Several vendors recently unveiled embedded processors containing large amounts of on-chip RAM—but not the SRAM usually found in embedded processor chips. Instead, these new processors integrate copious quantities of on-chip DRAM. Embedding DRAM with processing logic can greatly improve on-chip memory bandwidth by providing access to the wide internal bus structures present in DRAM. Combining embedded DRAM (eDRAM) with processing logic also creates an opportunity to choose between cost and performance. By fabricating the chip in a memory process, chip designers can achieve high memory densities—and hence lower chip costs—at the expense of lower speeds. Alternatively, designers can use a logic process to achieve high clock rates at the expense of reduced density and higher chip costs.
One example of the emerging eDRAM trend is Toshiba’s new TMPR9961 processor, which contains an 800 MHz MIPS64 core, 32 MB of logic process eDRAM, and a host of I/O interfaces (USB, DDR, etc.). The TMPR9961 targets graphics-oriented embedded applications like automotive navigation systems, set-top-boxes, and digital televisions. The eDRAM is used exclusively as video memory; the caches for the MIPS core use conventional SRAM. (Toshiba says future versions of the chip will implement the caches with eDRAM.) Toshiba claims that the eDRAM allows the TMPR9961 to achieve greatly improved pixel fill-rate performance compared to an off-chip DRAM configuration.
Memory giant Micron recently bolstered the argument for eDRAM with the announcement of its first processor. This processor, code-named Yukon, contains an array of 256 8-bit integer ALUs and
16 MB of eDRAM. According to Micron, Yukon is fabricated in a memory process with some additional fabrication steps added to enhance logic performance. Yukon embodies echoes of U.C. Berkeley’s Vector IRAM project. Like the IRAM project, Yukon combines a wide processing engine with a wide internal data buses—2,048 bits wide in the case of Yukon. Operating at 200 MHz, Yukon’s internal memory bandwidth of 25.6 Gbytes/s, or 128 bytes per cycle, is nearly an order of magnitude higher than that of typical high-performance DSPs.
As memory-intensive embedded applications like video processing continue to proliferate, eDRAM will likely gain increasing attention. Indeed, the new processors from Toshiba and Micron suggest that the embedded DRAM concept may have come of age.
Add new comment