Rob van Blommestein, S2C Inc. 2016-7-11 21:28:00 ARM Processors
SemiWiki recently published a book on FPGA-based prototyping titled “PROTOTYPICAL: The Emergence of FPGA-Based Prototyping for SoC Design.” Among other things the book explores ARM’s role in fpga prototyping technology.
Developing for ARM Architecture
Since ARM introduced its Cortex strategy, with A cores for application processors, R cores for real-time processors, and M cores for microcontrollers, designers have been able to choose price/performance points – and migrate software between them. How do designers, who are often doing co-validation of SoC designs with production software, prototype with these cores?
Some teams elect to use ARM’s hard macro IP offering, with optimized implementations of cores. ARM has a mixed prototyping solution with their CoreTile Express and LogicTile Express products. CoreTile Express versions are available for the Cortex-A5, Cortex-A7, Cortex-A9, and Cortex-A15 MPCore processors, based on a dedicated chip with the hardened core and test features. The LogicTile Express comes in versions with a single Xilinx Vertex-5, dual Virtex-6, or single Virtex-7 FPGAs, allowing loose coupling of peripheral IP.
Others try to attack the challenge entirely in software. Cycle-accurate and instruction-accurate models of ARM IP exist, which can be run in a simulator testbench along with other IP. With growing designs come growing simulation complexity, and with complexity comes drastic increases in execution time or required compute resources. Simulation supports test vectors well, but is not very good at supporting production software testing – a large operating system can take practically forever to boot in a simulated environment.
Full-scale hardware emulation has the advantage of accommodating very large designs, but at substantial cost. ARM has increased its large design prototyping efforts with the Juno SoC for ARMv8-A, betting on enabling designers with a production software-ready environment with a relatively inexpensive development board.
However, as we have seen SoC design is rarely about just the processor core; other IP must be integrated and verified. Without a complete pass at the full chip design with the actual software, too much is left to chance in committing to silicon. While useful, these other platforms do not provide a cost-effective end-to-end solution for development and debug with distributed teams. Exploration capability in a prototyping environment is also extremely valuable, changing out design elements in a search for better performance, power consumption, third-party IP evaluation, or other tradeoffs.
The traditional knock on FPGA-based prototyping has been a lack of capacity and the hazards of partitioning, which introduces uncertainty and potential faults. With bigger FPGAs and synthesizable RTL versions of ARM core IP, many of the ARM core offerings now fit in a single FPGA without partitioning. Larger members of the ARM Cortex-A core family have been successfully partitioned across several large FPGAs without extensive effort and adverse timing effects, running at speeds significantly higher than simulation but without the cost of full-scale hardware emulation.
A hybrid solution has emerged in programmable SoCs, typified by the Xilinx Zynq family. The Zynq UltraScale+ MPSoC has a quad-core ARM Cortex-A53 with a dual-core ARM Cortex-R5 and an ARM Mali-400MP GPU, plus a large complement of programmable logic and a full suite of I/O. If that is a similar configuration to the payload of the SoC under design, it may be extremely useful to jumpstart efforts and add peripheral IP as needed. If not, mimicking the target SoC design may be difficult.
True FPGA-based prototyping platforms offer a combination of flexibility, allowing any ARM core plus peripheral IP payload, and debug capability. Advanced FPGA synthesis tools provide platform-aware partitioning, automating much of the process, and are able to deal with RTL and packaged IP such as encrypted blocks. Debug features such as deep trace and multi-FPGA visibility and correlation speed the process of finding issues.
The latest FPGA-based prototyping technology adds co-simulation, using a chip-level interconnect such as AXI to download and control joint operations between a host-based simulator and the hardware-based logic execution. This considerably increases the speed of a traditional simulation and allows use of a variety of host-based verification tools. Using co-simulation allows faster turnaround and more extensive exploration of designs, with greater certainty in the implementation running in hardware.
Integration rollup is also an advantage of scalable FPGA-based prototyping systems. Smaller units can reside on the desk of a software engineer or IP block designer, allowing dedicated and thorough investigation. Larger units can support integration of multiple blocks or the entire SoC design. With the same synthesis, debug, and visualization tools, artifacts are reused from the lower level designs, speeding testing of the integrated solution and shortening the time-to-success.
Another consideration in ARM design is not all cores are stock. In many cases, hardware IP is designed using an architectural license, customized to fit specific needs. In these cases, FPGA-based prototyping is ideal to quickly experiment and modify designs, which may undergo many iterations. Turnaround time becomes very important and is a large productivity advantage for FPGA-based prototyping.