Intel’s Hybrid CPU-FPGA
Tom R. Halfhill, The Linley Group
The rising costs of designing and manufacturing ASICs have motivated several established semiconductor companies and startups to find substitutes. Of course, FPGAs are the old standbys, and they continue to gain popularity as the cost of reprogrammable logic gradually declines. Beyond low-volume applications, however, FPGAs remain an expensive solution, and their relatively high power consumption is a major obstacle.
Another drawback of FPGAs is that their CPU options are very limited. Implementing a synthesizable CPU core in the programmable- logic fabric is not difficult, but performance is comparatively poor, even in the fastest and most expensive FPGAs. In past years, Altera and Xilinx, the leading FPGA vendors, have offered a few FPGAs with hard CPU cores baked into the chip. The variety of these hybrid devices is quite narrow, however, and other disadvantages have stunted their adoption.
Nevertheless, the demand for an ASIC substitute remains strong, so companies keep striving to meet the challenge. The latest attempts are from Intel, STMicroelectronics, and Xilinx—and Altera is promising future announcements. All these companies are crafting new ways to combine hard CPU cores with programmable logic.
Intel’s solution is the Intel® Atom™ processor E600C Series (codename Stellarton), which pairs an Intel Atom processor SoC with an Altera FPGA in a multichip package. These products even have integrated graphics. But the key feature is programmable logic, which allows developers to add application-specific functions that normally would require spinning a custom ASIC.
At 1.3GHz, the Intel® Atom™ processor E665C is the fastest member of the series. In one package is a single-core dual-threaded Intel Atom processor SoC (codename Tunnel Creek) and an Altera Arria-II FPGA. The two die are bonded side-by-side and linked over a 1x1 PCI Express Gen1 interface. Notably, the Intel Atom processor E600C series is the only hybrid processor of this type supporting the Intel® architecture (x86).
The FPGA die is from Altera’s midrange family. It has 63,250 reprogrammable logic elements, the equivalent of about 759,000 ASIC gates—roomy enough for customization. It also has 5.3Mbits of block RAM and 312 18x18-bit multipliers (often called DSP blocks by FPGA vendors). Because the multipliers are hard wired, they free all the programmable logic for custom development. I/O interfaces include eight 3.125Gbps serdes lanes and 364 I/O pins capable of various signaling standards.
One limitation is that all communications between the FPGA and the Intel Atom processor must funnel through the 1x1 PCIe channel linking the two die. This bottleneck makes offloading some tasks from the CPU to the FPGA impractical. For many embedded applications, however, this channel is sufficient.
The integrated graphics core of the Intel Atom processor E600C is the Intel® Graphics Media Accelerator (Intel® GMA) 600, a version of the SGX-535 GPU core licensed from Imagination Technologies. It supports Microsoft’s DirectX 9 graphics and runs at 400MHz. That frequency is twice as fast as other SGX-535 implementations, and it can execute 1.6 gigaflops. Given that the slower SGX-535 in Apple’s iPhone 3GS can draw 7.7 million triangles per second, the performance of the Intel GMA 600 is about 15 million triangles per second. An integrated video encoder/decoder supports various formats, including H.264 at 1080p, 30fps. An audio accelerator provides up to eight 32-bit channels at 192kHz.
Although the highly integrated E600C processors cost more than Tunnel Creek processors, they are a bargain compared with the prices of FPGAs in low volumes. Power consumption is relatively low, too: about 7W thermal design power (TDP).
The Intel Atom processor E600C series is the obvious choice if x86 compatibility is paramount or if the application would benefit from integrated graphics. At its top speed of 1.3GHz, the Intel Atom processor E665C is the fastest hybrid processor of this type announced to date. All things considered, it is a feasible alternative to a conventional ASIC project. For embedded designers, the Intel Atom E600C processor series opens up the possibility of using a processor customized for a specific application instead of settling for a standard off-the-shelf processor.
Tom R. Halfhill is a senior analyst at The Linley Group and senior editor of Microprocessor Report. He has been a journalist since 1977 and has been providing in-depth technology coverage since 1982. Prior to serving as a senior analyst for In-Stat, he was a technology analyst at ARC Cores, a senior editor at BYTE Magazine, and editor at several other publications.