ABEL

  10. Decoder Encoder Encoders vs. Decoders

  11. Binary encoders

  12. Need priority in most applications

  13. 8-input priority encoder

  14. Priority-encoder logic equations

  15. 74x148 8-input priority encoder • Active-low I/O • Enable Input • “Got Something” • Enable Output

  16. 74x148circuit

  17. 74x148 Truth Table

  18. Cascading priority encoders • 32-inputpriority encoder

  19. 15-input priority encoder in ABEL • Declarations

  20. Constant expressions

  21. Outputs

  22. Alternative formulation • WHEN is very natural for priority function

  23. Multiplexers

  24. 74x1518-input multiplexer

  25. 74x151 truth table

  26. CMOS transmission gates • 2-input multiplexer

  27. Other multiplexer varieties • 2-input, 4-bit-wide • 74x157 • 4-input, 2-bit-wide • 74x153

  28. ABEL code for 74x153-like mux

  29. Easier ABEL multiplexer code

  30. 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

  31. 16 16-to-1 muxes 16-to-1 mux = 2 x 74x151 8-to-1 mux + NAND gate

  32. 4 16-bit 2-to-1 muxes 16-bit 2-to-1 mux = 4 x 74x157 4-bit 2-to-1 mux

  33. Properties of different approaches

  34. ABEL code for barrel shifter 20 inputs 16 outputs 16 product terms per output