Transcript CDC aware power reduction for Soft IPs

CDC aware power reduction
for Soft IPs
Ritesh Agarwal (Freescale™)
Amit Goldie
(Atrenta)
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DAC 2014 - IP Track Submission
Background and Motivation
Need for CDC aware power reduction
 Clock gating changes for power reduction that introduce clock domain crossings can
lead to chip killer CDC bugs and/or design re-spins. It can anyway lead to increased
iterations, last minute ECO fixes and great productivity loss if not detected early in the
design cycle
The one-pass cdc-aware power reduction flow provides power reduction benefits
when developing low-power, high-performance SoCs
 Designers using this flow are able to push the bar for reducing excess power on their IPs
without worrying about their downstream CDC issues which can go a long way in
helping them create power-efficient SoCs
SpyGlass® Power leverages already existing SpyGlass® CDC setup (clock domain
crossing reports) to generate CDC aware (correct-by-construction) clock-gating
changes in the RTL
DAC 2014 - IP Track Submission
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Traditional vs. CDC aware Power Reduction
Back & Forth
iterations to resolve
CDC issues
Power Reduction
Final RTL
Validate CDC
RTL
Feed CDC info
RTL
Review Clock Setup
Validate CDC
CDC report
CDC report
CDC-aware
Power Reduction
CDC Issues
Final RTL
DAC 2014 - IP Track Submission
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Clock Crossing Issue on Implicit Enable
Properly synchronized crossing
CDC unaware transformation leading to
faulty circuit using automated clock-gating
to “save power” – breaks functionality!
Ck1
Ck2
Metastable
Point
ICG
Ck1
Ck2
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Missing Synchronizer Detection in Power Reduction
Some opportunities seem safe…
EN
clk2
EN
New
enable by
backward
trace; no
domain
crossing
…but may introduce a CDC problem
Two flop
synchronizer
EN
clk1
clk2
EN
Metastable
net should
not be used
for any
purpose
 Any enable generated using meta-stable signal from the synchronizer crossing can lead to
clock domain crossing issues in the finally obtained low-power design
 Even though we get a low power optimized design using this flow, it may or may not be
functionally correct if such type of issues are not caught before hand
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Enabling CDC-aware Power Reduction Flow
Feed CDC info
RTL
Review Clock Setup
Validate CDC
CDC report
CDC-aware
Power Reduction
Scenario where meta-stable net is suitable
for a new enable is uncommon, but not
impossible
SpyGlass® CDC detects common
synchronizers with no additional setup
With proper CDC setup, tool can detect any
custom/unusual synchronizer such as
mux-based, enable-based synchronizers etc
SpyGlass® Power can make use of CDC
reports to identify low power, CDC safe
gating opportunities
Final RTL
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Out-of-box CDC aware Power Reduction Results
CDC-aware power reduction results summary:
Details
Total Registers in original RTL
Registers with no enable in original RTL
Registers with new enable in optimized RTL
Registers with no enable in optimized RTL
Registers with existing enable in original RTL
Registers with strengthened enable in optimized RTL
Registers with same enable in optimized RTL
Count
26461
1301
45
1256
25160
4804
20356
Benefits of using CDC aware power reduction solution:
 Traditional treatment results in 3 clock-domain, 1 convergence issue in the modified RTL
 Required additional steps of identifying the incorrect clock gating changes in the RTL which
lead to CDC issues
 One step CDC aware power reduction flow eliminates the back and forth iterations,
producing superior results with productivity gains
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Final Power Reduction Summary
Results
Original Power (Total) = 20.5mW
Modified Power (Total) = 15.9mW
Power Savings = 4.6mW (22.5%)
Results Validation
CDC run on power modified RTL showed zero violations
Gate level synthesis on both original and power-modified RTL was done using
downstream synthesis tool. No timing issues were found
Gate level simulation and power measurement at gate-level yielded around 18%
power reduction measured using 3rd party power estimation tool
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Conclusion
One-pass cdc-aware power reduction flow generates correct-by-construction
clock-gating changes in the RTL to deliver smart and reliable power savings
Need for several back-and-forth design iterations can be completely avoided
using single pass cdc-aware power reduction methodology as described earlier
Leads to greater designer confidence on the clock-gating changes recommended
by the tool
With power efficient IPs, creating low-power, high performance SoCs becomes
increasingly viable
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Future Work
Automatically detect reset crossings from existing SpyGlass® CDC reports
 Exclude meta-stable reset signals from participating in generation of clock-gating enable
logic
Not using asynchronous resets as a part of clock-gating enable equations for
Observability based clock-gating power reduction changes
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Thanks
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