Using PSL and FoCs for Functional Coverage Verification

Using PSL and FoCs for Functional Coverage
Verification

September 2006

Chuck Alley

Introduction
The Waternoose VSU team inherited a coverage strategy, and bugspray code, from
a previous project. This bugspray code may have provided a useful report card but
was not an adequate for functional coverage:

●The designer developed Bugspray code was almost entirely structural or code
coverage

● Did not focus on areas of maximum complexity or areas of the logic prone to bugs

The designer driven Bugspray coverage points were not documented in any test
●

plan or workbook

The VHDL of the Bugspray was hard to maintain as the design diverged from the
●

antecedent design

Our Method
● We were unhappy with this method so we made the following changes to fully
implement functional coverage

● The verification team took over the coverage plan

– This included maintenance of the old bugspray and development of new
coverage points
● We adopted PSL as the coverage language

● We translated the PSL code into VHDL bugspray code with FoCs

● We documented the coverage points in the test plan, and reviewed them as part
of our verification of the unit

Core Verification Concerns
Several concerns came to my attention from the Austin core verification team, as we
revealed this plan to them.

●Generated VHDL is bloated and inefficient and would seriously degrade the
simulation speed at the core and chip levels

Generated VHDL is hard to read and debug inhibiting anyone else from debugging
●

problems that may come up or be revealed by our code

●Only designers know enough about the logic to write effective monitors, so how are
you going to get your information and how will you maintain the bugspray

Bugspray Organization
The Waternoose project leadership had organized the bugspray into the following
priority scheme:

P1 Required for Unit Gold criteria
P2 Required for Unit RIT criteria
P3 Required for Core Gold criteria
P4 Required for Core RIT criteria
...

Unit P1 and P2 monitors were instantiated in the local shell files and not passed up
to the core team. The P3 and P4 monitors were in unit_top to be passed to the core.

Bugspray File Organization
For instance, the file fw_p3_vmx_load_forwarding_bugspray.vhdl needs to be
included and covered at the core level. However, the file
fw_p2_vector_float_bugspray.vhdl needs only to be run and covered in a unit
simulation environment, and will not be promoted to the core level for simulation.
Other features:

●Proper grouping of coverage events allows verification engineers to turn off
coverage checking when coverage levels are achieved

●All coverage events were included in the unit simulation environment. This was
because we wanted to develop def files capable of achieving all coverage points to
save time and effort at higher levels of simulation.

Identifying Coverage Events
● Meetings are required with the design team to help identify coverage points.

● We went from there to the available project documentation (specs and
workbooks) to find the specific details that we needed to code the monitors.

● We covered IO cases, areas of complexity, areas very likely to cause problems,
complex control scenarios that were handled by multiple units/designers

Writing Monitors
A simple example:

● We are interested in monitoring for several interesting forwarding scenarios. We
determine that the criteria are as follows:

Monitor that data is forwarded to a valid instruction from every possible source.
Make sure that the instruction that uses this data is not canceled in rf1, so that
the data is checked when it writes back to the VRF by IBI.

For this problem the PSL code would look like this:
cover {[*]; i1_vq10_vsu_instr_v; i1_vq11_vmx_b_mux_cntl(0..5) =
%{bypass_source}; true[*2]; i1_rf1_cancel=0};

Writing Monitors
● There are however, many sources from which the forwarding can occur So this
code can be very quickly extended:

vunit vector_bypass_i1Bmux{

%for bypass_source in { i0_by1, i0_by2, i0_vwb, i0_wb2, i0_wb3, i1_by1, i1_by2,
i1_vwb, i1_wb2, i1_wb3, load_ex2, load_ex3, VRF } do

assert "--!!count -t nclk -cn bypass_mux_monitors_p3 -vn
i1_Bmux_%{bypass_source};"


%end

}

Converting to VHDL with FoCs
● Neglecting the for loop, FoCs shows one of the monitors as:

Assertion 1 of vunit vector_bypass_i1bmux:
--!!count -t nclk -cn bypass_mux_monitors_p3 -vn i1_Bmux_1;
Never,
For zero or more cycles (1 )
and then
For one cycle (i1_vq10_vsu_instr_v )
and then
For one cycle (( i1_vq11_vmx_b_mux_cntl(0..5) ) = ( 1 ) )
and then
For 2 cycles (1 )
and then
For one cycle (( i1_rf1_cancel ) = ( 0 ) )

● The FoCs gui provides a very useful debug aid by simply converting your PSL into
english

● We required little if any further debugging for our monitors

Converting to VHDL with FoCs
● FoCs converted the previous example to about 78 lines of VHDL, including the process
statements, variables, and etc.

● FoCs converted the same example into 58 lines of verilog

● Handwritten verilog for this example would start at about 34 lines of verilog depending user
choices

So addressing the issue of code bloat, while this example would indicate that there is not a
significant code bloat issue with FoCs, especially given the work savings for development and
maintenance of the code. However, we have seen several cases in more complex state
machines where the FoCs solution is less optimal.

So yes generated code is somewhat more verbose than handwritten code. How does that
affect simulation speed? It will clearly reduce it. But it can be addressed, and we claim that
the given the current state of the tool, that the value to verification far out weighs the loss of
simulation speed.

Test Planning
● We found that the PSL/Sugar code could be added directly to the test plan

● Given very simple supporting documentation and sensible naming conventions,
the code can truly document itself.

● We found that one did not have to have much PSL expertise to review the code
for completeness and accuracy.

%for bypass_source in { i0_by1, i0_by2, i0_vwb, i0_wb2, i0_wb3, i1_by1, i1_by2,
i1_vwb, i1_wb2, i1_wb3, load_ex2, load_ex3, VRF } do


Results and Conclusion
● Closing the coverage loop improved our verification performance and quality

● Given that our functional monitors were conceived of, and implemented by both the
verification team and the design team, we claim that we had a well thought out and very
evenly applied coverage methodology

● The combination of these strategies allowed us to complete our portion of the design ahead
of schedule.

– We then reduced our simulation throughput, allowing units that still needed
simulation cycles to use “our” cycles
● We then spent the extra time reviewing our coverage plan, RTX and design.

We had no post RIT bugs in any of the logic that was covered by this plan.

We did however, have post RIT bugs in sections of the design that relied on the more traditional
system of custom environment hooks and random scenario generation to find bugs.

Using PSL and FoCs for Functional Coverage Verification

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Using PSL and FoCs for Functional Coverage Verification