Priority Arbiters

1
Priority Arbiters

• Alex Bystrov
• David Kinniment
• Alex Yakovlev

University of Newcastle upon Tyne, UK
2
Outline of presentation

• Need for different arbitration disciplines
• Types of arbiter
• A static priority arbiter
• A dynamic priority arbiter
• Speed improvements
• Results
• Conclusions

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Arbitration

• Complex systems may require that some requests
  overtake others
• Here three input channels require access to a
  single output port
• Each request may have a different priority
• Priority can be topologically fixed, or
  determined by a function

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Types of arbiter

• Topologically fixed
• priorities determined by structure, e.g.
  daisy-chain

• Static or dynamic priority
• determined by fixed hardware, or priority data
  supplied

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Static or dynamic priority
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Metastability and priority

• Lock the request pattern
• incoming requests cause Lock to go high
• following MUTEX ensures that request wins or
  loses
• Evaluate priorities with a fixed request pattern

?
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Static priority arbiter
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Quasi speed independent

• Assumptions
• s must occur before Lock
• The physics of the MUTEX are such that if r is
  before Lock, s must be asserted
• The three inputs to the Lock bistable are
  implemented as a single complex gate set.
• A faster non speed independent implementation in
  which the gate is separate is possible

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More than one request

• Priority needed if requests are competing
• Shared resource free
• resolution required only if second request
  arrives before the lock signal due to first
  request
• Shared resource busy
• Further requests may accumulate, and one may be
  higher priority

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Two more requests
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Dual-rail priority module

• Dual rail request inputs
• One-hot grant output

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Dynamic priority
P0lt0..3gt
P1lt0..3gt
Priority Module
P7lt0..3gt
Reset completion detector
res_done
done
Lock
s0-7
r0-7
R0-7
G0-7
Lock Register
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Accelerated grant

• Valid and Invalid signals are generated from the
  Lock register
• Tree computation of grant
• Only one channel needs to be valid for the node
  to be valid
• Not all nodes need data evaluation
• Data comparison uses dual rail or one hot
  techniques

Maximum Calculation Cells
Root MCC
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Concurrent PM reset

• Not speed independent.
• Assume that Lock reset is faster than the
  resource.
• Reset of the PM can take place concurrently with
  grant.

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Results

• 0.6m AMS Process DPA
• R0 only to G0 4.94nS
• R1 .. R7 arrive while processing R0, then R0
  reset
• 13.45nS
• Priority module
• 2.74nS (no priority data required)
• 7.63nS (all priority inputs compared)

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Conclusions

• Arbitrary priority discipline
• Resource allocation a function of parameters
  supplied by active requests (or fixed statically)
• Quasi speed independent request locking and
  priority evaluation
• Accelerated grant where possible
• Speed improvements possible with relative timing
  assumptions