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A Prototyping with FPGA Approach

Frank Schirrmeister, Group Director for Product Marketing of the System Development Suite, Cadence.

In general, the industry is experiencing the need for what now has been started being called the “shift left” in the design flow as shown Figure 1. Complex hardware stacks, starting from IP assembled into sub-systems, assembled into Systems on Chips (SoCs) and eventually integrated into systems, are combined with complex software stacks, integrating bare metal software and drivers with operating systems, middleware and eventually the end applications that determine the user experience.

From a chip perspective, about 60% into a project three main issues have to be resolved. First, the error rate in the hardware has to be low enough that design teams find confidence to commit to a tape out. Second, the chip has to be validated enough within its environment to be sure that it works within the system. Third, and perhaps most challenging, significant portions of the software have to be brought up to be confident that software/hardware interactions work correctly. In short, hardware verification, system validation and software development have to be performed as early as possible, requiring a “shift left” of development tasks to allow them to happen as early as possible.

Figure 1: A Hardware/Software Development Flow.

Prototyping today happens at two abstraction levels – using transaction-level models (TLM) and register transfer models (RTL) – using five basic engines.

  • Virtual prototyping based on TLM models can happen based on specifications earliest in the design flow and works well for software development but falls short when more detailed hardware models are required and is plagued by model availability and its creation cost and effort.
  • RTL simulation – which by the way today is usually integrated with SystemC based capabilities for TLM execution – allows detailed hardware execution but is limited in speed to the low KHz or even Hz range and as such is not suitable for software execution that may require billions of cycles to just boot an operating system. Hardware assisted techniques come to the rescue.
  • Emulation is used for both hardware verification and lower level software development as speeds can reach the MHz domain. Emulation is separated into processor based and FPGA based emulation, the former allowing for excellent at speed debug and fast bring-up times as long FPGA routing times can be avoided, the latter excelling at speed once the design has been brought up.
  • FPGA based prototyping is typically limited in capacity and can take months to bring up due to modifications required to the design itself and the subsequent required verification. The benefit, once brought up, is a speed range in the 10s of MHz range that is sufficient for software development.
  • The actual prototype silicon is the fifth engine used for bring up. Post-silicon debug and test techniques are finding their way into pre-silicon given the ongoing shift-left. Using software for verification bears the promise to better re-use verification across the five engines all the way into post-silicon.

Advantages Of Using FPGAs For ASIC Prototyping

FPGA providers have been pursuing aggressive roadmaps. Single FPGA devices now nominally hold up to 20 million ASIC gates, with utilization rates of 60%, 8 FPGA systems promise to hold almost 100 MG, which makes them large enough for a fair share of design starts out there. The key advantage of FPGA based systems is the speed that can be achieved and the main volume of FPGA based prototypes today is shipped to enable software development and sub-system validation. They are also relatively portable, so we have seen customers use FPGA based prototypes successfully to interact with their customers to deliver pre-silicon representations of the design for demonstration and software development purposes.

Factors That May Limit The Growth Of The Technique

There certainly is a fair amount of growth out there for FPGA based prototyping, but the challenge of long bring-up times often defies the purpose of early availability. For complex designs, requiring careful partitioning and timing optimization, we have seen cases in which the FPGA based prototype did not become available even until silicon was back. Another limitation is that the debug insight into the hardware is very limited compared to simulation and processor based emulation. While hardware probes can be inserted, they will then reduce the speed of execution because of data logging. Subsequently, FPGA based prototypes find most adoption in the later stages of projects during which RTL has become already stable and the focus can shift to software development.

The Future For Such Techniques

All prototyping techniques are more and more used in combination. Emulation and RTL simulation are combined to achieve “Simulation Acceleration”. Emulation and transaction-level models with Fast Models from ARM are combined to accelerate operating system bring-up and software driven testing. Emulation and FPGA based prototyping are combined to combine the speed of bring-up for new portions of the design in emulation with the speed of execution for stable portions of the design in FPGA based prototyping. Like in the recent introduction of the Cadence Protium FPGA based prototyping platform, both processor based emulation and FPGA based prototyping can share the same front-end to significantly accelerate FPGA based prototyping bring-up. At this point all major EDA vendors have announced a suite of connected engines (Cadence in May 2011, Mentor in March 2014 and Synopsys in September 2014).It will be interesting to see how the continuum of engines grows further together to enable most efficient prototyping at different stages of a development project.

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