In 1996, Mitsubishi took a standard 16-Mbit DRAM and wedged a RISC CPU into the middle. The M32R/D cost more than separate processor and DRAM, and it did not catch on. But seven years later, embedded DRAM is finally gaining popularity. Most ASIC vendors offer some sort of eDRAM process, and the leading ASIC vendor, IBM, claims that 30 percent of its customers' designs use eDRAM. Why the change?
First, the technology switched from embedding logic into DRAM (Mitsubishi's approach) to embedding DRAM into logic. Embedded logic uses DRAM-like transistors, which are slower than standard transistors and require new circuit designs. ASIC builders don't want to redesign everything just to add memory.
Embedding DRAM in logic has its own problems: half to a third the memory density of a true DRAM process, higher wafer cost and lower yield. These problems make it difficult to use eDRAM for system cost reduction.
The biggest benefit of eDRAM is bandwidth. For example, EZchip's NP-1c network processor, which IBM manufactures, achieves 62 Gbytes/ second of memory bandwidth from eight banks of embedded DRAM totaling 2,048 bits in width. Competitive NPUs that use external DRAM achieve 6 to 12 Gbytes/s from one to two banks of memory, usually 256 bits wide.
Network processors need lots of memory bandwidth to perform multiple table lookups per packet at 10 Gbits/s. With a 5x to 10x bandwidth advantage, the NP-1c can perform many more lookups than any other NPU. The 4 Mbytes of on-chip DRAM add about 30mm2 to the chip's die size, however, significantly increasing its manufacturing cost.
Embedded DRAM can also reduce system power, because on-chip memory accesses require far less juice than external ones. In fact, the first successful eDRAM product, NeoMagic's NM2070, integrated a graphics accelerator and frame buffer, reducing power consumption in notebook PCs.
Although eDRAM has a reputation as tricky and unreliable, that is changing. IBM is on its third generation of eDRAM technology and now has a reliable process. High-volume devices such as the Sony Playstation 2 and Nintendo Gamecube use eDRAM chips. Commodity DRAM still provides the cheapest transistors on the planet, and most applications will continue to use it. For ASIC designers seeking a bandwidth or power advantage, however, eDRAM is a good choice.
Linley Gwennap is founder and Principal Analyst of the Linley Group(www.linleygroup.com).