Clock gating
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Transcript of Clock gating
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Power Consumption Primitives
Characteristics of Power Consumption in
Microprocessors
Clock Gating and Power Reduction
Similar Approaches
Power Reduction Example
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P (total) = P (static) + P (dynamic)
Static Power
Currently Negligible But Considerable in Future
Dynamic Power
P (dynamic) = CL Vdd2.A.F
Major Part of Total Power (e.g. 95%)
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Clock Circuitry Power Consumption
15 to 45% of TotalP (clock circuitry) ~ Frequency
Activity of Functional Units
A (units) < 50% in Execution Time
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Clock tree consume more than 50 % of dynamic power. The components of this power are:
1)Power consumed by combinatorial logic whose values are changing on each clock edge
2) Power consumed by flip-flops and
3) The power consumed by the clock buffer tree in the design.
Clock Gating and Power Reduction
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There are two types of clock gating styles available. They are:
1)Latch-based clock gating
2) Latch-free clock gating.
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Latch free clock gating
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Latch based clock gating
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Main Idea Clock Circuitry Partitioning Shutting down Unused Partitions
Implementation Creating Local Clocks Buffers or Flip-Flops with enable signal
Net Effects Reduction of Unnecessary Switching Switched Capacitance Reduction of Clock Circuitry Power Consumption Reduction
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How to determine unused modules?
Dynamically in Decode StageStatically with Compiler Assistance
Disadvantages
Additional CircuitryMore Complicated Timing Analysis, Design, Test and
VerificationPossible High L x di/dt Noise
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• Data Gating
– Effective in Wide Modules Like ALUs
– Complicated Design
– Possible Increase of Critical Path Delay
• Powering Down Unused Modules
– Possible Long Wake-up Times
– Need of Compiler and/or OS Support
• Using Asynchronous Systems
– Unnecessary Activity Elimination
– Harder Design
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Components - flip-flops, latches, ALU, adder, and shifter
Functions - decode, execute, and load store unit
Power Reduction Example
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• Clock Gating as an Architectural Technique
• Turning Unused Parts of Circuit Off
• High P(dynamic)/P (total) Ratio
• Decrease of Activity to Reduce Power
• Considerable Power Reduction (up to 25%)
• Highly Used Technique
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Summary