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Overcome challenges of ASIC/SoC prototyping with FPGAs

Posted: 24 Oct 2011 ?? ?Print Version ?Bookmark and Share

Keywords:SoC? ASIC? ASSP? FPGA? prototype?

Silicon process technologies have now gone into the sub-30 nm domain, making it feasible to realize more than several billion transistors on a single chip. This, in turn, has made it possible to implement multiple extremely large and complex functions into a single SoC/ASIC/ASSP (hereafter referred to as SoC that applies to ASIC and ASSP) and conceive a myriad of applications catering to both consumer space and commercial usage. Furthermore, many consumer gadgets have a short life-cycle (some as low as 3 to 6 months) and are often differentiated by the software content that runs on them.

There are two primary requirements for a successful launch of these devices. First, the design must undergo thorough and comprehensive verification prior to the design tapeout. Second, an affordable hardware platform, representing the SoC prototype, for software development must be available prior to the arrival of first batch of silicon.

Silicon respin is not an option anymore as the mask cost alone runs into tens of millions of dollars and the lost market opportunity due to delayed product introduction may mean risking the business viability altogether.

This article first looks into various available solutions that can be used for functional verification and software development. It briefly compares various solutions and summarizes the pros and cons in using these solutions. The article focuses on FPGA-based prototyping and discusses various factors that must be taken into account to successfully implement a prototyping strategy.

There are broadly two categories of verification options available: one based on software simulation techniques and the second based on hardware platforms; the latter option is often referred to as hardware-assisted verification. The hardware platforms can be categorized as hardware acceleration, emulation, or FPGA-based prototyping. When compared in terns of ease of use, performance, and cost, each approach has its own pros and cons.

Simulation: RTL simulators are easy to setup and use, and they are relatively inexpensive. They have both high controllability and observability and hence offer 100% signal visibility into the design. The disadvantage is that the simulation performance is very low; typically in the range of 20Hz to 30Hz (slightly higher speeds are possible in regression mode in some simulators). Modern day designs are tens of millions of ASIC gates size, which requires the running of a very large number of test suites. Even if the simulations were run on a high-end computing machine, the effective performance is very low. Further, at this speed it is impractical to develop even low-level diagnostic type software. Also, it is not feasible to debug and verify the design in the context of the real hardware interface. Hence RTL simulators are often used only for block-level design development.

Hardware acceleration: With hardware acceleration, the design being verified is mapped into hardware (an array of FPGAs or custom processors) to accelerate the design performance. The real bottleneck in performance is that the test bench resides in the software simulator. The performance is directly proportional to the ratio of TB (test bench) activity and the design under test (DUT) activity. To speed up performance, the TB may be required to communicate at the application level, instead of the signal level, through synthesized transactors that are mapped to the hardware along with the DUT. Transaction-based verification (TBV) minimizes the interaction of the test bench to boost performance. One major limitation of accelerators is that they verify the design in isolation and not within the context of the system. Cost is another issue that makes this approach unaffordable for multiple software and hardware developers. Typical performance for hardware acceleration is in the 250Hz to 100kHz range. Once again, this speed is insufficient for software development.

Emulation: Emulation allows you to map both the design and the synthesizable test bench into the hardware (this often consists of an array of FPGAs or custom processors). You can verify the emulated design in the context of the actual system using in-circuit mode. The debugging capabilities offered by emulators are powerful; however the performance can be anywhere in the range of 500kHz to 750kHz C possibly as high as 1.5MHz when the design clocks are highly synchronous to each other. With these speeds, it is possible to run low-level diagnostic software, but not enough to develop application software or to bring up an operating system (that may require several hundred million CPU instructions). While emulators can be shared, the overall cost is very high C several hundred thousand to over a million dollars. This high cost of emulators prohibits creating multiple platforms (replicates) and limits the number of early users in the form of software developers and potential customers.

FPGA-Based Prototyping: The key benefits of FPGA based prototyping are low cost, high performance, and easy deployment. In addition to these benefits, the ability to interface with real-time devices allows the designer to observe the SoC prototype behavior in the context of a real system. Low system cost makes it affordable to create multiple pre-silicon hardware platforms for software development. FPGA based prototyping offers significantly high operating performance, typically in the range of 10MHz to 80MHz, thereby enabling verification of designs that are subjective in nature, such as video processors. Higher performance also makes it practical to develop software: device drivers, OS boot-up, and application software C which is not possible using other verification techniques. A small system footprint makes this approach easily deployable and portable to the field for demonstration and testing.

In summary their high performance and low cost makes FPGA-based rapid prototypes ideal hardware platforms for:
? Comprehensive design verification
? Pre-silicon software development platform
? Multiple, low cost replicates

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