A brief tutorial on how system-on-chip (SoC) developers can accurately predict timing issues in the interconnect design and avoid project delays

KURT SHULER, Vice President of Marketing

Back-end timing closure for on-chip SoC interconnects
is now a significant obstacle for engineers who are migrating to smaller
semiconductor geometries and FinFET transistors. So the industry needs to summon
its innovative resources to address an issue that is now adding weeks, and even
months, of delays to project schedules.

Perhaps the best path forward is to assign a higher
priority to the interconnect earlier in the SoC planning stages. So the connections
between IP blocks get designed for optimal functionality, location,
performance, latency, power, and area before the front-end design migrates to
the physical back end of the process.

In fact, design teams that commit to interconnect
topology optimization during the front-end design have slashed more than 30
days from their schedules. This new path stands in contrast to the traditional
treatment in which engineers throw interconnect RTL “over the wall” late in the
process and let back-end synthesis-place-and-route (SP&R) engineers try to
resolve the issues.

Timing failures
Problems with interconnect timing closure first
started popping up at the 28-nm process node, but now, at the 16-nm FinFET
node, issues have become more common and acute. Migration to 10-nm and 7-nm
processes will increase the number and significance of issues.

As transistors get smaller with each successive
process node, chips are getting denser. Processor speeds have increased, but
the interconnecting wires that link all of the IP blocks together have grown
more congested and longer in length. Therefore, while the number of IP blocks
in each design has ballooned, wire RC constraints have created more


Fig. 1: Obtaining
floorplan information earlier in the SoC design process helps discover
interconnect timing closure issues sooner.

The interconnect has the longest wires of any SoC IP
because it stretches across the chip like a spider web. It is also the last IP
to be configured because it depends on all of the other on-chip IPs with which
it connects.

Extended wire lengths combined with RC delays have
made the interconnect one of the principal sources of timing closure issues,
and even worse, these issues are usually not discovered until late in the
design schedule. Waiting until the back-end stage to address these matters
almost guarantees a schedule slip.

Planning for
However, if designers could predict where timing
issues might occur in the interconnect during the front end of the process, it’s
possible that they could use that knowledge to create a design that avoids many
problems that are popping up in greater numbers today.

Using this information up front will create better
timing paths in the back end. With fewer issues to resolve, a more predictable
SoC delivery schedule can be accomplished alongside the opportunity for
additional benefits like higher overall performance, reduced die size, and
lower power.

Interconnect issues need to be tackled when all IP
blocks are placed from a functional and logical perspective rather than waiting
for a final physical floorplan. This step traditionally occurs when the RTL
design process is completed and locked in.

Because topology placement has traditionally come
after this process, the interconnect IP gets constricted into suboptimal
connection paths. However, if designers feed the process with greater
information and planning during the front end, it enables better decision-making
for the place-and-route tools implementing interconnect.

intervention points
There are three steps in the design process in which
timing issues can be initially detected and addressed: 1) in the SoC
architecture and topology stage; 2) in the functional or RTL design stage; 3)
and during the layout stage. After each stage, engineers simulate their designs
and uncover and fix interconnect timing issues.

In today’s most sophisticated designs, timing issues
that are detected in the SoC architecture take the least amount of time to
resolve. By contrast, timing issues found in the functional or RTL stage can
take days to resolve, while problems found in the layout stage take several
weeks to rectify.


Fig. 2: It is
better to find and fix timing closure issues at the earliest stages of SoC

Because schedule delays can weaken a project’s market
viability, perhaps the best opportunity to address failed timing paths before
they happen is at the RTL stage.

Finding timing issues in the netlist or afterward
contributes to schedule delays. As the SoC design shifts to the physical design
team, timing issues trigger problems that force the project back to front-end
engineers who need to revise their plans to fix the structural problems.

Engineering change orders (ECOs) present an optional
resolution path to edit the netlist created by front-end teams. However,
innovative designers can emphasize forethought and advanced planning to devise
an intelligent topology that will avert back-end errors that delay project


Fig. 3: This
methodology works for the entire SoC and all of its interconnects.

The case for
earlier interconnect timing closure
Designers that consider interconnect IP during the RTL
and functional design phase will be in a better position to create low-latency
connections and determine the ideal placement of interconnect components like
network interface units, switches, gates, and other logic. Doing so creates the
ideal conditions that promote the best performance, reduced logic footprint,
fewer pipeline insertions, and a reduction in failed timing paths.

From a practical technology standpoint, when optimal
interconnect placement gets factored into front-end design, engineers are given
a better opportunity to create optimal timing routes. In fact, designers should
be able to obtain early versions of the SoC floorplan to analyze the
interconnect design and pre-determine issues that might contribute to failed
timing closure later in the design flow process.

A user interface that reports on potential failures at
this stage is essential in automating the error-resolution process by feeding
die location, performance, and timing information into the back-end
place-and-route tools, allowing the back-end tools to lay out a more optimized

Putting more forethought
into the SoC interconnect IP to avoid delays is important for developers who
are forging ahead into sub-16-nm FinFET process technology. Delivering IP
intelligence into the front end ultimately slashes time-to-market and results
in a higher-performing SoC.