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ECE 3110: Introduction to Digital Systems Chapter 6 Combinational Logic Design Practices

ECE 3110: Introduction to Digital Systems Chapter 6 Combinational Logic Design Practices. Three-state devices Multiplexers. Three State Buffers/Drivers. A buffer/inverter with enable input

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ECE 3110: Introduction to Digital Systems Chapter 6 Combinational Logic Design Practices

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  1. ECE 3110: Introduction to Digital SystemsChapter 6Combinational Logic Design Practices Three-state devices Multiplexers

  2. Three State Buffers/Drivers • A buffer/inverter with enable input • Buffer Buffer Inverter Inverter Active High Enable Active Low Enable Active High Enable Active Low Enable • The device behaves like an ordinary buffer/inverter when the enable input is asserted. • The output is floating ( High Impedance, Hi-Z ) when the enable input is deasserted ( The input is isolated from the output, behaves as if it did not exist) • Application: Controlling the access of a single line/bus by multiple devices

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

  4. 8 sources share a three-state party line

  5. Timing considerations

  6. Standard SSI/MSI 3-state buffers SSI: 74x125, 74x126 (independent enable inputs) MSI: 74x541 and varieties such as 74x540, 74x240, 74x241

  7. Octal non-inverting 3-state buffer Hysteresis

  8. Driver application

  9. Three-state transceiver • Pairs of 3-state buffers connected in opposite directions between each pair of pins, so data can be transferred in either direction. • DIR determines the direction of transfer (A-->B or B-->A)

  10. Transceiver application Bidirectional buses

  11. Multiplexers (mux) E.g. Put between Processor’s registers and ALU A 16-bit processor where 3-bit field specifies one of 8 registers. The 3-bit field is connected to the select inputs of an 8-input, 16-bit mux. Select one of n sources of data to transmit on a bus.

  12. MSI: 74x1518-input 1-bit multiplexer

  13. 74x151 truth table

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

  15. Expanding Multiplexers 32-to-1 mux

  16. Demultiplexers • A mux is used to select one of n sources of data to transmit on a bus. • A demultiplexer can be used to route the bus data to one of m destinations. Just the inverse of a mux. • A binary decoder with an enable input can be used as a demux. E.g. 74x139 can be used as a 2-bit, 4-output demux.

  17. Homework #8 • Notes: On all timing calculation problems, describe the circuit path used and show each number in the calculation. For problem 5.19, use real 74x parts ONLY and include specific 74x part numbers for all components used on the diagram.

  18. Next… • X-OR gates and Parity circuits • Comparators • Reading Wakerly CH-6.8-6.9

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