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Three-state buffers

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74x148 8-input priority encoder. Active-low I/O. Enable Input 'Got Something' Enable Output ... 15-input priority encoder in ABEL. Declarations. 20. Constant ... – PowerPoint PPT presentation

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Title: Three-state buffers


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
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
  • DIN150, DOUT150, S30 (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
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