There are designs in which you need a CPU core, but not an ARM core. The issue may simply be avoiding royalties, or export restrictions, or complying with a corporate policy. Or it may be that you are trying to hit a speed-area-power point that is a stretch for ARM’s current products, and you don’t want to invest in an expert in processor-core optimization. This situation can come up easily when a legacy design using an 8- or 16-bit core runs out of steam, for example, and needs to migrate to 32 bits.
In such cases there are options. One would be to use an open-source CPU core such as the open-SPARC S1 or the LEON, or the more specialized LatticeMico 32. But with such open-source cores you may be on your own for verification and integration.
Another alternative would be a commercial core such as one based on the BA2 architecture from Slovenian IP vendor Beyond Semiconductor. This particular choice may have become more attractive this week with the announcement that three pre-configured versions of Beyond’s BA22 core are now available under royalty-free license from IP supplier CAST.
The BA22 family, according to CAST vice-president of marketing Nikos Zervas, is what you’d get if you started out with a clean sheet of paper, without constraints from a decades-old instruction-set architecture, and with the latest thinking in compact, low-power design. In this scenario you would have few enough constraints to produce some pretty impressive results. And it appears that Beyond has done so.
Zervas said a Linley Group study found the BA22 instruction stream 5 to 20 percent denser than ARM’s Thumb-2. The company goes on to claim implementations as small as 12k gates, operating dynamic power consumption of as little as 23 microW/MHz in a—presumably lp—65nm process, and 1.4 DMIPS/MHz—all from a fully synthesizable core.
Specifically, CAST this week released three preconfigured cores. The BA22-BASE is the compact member of the family. The company says it synthesizes to under 15k gates and can reach 350 MHZ at 65nm. The configuration includes either an AMBA/AHB or a Wishbone bus controller, dedicated ports for on-chip instruction and data memories, and an integral interrupt controller and power-management engine. The latter manages power modes through a combination of dynamic clock gating at the unit level and software-controlled clock-throttling—nothing that should present huge integration challenges. There is an optional debug engine as well. The point of this configuration, Zervas said, is to give deeply-embedded controller applications—especially those moving up from 8- or 16-bit architectures—easy access to 32-bit tools and performance.
The second offering is the BA22-ADV. This is essentially the same BASE core with the addition of 16 more general registers—for a total of 32—and instruction and data caches. It is aimed at larger systems that require off-chip memory—hence the caches—and those that will run an RTOS. CAST puts the core at about 19k gates and up to 300 MHz at 65nm. Ports of the eCOS and uClinux RTOS kernels are available.
Third, the BA22-AP adds one more element, instruction and data MMUs, to the ADV configuration. The AP also offers the option of floating-point, divider, and multiply-accumulate units. The core synthesizes to 35k gates in a 90nm process, CAST claims, although it’s not clear what all is included in that number. Once suspects the added arithmetic units, the TLBs, and certainly the caches are not. In the benchmark 65nm process the core is said to hit 300 MHz.
The AP is intended to be an application processor rather than a deeply-embedded controller. Accordingly, Zervas said, ports of Linux and Android are available. The executive claimed the core has application throughput similar to that of a Cortex R4.
All the BA22 cores come with a gcc-based Eclipse environment. CAST offers a variety of optional modules for the cores, and a fairly complete range of compatible peripheral and accelerator cores. The company can provide preconfigured subsystems and integration support. While the BA22 may lack the exhaustive processor validation programs of say, the ARM cores, they are silicon-proven, Zervas said, at major houses such as Omnivision and ST Microelectronics. The three BA22 cores are available now, as is the development environment for Linux. A Windows port of the environment is expected this Fall.