Everything You Wanted to Know About SOC Memory

Everything You Wanted to Know About SOC Memory*

* But were Afraid to Ask

If you are a member of an SOC design team, or if you manage one, then memory is critically important to you. On today's multicore SOC designs, more on-chip silicon is devoted to memory than to anything else on the chip and yet memory is often added as an afterthought. Don't let that happen to your team.

This White Paper discusses the many alternatives for on-chip and off-chip memory usage that SOC designers must understand to develop successful multicore SOCs. It discusses the essentials of SOC memory organizations for multicore designs, on-chip SRAM and DRAM, local memories and caches, on-chip non-volatile memories, and memory controllers for off-chip memory. IT covers the difference between 6T and 4T SRAM designs, the system design ramifications of NAND and NOR Flash ROM, and how DDR2 and DDR3 SDRAMS compare (the differences might surprise you).

The State of SOC Memory

Consider what's happening to the SOC's on-chip memory. Figure 1 shows a 4-year-old graph predicting that the average SOC would devote more than 80% of its silicon to memory by the year 2008. This percentage was predicted to grow until logic made up less than 10% of an average SOC. Now these figures are just averages and of course, future predictions are just a guess. This particular guess was synthesized from data provided by IC Insights, the SIA roadmap, and other sources.

Figure 2 shows a more recent and more conservative prediction of memory use in SOCs made by Semico in 2007. The analysts at Semico predicted that only a little more than half of an SOC designed in 2008 would consist of memory and they don't even agree with the prediction shown in Figure 1 with respect to past SOC memory usage.

Your SOC will no doubt be different than either of the two predictions shown-but that really isn't important. Whether the total amount of area devoted to memory on your SOC is 50% or 80%, it's still a big part of your chip. There's an increasing amount of data to process in today's applications and that data requires more on-chip memory. How much more is determined during the design of the SOC.

SOC Evolution

Before discussing where we are today with respect to on-chip SOC memory, let's set the stage by discussing how we got here. Let's briefly review the evolution of the SOC.

Figure 3 depicts the evolution of the SOC. The 1980s saw the real arrival of ASICs-actually gate arrays, which were custom chips that vacuumed up all of the glue logic on a board-level design. Back then, there weren't enough gates on an ASIC to implement a processor, so there were no SOCs. SOCs-by definition-are ASICs that incorporate one or more processors.