Selecting the best memory solution in terms of system performance often means designers must trade memory density versus performance.

As today's processors continue to speed up, main memory grows larger. Unfortunately, memory access time (latency) has seen little improvement. In fact, with each succeeding generation, main memory access takes longer in terms of processor cycles. Although other variables are involved, memory latency is closely tied to overall system performance. Therefore, the choice of memory technology can dramatically impact system performance as well as cost.

Memory tradeoffs

Traditionally, SRAM embedded memory has been the designer's choice for fast memory. This choice, however, comes at the expense of cost and silicon area. Alternatives such as embedded DRAM or Z-RAM are much lower cost options, but they have higher latency and are typically used further from the processor. In spite of this, having more memory closer to the processor often generates a performance advantage—even if the raw memory latency is higher. With such opportunities for lower cost and higher performance, alternative memories are now replacing SRAM in what was traditionally sacred SRAM territory.

One alternative memory option—Z-RAM technology—is a new player in this market and offers some compelling features:

• Simple process integration: Z-RAM requires no process modification for logic processes built on SOI. By comparison, DRAM is difficult to build on a logic process.

• Minimal standby power: When the Z-RAM array isn't in use, all nodes in each memory cell are tied to ground. As a result, there's no array leakage; only refresh current. And the refresh current is small compared to typical SRAM array leakage. Consequently, Z-RAM can offer orders of magnitude lower standby power.

• Low soft-error rate: Z-RAM soft-error rates are consistent with embedded DRAM, and are an order of magnitude better than SRAM.

• Better value proposition: At a system level, designers can use the same amount of real estate in Z-RAM as with SRAM, but with 4x higher density. Even if Z-RAM is slower, the overall memory performance can improve.

• Cost savings: Cost savings are proportional to the amount of real estate occupied. In terms of mm² cost, if the real estate is cut down by a factor of 4, the cost of memory goes down by at least a factor of 4 as well. An important note: yield will always go up as a result of using smaller real estate, which provides added gains.