Managing Metastability with Quartus II Software ( 英語版 ・PDF)

Managing Metastability with Quartus II Software ( 英語版 ・PDF)
14. Managing Metastability with the
Quartus II Software
June 2012
QII51018-12.0.0
QII51018-12.0.0
This chapter describes the industry-leading analysis, reporting, and optimization
features that can help you manage metastability in Altera® devices. You can use the
Quartus® II software to analyze the average mean time between failures (MTBF) due
to metastability caused by synchronization of asynchronous signals, and optimize the
design to improve the metastability MTBF. This chapter explains how to take
advantage of these features in the Quartus II software, and provides guidelines to
help you reduce the chance of metastability effects caused by signal synchronization.
Introduction
All registers in digital devices, such as FPGAs, have defined signal-timing
requirements that allow each register to correctly capture data at its input ports and
produce an output signal. To ensure reliable operation, the input to a register must be
stable for a minimum amount of time before the clock edge (register setup time or tSU)
and a minimum amount of time after the clock edge (register hold time or tH). The
register output is available after a specified clock-to-output delay (tCO).
If the data violates the setup or hold time requirements, the output of the register
might go into a metastable state. In a metastable state, the voltage at the register
output hovers at a value between the high and low states, which means the output
transition to a defined high or low state is delayed beyond the specified tCO. Different
destination registers might capture different values for the metastable signal, which
can cause the system to fail.
In synchronous systems, the input signals must always meet the register timing
requirements, so that metastability does not occur. Metastability problems commonly
occur when a signal is transferred between circuitry in unrelated or asynchronous
clock domains, because the signal can arrive at any time relative to the destination
clock.
The MTBF due to metastability is an estimate of the average time between instances
when metastability could cause a design failure. A high MTBF (such as hundreds or
thousands of years between metastability failures) indicates a more robust design.
You should determine an acceptable target MTBF in the context of your entire system
and taking in account that MTBF calculations are statistical estimates.
The metastability MTBF for a specific signal transfer, or all the transfers in a design,
can be calculated using information about the design and the device characteristics.
Improving the metastability MTBF for your design reduces the chance that signal
transfers could cause metastability problems in your device.
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Chapter 14: Managing Metastability with the Quartus II Software
Metastability Analysis in the Quartus II Software
f For more information about metastability due to signal synchronization, its effects in
FPGAs, and how MTBF is calculated, refer to the Understanding Metastability in FPGAs
white paper on the Altera website. Your overall device MTBF is also affected by other
FPGA failure mechanisms that you cannot control with your design. For information
about Altera device reliability, refer to the Reliability Report on the Altera website.
The Quartus II software provides analysis, optimization, and reporting features to
help manage metastability in Altera designs. These metastability features are
supported only for designs constrained with the Quartus II Timing Analyzer. Both
typical and worst-case MBTF values are generated for select device families.
h For information about device and version support for the metastability features in the
Quartus II software, refer to the Quartus II Help.
This chapter contains the following topics:
■
“Metastability Analysis in the Quartus II Software”
■
“Metastability and MTBF Reporting” on page 14–5
■
“MTBF Optimization” on page 14–8
■
“Reducing Metastability Effects” on page 14–9
■
“Scripting Support” on page 14–11
Metastability Analysis in the Quartus II Software
When a signal transfers between circuitry in unrelated or asynchronous clock
domains, the first register in the new clock domain acts as a synchronization register.
To minimize the failures due to metastability in asynchronous signal transfers, circuit
designers typically use a sequence of registers (a synchronization register chain or
synchronizer) in the destination clock domain to resynchronize the signal to the new
clock domain and allow additional time for a potentially metastable signal to resolve
to a known value. Designers commonly use two registers to synchronize a new signal,
but a standard of three registers provides better metastability protection.
The timing analyzer can analyze and report the MTBF for each identified
synchronizer that meets its timing requirements, and can generate an estimate of the
overall design MTBF. The software uses this information to optimize the design
MTBF, and you can use this information to determine whether your design requires
longer synchronizer chains.
This section contains the following subsections:
■
“Synchronization Register Chains”
■
“Identifying Synchronizers for Metastability Analysis” on page 14–4
■
“How Timing Constraints Affect Synchronizer Identification and Metastability
Analysis” on page 14–4
For information about the reports generated by the timing analyzer, refer to
“Metastability and MTBF Reporting” on page 14–5. For more information about
optimizing the MTBF, refer to “MTBF Optimization” on page 14–8.
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Metastability Analysis in the Quartus II Software
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Synchronization Register Chains
A synchronization register chain, or synchronizer, is defined as a sequence of registers
that meets the following requirements:
■
The registers in the chain are all clocked by the same clock or phase-related clocks.
■
The first register in the chain is driven asynchronously or from an unrelated clock
domain.
■
Each register fans out to only one register, except the last register in the chain.
The length of the synchronization register chain is the number of registers in the
synchronizing clock domain that meet the above requirements. Figure 14–1 shows a
sample two-register synchronization chain.
Figure 14–1. Sample Synchronization Register Chain
Synchronization Chain
Clock 1 Domain
Data
Clock 2 Domain
D
Q
Clock 1
D
Q
D
Q
Output
Registers
Clock 2
The path between synchronization registers can contain combinational logic as long
as all registers of the synchronization register chain are in the same clock domain.
Figure 14–2 shows an example of a synchronization register chain that includes logic
between the registers.
Figure 14–2.
Sample Synchronization Register Chain Containing Logic
Synchronization Chain
Clock 1 Domain
Data
Clock 1
D
Clock 2 Domain
Q
D
Q
Clock 2
Data
D
Q
Output
Registers
Clock 2
D
Q
Clock 2
The Quartus II software uses the design timing constraints to determine which
connections are asynchronous signal transfers, as described in “How Timing
Constraints Affect Synchronizer Identification and Metastability Analysis” on
page 14–4.
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Chapter 14: Managing Metastability with the Quartus II Software
Metastability Analysis in the Quartus II Software
The timing slack available in the register-to-register paths of the synchronizer allows a
metastable signal to settle, and is referred to as the available settling time. The
available settling time in the MTBF calculation for a synchronizer is the sum of the
output timing slacks for each register in the chain. Adding available settling time with
additional synchronization registers improves the metastability MTBF.
Identifying Synchronizers for Metastability Analysis
The first step in enabling metastability MTBF analysis and optimization in the
Quartus II software is to identify which registers are part of a synchronization register
chain. You can apply synchronizer identification settings globally to automatically list
possible synchronizers with the Synchronizer identification option on the Timing
Analyzer page in the Settings dialog box.
Synchronization chains are already identified within most Altera intellectual property
(IP) cores.
h For more information about how to enable metastability MTBF analysis and
optimization in the Quartus II software, and more detailed descriptions of the
synchronizer identification settings, refer to Identifying Synchronizers for Metastability
Analysis in Quartus II Help.
How Timing Constraints Affect Synchronizer Identification and
Metastability Analysis
The timing analyzer can analyze metastability MTBF only if a synchronization chain
meets its timing requirements. The metastability failure rate depends on the timing
slack available in the synchronizer’s register-to-register connections, because that
slack is the available settling time for a potential metastable signal. Therefore, you
must ensure that your design is correctly constrained with the real application
frequency requirements to get an accurate MTBF report.
In addition, the Auto and Forced If Asynchronous synchronizer identification
options use timing constraints to automatically detect the synchronizer chains in the
design. These options check for signal transfers between circuitry in unrelated or
asynchronous clock domains, so clock domains must be related correctly with timing
constraints.
The timing analyzer views input ports as asynchronous signals unless they are
associated correctly with a clock domain. If an input port fans out to registers that are
not acting as synchronization registers, apply a set_input_delay constraint to the
input port; otherwise, the input register might be reported as a synchronization
register. Constraining a synchronous input port with a set_max_delay constraint for a
setup (tSU) requirement does not prevent synchronizer identification because the
constraint does not associate the input port with a clock domain.
Instead, use the following command to specify an input setup requirement associated
with a clock:
set_input_delay -max -clock <clock name> <latch – launch – tsu requirement> <input
port name>
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Metastability and MTBF Reporting
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Registers that are at the end of false paths are also considered synchronization
registers because false paths are not timing-analyzed. Because there are no timing
requirements for these paths, the signal may change at any point, which may violate
the tSU and tH of the register. Therefore, these registers are identified as
synchronization registers. If these registers are not used for synchronization, you can
turn off synchronizer identification and analysis. To do so, set Synchronizer
Identification to Off for the first synchronization register in these register chains.
Metastability and MTBF Reporting
The Quartus II software reports the metastability analysis results in the Compilation
Report and Timing Analyzer reports as described in “Metastability Reports”. The
MTBF calculation uses timing and structural information about the design, silicon
characteristics, and operating conditions, along with the data toggle rate described in
“Synchronizer Data Toggle Rate in MTBF Calculation” on page 14–7.
If you change the Synchronizer Identification settings, you can generate new
metastability reports by rerunning the timing analyzer. However, you should rerun
the Fitter first so that the registers identified with the new setting can be optimized for
metastability MTBF. For information about metastability optimization, refer to “MTBF
Optimization” on page 14–8.
For more information about how metastability MTBF is calculated, refer to the
Understanding Metastability in FPGAs white paper.
Metastability Reports
Metastability reports provide summaries of the metastability analysis results. In
addition to the MTBF Summary and Synchronizer Summary reports, the Timing
Analyzer tool reports additional statistics in a report for each synchronizer chain.
h For more information about how to access metastability reports in the Quartus II
software, refer to Viewing Metastability Reports in Quartus II Help.
1
If the design uses only the Auto Synchronizer Identification setting, the reports list
likely synchronizers but do not report MTBF. To obtain an MTBF for each register
chain, force identification of synchronization registers as described in “Identifying
Synchronizers for Metastability Analysis” on page 14–4.
1
If the synchronizer chain does not meet its timing requirements, the reports list
identified synchronizers but do not report MTBF. To obtain MTBF calculations, ensure
that the design is properly constrained and that the synchronizer meets its timing
requirements, as described in “How Timing Constraints Affect Synchronizer
Identification and Metastability Analysis” on page 14–4.
MTBF Summary Report
The MTBF Summary reports an estimate of the overall robustness of cross-clock
domain and asynchronous transfers in the design. This estimate uses the MTBF
results of all synchronization chains in the design to calculate an MTBF for the entire
design.
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Chapter 14: Managing Metastability with the Quartus II Software
Metastability and MTBF Reporting
The MTBF Summary Report reports the Typical MTBF of Design and the Worst-Case
MTBF of Design for supported fully-characterized devices. The typical MTBF result
assumes typical conditions, defined as nominal silicon characteristics for the selected
device speed grade, as well as nominal operating conditions. The worst case MTBF
result uses the worst case silicon characteristics for the selected device speed grade.
When you analyze multiple timing corners in the timing analyzer, the MTBF
calculation may vary because of changes in the operating conditions, and the timing
slack or available metastability settling time. Altera recommends running
multi-corner timing analysis to ensure that you analyze the worst MTBF results,
because the worst timing corner for MTBF does not necessarily match the worst
corner for timing performance.
h For more information about turning on multicorner timing analysis in the Quartus II
software, refer to the Timing Analyzer page in Quartus II Help.
The MTBF Summary report also lists the Number of Synchronizer Chains Found
and the length of the Shortest Synchronizer Chain, which can help you identify
whether the report is based on accurate information. If the number of synchronizer
chains found is different from what you expect, or if the length of the shortest
synchronizer chain is less than you expect, you might have to add or change
Synchronizer Identification settings for the design. The report also provides the
Worst Case Available Settling Time, defined as the available settling time for the
synchronizer with the worst MTBF.
You can use the reported Fraction of Chains for which MTBFs Could Not be
Calculated to determine whether a high proportion of chains are missing in the
metastability analysis. A fraction of 1, for example, means that MTBF could not be
calculated for any chains in the design. MTBF is not calculated if you have not
identified the chain with the appropriate Synchronizer identification option, or if
paths are not timing-analyzed and therefore have no valid slack for metastability
analysis. You might have to correct your timing constraints to enable complete
analysis of the applicable register chains.
Finally, the MTBF Summary report specifies how an increase of 100ps in available
settling time increases the MTBF values. If your MTBF is not satisfactory, this metric
can help you determine how much extra slack would be required in your
synchronizer chain to allow you to reach the desired design MTBF.
Synchronizer Summary Report
The Synchronizer Summary lists the synchronization register chains detected in the
design depending on the Synchronizer Identification setting. The Source Node is the
register or input port that is the source of the asynchronous transfer. The
Synchronization Node is the first register of the synchronization chain. The Source
Clock is the clock domain of the source node, and the Synchronization Clock is the
clock domain of the synchronizer chain.
This summary reports the calculated Worst-Case MTBF, if available, and the Typical
MTBF, for each appropriately identified synchronization register chain that meets its
timing requirement. To see more detail about each synchronizer, refer to the statistics
report described in the following section.
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Metastability and MTBF Reporting
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Synchronizer Chain Statistics Report in the Timing Analyzer
The timing analyzer provides an additional report for each synchronizer chain. The
Chain Summary tab matches the Synchronizer Summary information described in
the previous section, while the Statistics tab adds more details, including whether the
Method of Synchronizer Identification was User Specified (with the Forced if
Asynchronous or Forced settings for the Synchronizer Identification setting), or
Automatic (with the Auto setting). The Number of Synchronization Registers in
Chain report provides information about the parameters that affect the MTBF
calculation, including the Available Settling Time for the chain and the Data Toggle
Rate Used in MTBF Calculation.
1
For information about the toggle rate, see “Synchronizer Data Toggle Rate in MTBF
Calculation” on page 14–7.
The following information is also included to help you locate the chain is in your
design:
■
Source Clock and Asynchronous Source node of the signal.
■
Synchronization Clock in the destination clock domain.
■
Node names of the Synchronization Registers in the chain.
Synchronizer Data Toggle Rate in MTBF Calculation
The MTBF calculations assume the data being synchronized is switching at a toggle
rate of 12.5% of the source clock frequency. That is, the arriving data is assumed to
switch once every eight source clock cycles. If multiple clocks apply, the highest
frequency is used. If no source clocks can be determined, the data rate is taken as
12.5% of the synchronization clock frequency.
If you know an approximate rate at which the data changes, specify it with the
Synchronizer Toggle Rate assignment in the Assignment Editor. You can also apply
this assignment to an entity or the entire design. Set the data toggle rate, in number of
transitions per second, on the first register of a synchronization chain. The timing
analyzer takes the specified rate into account when computing the MTBF of that
particular register chain. If a data signal never toggles and does not affect the
reliability of the design, you can set the Synchronizer Toggle Rate to 0 for the
synchronization chain so the MTBF is not reported. To apply the assignment with Tcl,
use the following command:
set_instance_assignment -name SYNCHRONIZER_TOGGLE_RATE <toggle rate in
transitions/second> -to <register name>
1
There are two other assignments associated with toggle rates, which are not used for
metastability MTBF calculations. The I/O Maximum Toggle Rate is only used for
pins, and specifies the worst-case toggle rates used for signal integrity purposes. The
Power Toggle Rate assignment is used to specify the expected time-averaged toggle
rate, and is used by the PowerPlay Power Analyzer to estimate time-averaged power
consumption.
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Chapter 14: Managing Metastability with the Quartus II Software
MTBF Optimization
MTBF Optimization
In addition to reporting synchronization register chains and MTBF values found in
the design, the Quartus II software can also protect these registers from optimizations
that might negatively impact MTBF and can optimize the register placement and
routing if the MTBF is too low. Synchronization register chains must first be explicitly
identified as synchronizers, as described in “Identifying Synchronizers for
Metastability Analysis” on page 14–4. Altera recommends that you set Synchronizer
Identification to Forced If Asynchronous for all registers that are part of a
synchronizer chain.
Optimization algorithms, such as register duplication and logic retiming in physical
synthesis, are not performed on identified synchronization registers. The Fitter
protects the number of synchronization registers specified by the Synchronizer
Register Chain Length option which is described in the next section.
In addition, the Fitter optimizes identified synchronizers for improved MTBF by
placing and routing the registers to increase their output setup slack values. Adding
slack in the synchronizer chain increases the available settling time for a potentially
metastable signal, which improves the chance that the signal resolves to a known
value, and exponentially increases the design MTBF. The Fitter optimizes the number
of synchronization registers specified by the Synchronizer Register Chain Length
option.
Metastability optimization is on by default. To view or change the option, on the
Assignments menu, click Settings. Under Fitter Settings, click More Settings. From
the More Settings dialog box, you can turn on or off the Optimize Design for
Metastability option. To turn the optimization on or off with Tcl, use the following
command:
set_global_assignment -name OPTIMIZE_FOR_METASTABILITY <ON|OFF>
Synchronization Register Chain Length
The Synchronization Register Chain Length option specifies how many registers
should be protected from optimizations that might reduce MTBF for each register
chain, and controls how many registers should be optimized to increase MTBF with
the Optimize Design for Metastability option. For example, if the Synchronization
Register Chain Length option is set to 2, optimizations such as register duplication or
logic retiming are prevented from being performed on the first two registers in all
identified synchronization chains. The first two registers are also optimized to
improve MTBF when the Optimize Design for Metastability option is turned on.
The default setting for the Synchronization Register Chain Length option is 2. The
first register of a synchronization chain is always protected from operations that
might reduce MTBF, but you should set the protection length to protect more of the
synchronizer chain. Altera recommends that you set this option to the maximum
length of synchronization chains you have in your design so that all synchronization
registers are preserved and optimized for MTBF.
To change the global Synchronization Register Chain Length option, on the
Assignments menu, click Settings. Under Analysis & Synthesis Settings, click More
Settings. From the More Settings dialog box, you can set the Synchronization
Register Chain Length.
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Reducing Metastability Effects
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You can also set the Synchronization Register Chain Length on a node or an entity in
the Assignment Editor. You can set this value on the first register in a synchronization
chain to specify how many registers to protect and optimize in this chain. This
individual setting is useful if you want to protect and optimize extra registers that you
have created in a specific synchronization chain that has low MTBF, or optimize less
registers for MTBF in a specific chain where the maximum frequency or timing
performance is not being met. To make the global setting with Tcl, use the following
command:
set_global_assignment -name SYNCHRONIZATION_REGISTER_CHAIN_LENGTH
<number of registers>
To apply the assignment to a design instance or the first register in a specific chain
with Tcl, use the following command:
set_instance_assignment -name SYNCHRONIZATION_REGISTER_CHAIN_LENGTH
<number of registers> -to <register or instance name>
Reducing Metastability Effects
You can check your design's metastability MTBF in the Metastability Summary report
described in “Metastability Reports” on page 14–5, and determine an acceptable
target MTBF given the context of your entire system and the fact that MTBF
calculations are statistical estimates. A high metastability MTBF (such as hundreds or
thousands of years between metastability failures) indicates a more robust design.
This section provides guidelines to ensure complete and accurate metastability
analysis, and some suggestions to follow if the Quartus II metastability reports
calculate an unacceptable MTBF value. The Timing Optimization Advisor (available
from the Tools menu) gives similar suggestions in the Metastability Optimization
section.
Apply Complete System-Centric Timing Constraints for the Timing Analyzer
To enable the Quartus II metastability features, make sure that the timing analyzer is
used for timing analysis.
Ensure that the design is fully timing constrained and that it meets its timing
requirements. If the synchronization chain does not meet its timing requirements,
MTBF cannot be calculated. If the clock domain constraints are set up incorrectly, the
signal transfers between circuitry in unrelated or asynchronous clock domains might
be identified incorrectly.
Use industry-standard system-centric I/O timing constraints instead of using
FPGA-centric timing constraints. As described in “How Timing Constraints Affect
Synchronizer Identification and Metastability Analysis” on page 14–4, you should use
set_input_delay constraints in place of set_max_delay constraints to associate each
input port with a clock domain to help eliminate false positives during
synchronization register identification.
Force the Identification of Synchronization Registers
Use the guidelines in “Identifying Synchronizers for Metastability Analysis” on
page 14–4 to ensure the software reports and optimizes the appropriate register
chains.
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Reducing Metastability Effects
In summary, identify synchronization registers with the Synchronizer Identification
set to Forced If Asynchronous in the Assignment Editor. If there are any registers that
the software detects as synchronous but you want to be analyzed for metastability,
apply the Forced setting to the first synchronizing register. Set Synchronizer
Identification to Off for registers that are not synchronizers for asynchronous signals
or unrelated clock domains.
To help you find the synchronizers in your design, you can set the global
Synchronizer Identification setting on the Timing Analyzer page of the Settings
dialog box to Auto to generate a list of all the possible synchronization chains in your
design.
Set the Synchronizer Data Toggle Rate
The MTBF calculations assume the data being synchronized is switching at a toggle
rate of 12.5% of the source clock frequency. To obtain a more accurate MTBF for a
specific chain or all chains in your design, set the Synchronizer Toggle Rate as
described in “Synchronizer Data Toggle Rate in MTBF Calculation” on page 14–7.
Optimize Metastability During Fitting
Ensure that the Optimize Design for Metastability setting described in “MTBF
Optimization” on page 14–8 is turned on.
Increase the Length of Synchronizers to Protect and Optimize
Increase the Synchronizer Chain Length parameter to the maximum length of
synchronization chains in your design, as described in “Synchronization Register
Chain Length” on page 14–8. If you have synchronization chains longer than 2
identified in your design, you can protect the entire synchronization chain from
operations that might reduce MTBF and allow metastability optimizations to improve
the MTBF.
Set Fitter Effort to Standard Fit instead of Auto Fit
If your design MTBF is too low after following the previous guidelines in this section,
you can try increasing the Fitter effort to perform more metastability optimization.
The default Auto Fit setting reduces the Fitter’s effort after meeting the design’s
timing and routing requirements to reduce compilation time. This effort reduction can
result in less metastability optimization if the timing requirements are easy to meet. If
Auto Fit reduces the Fitter’s effort during your design compilation, setting the Fitter
effort to Standard Fit might improve the design’s MTBF results. In the Settings dialog
box, on the Fitter Settings page, set Fitter effort to Standard Fit.
Increase the Number of Stages Used in Synchronizers, If Possible
Designers commonly use two registers in a synchronization chain to minimize the
occurrence of metastable events, and a standard of three registers provides better
metastability protection. However, synchronization chains with two or even three
registers may not be enough to produce a high enough MTBF when the design runs at
high clock and data frequencies.
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Scripting Support
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If a synchronization chain is reported to have a low MTBF, consider adding an
additional register stage to your synchronization chain. This additional stage
increases the settling time of the synchronization chain, allowing more opportunity
for the signal to resolve to a known state during a metastable event. Additional
settling time increases the MTBF of the chain and improves the robustness of your
design. However, adding a synchronization stage introduces an additional stage of
latency on the signal.
If you use the Altera FIFO megafunction with separate read and write clocks to cross
clock domains, increase the metastability protection (and latency) for better MTBF. In
the MegaWizard™ Plug-In Manager for the DCFIFO function, choose the Best
metastability protection, best fmax, unsynchronized clocks option to add three or
more synchronization stages. You can increase the number of stages to more than
three using the How many sync stages? setting.
Select a Faster Speed Grade Device, if Possible
The design MTBF depends on process parameters of the device used. Faster devices
are less susceptible to metastability issues. If the design MTBF falls significantly
below the target MTBF, switching to a faster speed grade can improve the MTBF
substantially.
Scripting Support
You can run procedures and make settings described in this chapter in a Tcl script.
You can also run some procedures at a command prompt. For detailed information
about scripting command options, refer to the Quartus II Command-Line and Tcl API
Help browser. To run the Help browser, type the following command at the command
prompt:
quartus_sh --qhelp r
f For more information about Tcl scripting, refer to the Tcl Scripting chapter in volume 2
of the Quartus II Handbook. For more information about settings and constraints in the
Quartus II software, refer to the Quartus II Settings File Reference Manual. For more
information about command-line scripting, refer to the Command-Line Scripting
chapter in volume 2 of the Quartus II Handbook and About Quartus II Scripting in
Quartus II Help.
Identifying Synchronizers for Metastability Analysis
To apply the global Synchronizer Identification assignment described in “Identifying
Synchronizers for Metastability Analysis” on page 14–4, use the following command:
set_global_assignment -name SYNCHRONIZER_IDENTIFICATION
<OFF|AUTO|"FORCED IF ASYNCHRONOUS">
To apply the Synchronizer Identification assignment to a specific register or instance,
use the following command:
set_instance_assignment -name SYNCHRONIZER_IDENTIFICATION
<AUTO|"FORCED IF ASYNCHRONOUS"|FORCED|OFF> -to <register or instance
name>
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Scripting Support
Synchronizer Data Toggle Rate in MTBF Calculation
To specify a toggle rate for MTBF calculations as described on page “Synchronizer
Data Toggle Rate in MTBF Calculation” on page 14–7, use the following command:
set_instance_assignment -name SYNCHRONIZER_TOGGLE_RATE <toggle rate in
transitions/second> -to <register name>
report_metastability and Tcl Command
If you use a command-line or scripting flow, you can generate the metastability
analysis reports described in “Metastability Reports” on page 14–5 outside of the
Quartus II and user interfaces. Table 14–1 describes the options for the
report_metastability and Tcl command.
Table 14–1.
report_metastabilty Command Options
Option
Description
-append
If output is sent to a file, this option appends the result to that file.
Otherwise, the file is overwritten.
-file <name>
Sends the results to an ASCII or HTML file. The extension specified
in the file name determines the file type—either *.txt or *.html.
-panel_name <name>
Sends the results to the panel and specifies the name of the new
panel.
-stdout
Indicates the report be sent to the standard output, via messages.
This option is required only if you have selected another output
format, such as a file, and would also like to receive messages.
MTBF Optimization
To ensure that metastability optimization described on page “MTBF Optimization” on
page 14–8 is turned on (or to turn it off), use the following command:
set_global_assignment -name OPTIMIZE_FOR_METASTABILITY <ON|OFF>
Synchronization Register Chain Length
To globally set the number of registers in a synchronization chain to be protected and
optimized as described on page “Synchronization Register Chain Length” on
page 14–8, use the following command:
set_global_assignment -name SYNCHRONIZATION_REGISTER_CHAIN_LENGTH
<number of registers>
To apply the assignment to a design instance or the first register in a specific chain,
use the following command:
set_instance_assignment -name SYNCHRONIZATION_REGISTER_CHAIN_LENGTH
<number of registers> -to <register or instance name>
Quartus II Handbook Version 13.1
Volume 1: Design and Synthesis
June 2012
Altera Corporation
Chapter 14: Managing Metastability with the Quartus II Software
Conclusion
14–13
Conclusion
Altera’s Quartus II software provides industry-leading analysis and optimization
features to help you manage metastability in your FPGA designs. Set up your
Quartus II project with the appropriate constraints and settings to enable the software
to analyze, report, and optimize the design MTBF. Take advantage of these features in
the Quartus II software and follow the guidelines in this chapter to make your design
more robust with respect to metastability.
Document Revision History
Table 14–2 shows the revision history for this document.
Table 14–2. Document Revision History
Date
Version
Changes
June 2012
12.0.0
Removed survey link.
November 2011
10.0.2
Template update.
December 2010
10.0.1
Changed to new document template.
July 2010
10.0.0
November 2009
9.1.0
March 2009
9.0.0
Technical edit.
Clarified description of synchronizer identification settings.
Minor changes to text and figures throughout document.
Initial release.
f For previous versions of the Quartus II Handbook, refer to the Quartus II Handbook
Archive.
June 2012 Altera Corporation
Quartus II Handbook Version 13.1
Volume 1: Design and Synthesis
14–14
Quartus II Handbook Version 13.1
Volume 1: Design and Synthesis
Chapter 14: Managing Metastability with the Quartus II Software
Document Revision History
June 2012
Altera Corporation
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