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

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ECE 3110 Introduction to Digital
SystemsChapter 6 Combinational Logic Design
Practices
Encoders
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74x138/139 decoders
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Encoders

• Multiple-input/multiple-output device.
• Performs the inverse function of a Decoder.
• Outputs ( m ) are less than inputs ( n ).
• Converts input code words into output code
  words.

input code
output code
ENCODER
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Encoders vs. Decoders
Binary decoders/encoders

• 2n-to-n encoder
• Input code 1-out-of-2n.
• Output code Binary Code

• n-to-2n
• Input code Binary Code
• Output code 1-out-of-2n.

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Binary Encoder
Binary encoder
I0
I1
I2
Y0
I3
Y1

• 2n-to-n encoder 2n inputs and n outputs.
• Input code 1-out-of-2n.
• Output code Binary Code
• Example n3, 8-to-3 encoder Inputs
  
  Outputs
• 
  I0 I1 I2 I3
  I4 I5 I6 I7 Y2
  Y1 Y0 1 0 0 0 0
  0 0 0 0 0
  0 0 1 0 0 0 0
  0 0 0 0 1 0
  0 1 0 0 0 0
  0 0 1 0 0
  0 0 1 0 0 0 0
  0 1 1 0 0
  0 0 1 0 0 0
  1 0 0 0 0 0
  0 0 1 0 0
  1 0 1 0 0 0 0
  0 0 1 0 1
  1 0 0 0 0 0 0
  0 0 1 1 1
  1

Y2
I4
I5
I6
I7
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8-to-3 encoder Implementation
I0

• Simplified implementation- From the truth
  table Y0 I1 I3 I5 I7 Y1 I2 I3
  I6 I7 Y2 I4 I5 I6 I7
• Limitations - I0 has no effect on the output-
  Only one input can be activated
• ApplicationHandling multiple devices requests
  But, no simultaneous requests
• Establishing priorities solve the problem of
  multiple requests

Y2
I1
I2
Y1
I3
I4
I5
Y0
I6
I7
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Need priority in most applications
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Priority Encoder

• Assign priorities to the inputs
• When more than one input are asserted, the output
  generates the code of the input with the highest
  priority
• Priority Encoder H7I7
  (Highest Priority) H6I6.I7 H5I5.I6.I7
  H4I4.I5.I6.I7 H3I3.I4.I5.I6.I7
  H2I2.I3.I4.I5.I6.I7 H1I1.
  I2.I3.I4.I5.I6.I7 H0I0.I1.
  I2.I3.I4.I5.I6.I7 IDLE I0.I1.
  I2.I3.I4.I5.I6.I7 - Encoder A0Y0 H1
  H3 H5 H7 A1Y1 H2 H3 H6 H7
  A2Y2 H4 H5 H6 H7

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8-input priority encoder

• I7 has the highest priority,I0 least
• A2-A0 contain the number of the highest-priority
  asserted input if any.
• IDLE is asserted if no inputs are asserted.

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74x148 8-input priority encoder
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74x148 8-input priority encoder

• Active-low I/O
• Enable Input
• Got Something" Group Select
• Enable Output

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74x148 Truth Table
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74x148circuit
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Cascading priority encoders

• 32-inputpriority encoder

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Next

• Three state devices
• Multiplexers
• Reading Wakerly CH-6.6-6.7