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Embedded memories multiply in system-on-chips

Posted: 18 Aug 2003 ?? ?Print Version ?Bookmark and Share

Keywords:embedded memory? dram? sram? soc? memory?

Developments in embedded-memory technology have made large DRAMs and SRAMs commonplace in today's SoCs. Trade-offs between small and large memories make all sizes practical, enabling SoCs to resemble board-level systems. The latest embedded memories are even bringing the added benefits of low-power operation to handheld systems.

Large embedded memories give SoC benefits like improved bandwidth and considerable power savings that can only be achieved through the use of embedded technologies. The practicality and success of including embedded DRAM and/or large SRAM blocks in an SoC depends mainly on manufacturability. Highly manufacturable memory structures resolve issues of cost, time-to-market and risk that affect all SoC designs.

SRAMs have long been an SoC mainstay, but the size of the SRAM blocks and the number of them in a single SoC has begun to explode in the past year or so. It is common to see chips that have as many as 150 SRAM blocks, with some of the cores ranging from 1Mb to 8Mb.

At the same time, improvements in DRAM manufacturability have caused a boom in the use of large DRAM blocks. Even ASICs for commodity products such as game machines and camcorders include DRAM cores.

In Toshiba's case, the embedded-DRAM systems are frequently the early adopters of new-generation fabrication technologies. The number and size of the embedded DRAM cores in an SoC has tended to increase as chips move down the technology curve. At 180nm, system ASICs were typically using two blocks of DRAM and up to about 64Mb of total memory capacity. Now at 130nm and 90nm, typical systems are using four or more blocks with 120Mb (or higher) of DRAM cores.

From a fabrication point of view, large memory blocks have to be just as manufacturable as small ones. However, trade-offs between small and large memories can have some effect on performance and die size. With small memories, you pay a somewhat higher area penalty for overhead circuitry like sense amps. Large memories, on the other hand, have beneficial overhead in the form of redundancy to ensure manufacturability. The trade-offs are not simple, so if you have a choice between a few large memory blocks or many small ones, consult your semiconductor vendor's application engineers.

Even before reaching fabrication, large blocks have to work well with back-end layout requirements. The ability to route over the top of big memory blocks has now made them friendlier to the layout environment. So are test schemes with features like common BIST blocks. Today, you can choose among a variety of test schemes for embedded memories, some requiring wafer-level memory testers and some relying heavily on BIST structures. Choosing the best test scheme for a given design requires detailed discussions with your silicon vendor.

The boom in embedded memories is all due to the success of new integrated process technologies that have been developed from the beginning with large memories in mind. The process steps have been refined generation after generation to ensure high yields on every chip that contains SRAM or DRAM. Only with this level of predictability can an ASIC vendor offer embedded memories at a cost-effective price.

Hence, you can expect that SRAM and embedded-trench-based DRAM will be the prevalent SoC memories of choice going forward. In addition to reducing costs, the dependability of these memories minimizes your time-to-market and design risk. To get these benefits, you need to take advantage of your process vendor's memory IP because the fab process has been tuned to work with specific memory structures.

For non-volatile requirements, Toshiba has found that combining an off-the-shelf flash die with an SoC in a stacked-die package works well and costs less than integrating flash into the fabrication process. Otherwise, SRAMs are ideal for small, fast SoC memories while embedded-trench-based DRAMs handle needs for large memory blocks.

- Frank R. Ramsay

Director of Technology and Business Strategy, System LSI Group

Toshiba America Electronic Components Inc.

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