332:437 Lecture 16 FSM Synchronizers

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332437 Lecture 16 FSM Synchronizers

• Synchronization failure
• Runt pulses
• Simple synchronizers
• Synchronizer timing
• Handshake interface techniques
• Summary

Material from An Engineering Approach to Digital
Design, by William I. Fletcher, Englewood Cliffs,
NJ Prentice-Hall
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Synchronization of Systems

• Needed when merging independent systems on
  different clocks
• Problem System controller needs to know when or
  within what time frame it can expect input
  changes from controlled/controlling systems

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Consequences of Synchronization Failure

• Input Changes missed by system controller
• Undefined state transitions occur

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Input changing Near Triggering Edge of Clock
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Output Cell in Meta-Stable Condition from Runt
Pulse
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Meta-Stable Condition of Flip-Flop
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Example Synchronizer Catching Cell

• Catching cell converts Pulse into level

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Synchronizer with Explicit Reset from System
Controller
SYNCHD INPUT(H)
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Synching Operation of the Pulse-Catching Circuits
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Synchronizer Timing Considerations

• May be impractical to increase system clock
  frequency for synchronization
• Need to instead catch input pulse hold it until
  system controller can service it.
• fp frequency of input pulse
• fc Clock frequency
• tp Period of input pulse
• tc Period of Clock
• First assume that fp lt fc

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Synchronizer Based on These Assumptions

• tp lt tc So, tp short infrequent in relation
  to system clock
• State changes of system controller made on rising
  edge of system clock
• Time period between falling rising edge of
  system clock gt systems setting time

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Missed Short Asynchronous Input

• Compared to system clock

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Level Synchronization When tp gtgt tc

• Catching level that changes asynchronously with
  respect to system clock
• Use same circuit as before, but omit catching cell

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Problem with Asynchronous Inputs

• Changes in inputs may cause outputs of next state
  decoder to change during set-up hold times of
  flip-flops
• Causes erratic behavior of present-state register

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Handshake Interface Technique

• One party stimulates second party
• Second party signals first to acknowledge receipt
  of signal
• First party can now initiate another transaction

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Handshake Between Systems Operating Asynchronously
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Problem

• Internal input changes in one of the systems
  being synchronized can cause transient electrical
  noise on output control lines.
• May have to design special circuits to sense
  noise transient and delay action until it damps
  out.

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Summary

• Synchronization failure
• Runt pulses
• Simple synchronizers
• Synchronizer timing
• Handshake interface techniques