Asynchronous in a synchronous world - Part 2

Performing static timing analysis is the process of verifying that every signal path in a design meets required clock-cycle timing, whether or not all of the signal paths are even possible. Static timing analysis is not used to verify the functionality of the design, only that the design meets timing goals. In theory, timing verification could be accomplished by running exhaustive gatelevel simulations with SDF backannotation of actual timing values after a design is placed and routed. This is often referred to as dynamic timing verification.

Static timing analysis has three principal advantages over dynamic timing verification:
  • static timing analysis tools verify every single path between any two sequential elements,
  • static timing analysis does not require the generation of any test vectors, and
  • static timing analysis tools are orders of magnitude faster than trying to do timing verification running exhaustive gatelevel simulations.
Timing analysis using Synopsys tools on a completely synchronous design is relatively easy to perform using either DesignTime within the Synopsys Design Compiler or Design Analyzer environments, or by using PrimeTime.
  • Timing analysis on modules with two or more asynchronous clocks is error prone, more difficult and can be time consuming.
  • Static timing analysis on signals generated from one clock domain and latched into sequential elements within a second, asynchronous clock domain is inaccurate and for the most part worthless.
  • The timing information for a signal latched by a clock that is asynchronous to the latched signal is inaccurate because the phase relationship between the signal and the asynchronous clock is always changing; therefore, the static timing analysis tool would have to check an infinite number of phase relationships between the signal and asynchronous clock.
The fact is, one must assume that signals that pass from one clock domain to another at some point will violate either setup or hold times on the destination sequential element. There is no good reason to perform timing analysis on signals that are generated in one clock domain and registered in another asynchronous clock domain. It is a given that these signals DO violate setup and hold times on the destination register. This is why synchronizers (see section 3.0) are needed, to alleviate the problems that can occur when a signal is passed from one clock domain to another.

For RTL modules that have two or more asynchronous clocks as inputs, a designer will be required to indicate to the static timing analysis tool which signal paths should be ignored. This is accomplished by "setting false paths" on signals that cross from one clock domain to another. This can be a tedious and error prone job unless the guidelines in the next two sections are followed.

Clock Naming Conventions
Guideline: Use a clock naming convention to identify the clock source of every signal in a design.
Reason: A naming convention helps all team members to identify the clock domain for every signal in a design and also makes grouping of signals for timing analysis easier to do using regular expression "wild-carding" from within a synthesis script. A number of useful clock naming conventions have been used by various design teams. Examples included: uClk for the microprocessor clock, vClk for the video clock and dClk for the display clock. Each signal was synchronized to one of the clock domains in the design and each signal-name had to include a prefix character identifying the clock domain for that signal. Any signal that was clocked by the uClk would have a u-prefix in the signal name, such as uaddr, udata, uwrite, etc. Any signal that was clocked by the vClk would similarly have a v-prefix in the signal name, such as vdata, vhsync, vframe, etc. The same signal naming convention was used for all signals generated by any of the other clocks in the design.

Using this technique, any engineer on the ASIC design team could easily identify the clockdomain source of any signal in the design and either use the signals directly or pass the signals through a synchronizer so that they could be used within a new clock domain. The naming convention alone contributed significantly to the productivity of the design team. How do we know there was a productivity gain? One of the design engineers started his part of the ASIC design using his own naming convention, ignoring the convention in use by the other design team members. After much confusion about the signals entering and leaving his design partition, a team meeting was called and the non-compliant designer was "strongly encouraged" to rename the signals in his part of the design to conform to the team naming convention. After the signal names were changed, it became easier to interface to the partition in question. Fewer questions and less confusions occurred after the change.

Design Partitioning
Guideline: Only allow one clock per module.
Reason: Static timing analysis and creating synthesis scripts is more easily accomplished on single-clock modules or groups of single-clock modules.

Guideline: Create a synchronizer module for each set of signals that pass from just one clock domain into another clock domain.
Reason: It is given that any signal passing from one clock domain to another clock domain is going to have setup and hold time problems. No worst-case (max time) timing analysis is required for synchronizer modules. Only best case (min time) timing analysis is required between first and second stage flip-flops to ensure that all hold times are met. Also, gate-level simulations can more easily be configured to ignore setup and hold time violations on the first stage of each synchronizer.

By partitioning a design to permit only one clock per module, static timing analysis becomes a significantly easier task. The next logical step was to partition the design so that every input module signal was already synchronized to the same clock domain before entering the module. Why is this significant? If all
signals entering and leaving the module are synchronous to the clock used in the module, the design is now completely synchronous! Now the entire module can be static timing analyzed without any "false paths" and Design Compiler can be used to "group" all of the same-clock synchronous modules to perform complete, sequential static timing analysis within each clock domain.

There is one exception to the above recommendation. Multi-clock designs require at least some RTL modules to pass signals from one clock domain to modules that are clocked within a different clock domain.

For example, someone created separate synchronizer modules that permitted signals from one and only one clock domain to be passed into a module
that synchronized the signals into a new clock domain. Using the naming convention described earlier, all processor-clock generated signals (usignals)
would be used as inputs to a module that might be clocked by the video clock. This module was called the "sync_u2v" module and the RTL code did nothing more than take each usignal input and run it through a pair of flip-flops clocked by vClk. Aside from the vClk and reset inputs, every other input signal to the "sync_u2v" module had a "u" prefix and every output signal from that same module had a "v" prefix.

No worst-case timing analysis is required on the "sync" modules because we know that every input signal to these modules will have timing problems; otherwise, we would not have to pass the signals through synchronizers. The only timing analysis that we need to perform within synchronizer modules is min-time (hold time) analysis between the first and second flip-flop stages for each signal. In general, if there are n asynchronous clock domains, the design will require n(n-1) synchronizer modules, two for each pair of clock signals (example: using the uClk and vClk signals: the two synchronizer modules required would be sync_u2v and sync_v2u). Only if there are no signals that pass between two specific clock domains will a pair of synchronizer modules not be required.

After modifying all of the RTL files to create either completely synchronous modules or synchronizer modules, the task of generating synthesis scripts becomes trivial. All of the script files which previously included "set_false_path" commands were either deleted or significantly simplified. All timing problems
were easily identified and fixed (because they were all within single-clock domain groupings) and the final synthesis runs completed two weeks earlier than anticipated, putting the project back on schedule and completely justifying the decision to repartition the design.

Synthesis Scripts & Timing Analysis
Following the guidelines of the previous section, to only permit one clock per module, to require that all signals entering non-synchronizer modules are also in the same clock domain that is used to clock that module and to require that synchronizer modules only permit input signals from one other clock domain, helps to simplify the timing analysis and synthesis scripting tasks associated with a multi-clock design.

Synthesis script commands used to address multiple clock domain issues now become a matter of grouping, identifying false paths and performing min-max timing analysis.

Group together all non-synchronizer modules that are clocked within each clock domain. One group should be formed for each clock domain in the design. These groups will be timing verified as if each were a separate, completely synchronous design.

Identifying False Paths
In general, only the inputs to the synchronizer modules require "set_false_path" commands. If a clock-prefix naming scheme is used, then wild-cards can be used to easily identify all asynchronous inputs. For example, the sync_u2v module should have inputs that all start with the letter "u". The following dc_shell command should be sufficient to eliminate all asynchronous inputs from timing analysis:
set_false_path -from { u* }

Performing Min-Max Timing Analysis
Each grouped set of modules for each clock domain is now a completely synchronous sub-design and tools such as DesignTime or PrimeTime can be used to verify worst case timing (including setup time checks) and best case timing (including hold time checks). The synchronizer blocks are timing verified separately. Worst case timing checks are not required because these modules are just composed of flip-flops to synchronize asynchronous input signals; therefore, there are no long path delays and the outputs are fully registered. After setting false paths on all of the asynchronous inputs, best case (minimum) timing verification is conducted to insure that hold times are met on all signals that are passed from the first to second stage synchronizing flip-flops.

Synchronizing Fast Signals Into Slow Clock Domains
A general problem associated with synchronizers is the problem that a signal from a sending clock domain might change values twice before it can be sampled into a slower clock domain. This problem must be considered any time signals are sent from one clock domain to another. Synchronizing slower control signals into a faster clock domain is generally not a problem since the faster clock signal will sample the slower control signal one or more times. Recognizing that sampling slower signals into faster clock domains causes fewer potential problems than sampling faster signals into slower clock domains, a designer might want to take advantage of this fact and try to steer control signals towards faster clock domains.
This has been explained in Part 1 of this article.

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