report_synchronizer_mtbf - 2020.2 English

report_synchronizer_mtbf [‑file <arg>] [‑append] [‑return_string]
    [‑warn_if_mtbf_below <arg>] [‑quiet] [‑no_header] [‑report_endpoints]
    [‑verbose]

Asynchronous clock domain crossings (CDCs) can fail due to metastability as data is captured asynchronously and may settle to different values on different loads in the circuit. Synchronizer registers are used to improve overall circuit reliability for designs which contain multiple clock domains, in which asynchronous data transfers occur, or in which external asynchronous signals are captured with an internal clock. A synchronizer chain consists of two or more registers connected sequentially with the first stage capturing the data signal from the asynchronous clock domain. The successive register stages provide additional settling time for metastable events and increase MTBF. The synchronizer registers must have ASYNC_REG properties with values of TRUE. Besides reporting MTBF, the ASYNC_REG properties instruct synthesis, simulation and implementation tools to optimize for increased MTBF and improve overall behavior of the synchronizer circuit.

This command returns the MTBF report, or returns an error if it fails. The command issues a warning message when the MTBF cannot be calculated correctly, for example when a CDC is improperly constrained. The following conditions result in an UNDEFINED synchronizer MTBF value:

• One or both clocks of the CDC are unconstrained.
• There is a timing violation involving registers in the synchronizer chain.
• There is a zero toggle rate detected for the CDC data.

In the case of a zero toggle rate, it may be necessary to use the set_switching_activity command to manually override the toggle rate on the CDC net with a realistic value. This involves assigning the Toggle Rate and the Static Probability:

• Toggle Rate: The number of CDC data signal transitions measured in Million Transitions per Second.
• Static Probability: The percentage of time during which the CDC data signal is driven at a high logic level.

Example: to assign a toggle rate of 12.5% with 0.5 static probability on a CDC net named resync[0]:

set_switching_activity -toggle_rate 12.5 -static_probability 0.5 \  
[get_nets resync[0]]

The report contents include the following data for each synchronizer chain in the design:

• MTBF: The Mean Time Between Failures for the CDC synchronizer reported in dynamic time units, from seconds to years. An invalid MTBF value is reported as UNDEFINED.
• Data Toggle Rate: The rate at which the CDC data switches, based on the default switching activity for the design as reported by report_switching_activity. Measured in (Mts) Millions of Transitions per Second. The rate can be overridden using the set_switching_activity command targeting the CDC net object.
• Data Sample Rate: The rate at which the CDC data is sampled, equivalent to the synchronizer chain frequency, measured in MHz.
• Settling Time: The total amount of positive slack in nanoseconds on the timing paths from synchronizer register outputs. Higher Settling Time increases MTBF.
• Sending Domain: The clock domain of the source of the CDC data. A value of UNCONSTRAINED is reported if the source clock is not defined.
• Receiving Domain: The clock domain of the destination of the CDC data. A value of UNCONSTRAINED is reported if the destination clock is not defined.
• Number of Stages: This is the length of the synchronizer chain, which equals the number of registers with ASYNC_REG value of TRUE. The MTBF calculation will determine the likelihood that the output register or registers (should the fanout be greater than 1) will experience a metastable event. For example in a typical synchronizer containing 2 registers with the ASYNC_REG property set, the MTBF calculation indicates the probability that the output register(s) following the last ASYNC_REG register will capture an incorrect value resulting from the metastable event. When a synchronizer is connected to more than 1 output register, the minimum slack from all the paths will be used in the MTBF calculation to ensure that all registers capture the same logic level.
• CDC Net Name: This is the logical net name of the CDC data, the data that is captured asynchronously.

-file <arg> - (Optional) Write the report into the specified file. This command will overwrite any files of the same name without warning.

-append - (Optional) Append the output of the command to the specified file rather than overwriting it.

-return_string - (Optional) Directs the output to a Tcl string rather than to the standard output. The Tcl string can be captured by a variable definition and parsed or otherwise processed.

-quiet - (Optional) Execute the command quietly, returning no messages from the command. The command also returns TCL_OK regardless of any errors encountered during execution.

-verbose - (Optional) Temporarily override any message limits and return all messages from this command.

The following example writes the MTBF report to the specified file:

report_synchronizer_mtbf -file C:/Data/mtbf_report.txt