Breaking The Three Laws

 

Automated Timing Biased Partitioning

I spent a lot of time this week talking about timing biased automated partitioning which is the ability of the ProtoCompiler partition engine to generate a partition and pin multiplexing implementation with the goal of maximizing performance of the final prototype implementation. As it’s getting close to Easter I thought it was fitting to discuss ProtoCompiler’s Multi-Hop optimization algorithm. (Easter, Easter bunny, hop, you see what I did there?)

Firstly what is a Hop you ask? In the image below you can see the ASIC design example with combinatorial logic between two register points

ASIC Design with logic clouds between register points

It’s possible during partitioning for FPGA-based prototyping that these two registers end up in different FPGA’s as seen in the image below.

Example of a single "Hop" between FPGAs on a physical prototype

This is what is known as a single hop. The design has been split up with the starting and ending register points in separate FPGA’s. However, without timing biased capabilities it’s possible that a partition is created that spans multiple FPGA’s. An example of this Multi-Hop is represented in the image below

Example of what a multi-hop is

In our example above FPGA B has basically become a feed through FPGA and while this helps a partition engine get to a partition solution it has a dramatic effect on the overall performance of a prototype platform. To explain the timing impact lets go back to our original ASIC design and review the timing between the register points

Timing delay of this logic in ASIC implementation

In our made up example the combinatorial logic has a timing impact of about 10ns. However in our FPGA partition where you have the critical path split up over multiple FPGA’s timing becomes much more significant. The reason for this is that you typically have to apply pin multiplexing between FPGA’s as you have more signals than you have physical pins (One of the three ASIC Prototyping three laws which are the grounding facts driving this blog) Now if you review the timing impact with a typical pin multiplexing scheme inserted you suddenly see the impact of a Multi-Hop

Tming impact of multi-hops and pin multiplexing

But not to worry if you are using ProtoCompiler as the partition engine is not only fast but it’s timing biased and includes a Multi-Hop optimization algorithm.

When Multi-Hop optimization is enabled ProtoCompiler partition engine will:

  • Focus on reducing the number of multi-hops, with a goal of zero
  • If multi-hops are needed to complete the partition the focus turns to reducing the path length of the multi-hop
  • Avoids pin-multiplexing on multi-hop path
  • If pin-multiplexing is needed the focus turns to using the lowest pin-multiplexing ratio on the multi-hop path
  • Selects pin-multiplexing ratio based on timing slack

Knowing that eliminating multi-hops would lead to higher prototype performance you might think that by default the partition engine should not allow any multi-hops. However multi-hops do play a vital role sometimes in enabling an automated partition solution to be found.

ProtoCompiler’s timing biased multi-hop optimization is making a huge impact on the resulting HAPS prototyping performance. Across a suite of over 40 ASIC designs ProtoCompilers timing biased optimizations improved the clock period by an average of 50ns. HUGE improvement in resulting performance. Across this suite of designs, ProtoCompiler reduced the number of nets that are included in multi-hop paths of length two or greater by up to 80%. For most designs in the suite, the number of paths of length three or greater was reduced to ZERO. Also, for most designs in the suite, the pin multiplexing ratio of the multi-hop path nets required to get feasible automated partition was reduced to one (i.e. no pin-multiplexing required). Fantastic. Not only is ProtoCompiler’s partition engine super-fast running in minutes for multi-million ASIC gate design but the out of the box results are phenomenal.

I’m out on vacation for a week (yes even I need time off once in a while) so no blog next week.

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