Emerging%20Technologies%20of%20Computation

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Emerging Technologies of Computation

• Montek Singh
• COMP790-084
• Nov 1, 2011

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Today Basics of Metastability/Synchronization

• Review
• What is metastability
• Can we avoid it?
• How do we deal with it?

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Metastability

• What is metastability?
• indecision

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Any physical system

• in which a discrete decision must be made based
  on analog inputs
• can never be guaranteed to produce a correct
  output in bounded time
• wow!

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Background flip-flops

• Typically sample values at clock edges

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Sampling when data is changing

• When clock edge and data transition are close
• output can take a long time to settle
• closer the call, harder the decision

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Lets go analog

• Analog model

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Resolution time

• Closer the call, the longer the decision time!

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Resolution time (contd.)
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Synchronizers

• Practical circuits that reduce the chance of
  persisting metastability
• cannot eliminate!
• give a full clock cycle for metastability to
  resolve

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Failure probability

• Always a finite probability that metastability
  persists for an arbitrarily long time
• diminishes fast with time
• Mean time to failure

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Synchronizer scenarios

• Several scenarios possible
• depending on data and clock timing

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Full handshake synchronizer

• Sender-receiver pair

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Full handshake synchronizer

• (Note error in figure b)

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FIFO synchronizer

• frequently, buffering needed along with
  synchronization

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Ring synchronization

• well suited for mesochronous clocks
• same frequency, but different phases

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Classic Seitz circuit

• best-known circuit for resolving metastability
• backbone of many modern arbiter circuits
• instead of waiting for a fixed time, includes
  metastability detection
• of course, no guarantees here either!