regular readers know from previous posts,
I have a lot of background in assertion-based verification (ABV) and how
assertions are used in simulation and formal analysis. There has been a lot of
growth in the use of both assertions and formal since I was first involved in
these technologies in 1999, but it would be disingenuous of me to suggest that
ABV is as mainstream as I hoped it would be by now. Assertion-based tools are
much easier to use, and are no longer appropriate only for those with a PhD in formal
methods. Every major simulator supports assertions well, as do most of the
hardware acceleration and emulation platforms.
what's the problem? Why isn't everyone using assertions everywhere?
The answer is simply that writing assertions, as valuable as they are in many
ways, takes time and effort. The best assertions usually come from designers
documenting their assumptions about their designs. In some cases, especially
for well-defined buses and interfaces, verification engineers typically write
the assertions. I've heard the same question hundreds of times over the years,
sometimes from verification engineers but especially from designers: "Why can't
you write my assertions for me?"
simple answer, of course, is that assertions are supposed to capture the
assumptions in the designers' heads and no EDA tool (at least so far) can read
their minds. High-quality assertions are expressions of design intent that are
orthogonal to the specific implementation. Thus, any tool that generates
assertions from the design itself is suspect. I maintain that no automatic
method for generating assertions will ever completely replace the need for
engineer-written assertions. There is always information in the heads of the
design and verification engineers that cannot possibly be gleaned from the
said this, various forms of automatic assertions are in common use and do have
considerable value. These generally fall into three categories. The first is assertions
derived directly from the design itself, including overflow and underflow
checks for arithmetic expressions, bounds checks for arrays, and checks for proper synchronization
of asynchronous clocks. Examples in the Cadence product line include the
Automatic Formal Analysis (AFA) features of Incisive Formal Verifier (IFV) and
Incisive Enterprise Verifier (IEV) as well as the clock-domain checks performed
by Conformal Constraint Designer. These tend to be very accurate, requiring
little review by the engineers before being used.
assertions can be generated by making some reasonable assumptions about the
design, perhaps based on naming rules as well as the design structure. These
assertions tend to be suggestions, often requiring a bit of analysis by the
designer to ensure accuracy. For example, IFV can identify most forms of FSMs
and generate checks for dead-end or unreachable states; the designer need only check
that the identified structures were intended as FSMs. Another example is
overflow and underflow checks on stacks and FIFOs. There is no sure way to
identify these structures in a design, but signals containing the string "stack" or "fifo"
might be a good start.
final category is assertions derived from the design and from supplemental
files that express some aspect of design intent. For example, Conformal
Constraint Designer can cross-check the design constraints specified in a
constraint file against the design itself. IFV can formally verify pin
connectivity by generating assertions based on the design and connectivity
intent as captured by the designer in a spreadsheet. For low-power designs, Incisive
Enterprise Simulator (IES) can generate assertions related to power domain
transitions based on the power intent expressed in a Common Power Format (CPF)
file. The resulting assertions can be run in IES, IFV, IEV, or the Palladium XP
verification computing platform.
the most exciting example of assertion generation using supplemental files is
the approach taken by NextOp, which analyzes simulation traces to "learn" about
a design's behavior and generates appropriate assertions (and coverage). We've
discussed NextOp's "assertion synthesis" in the Cadence Community before; Joe Hupcey recently published an update to his video with NextOp CEO Yunshan Zhu; Richard
Goering published an interview as well. I also
mentioned them briefly in my last blog post about a DVCon
panel. For my next post, I'm going to discuss my thoughts on NextOp and
assertion synthesis. See you there!
The truth is out there...sometimes
it's in a blog.