Feature demand strains portable memory design

Handset models are changing every year as designers strive to strike a balance between what end users want and what can actually be designed in. Many trade-offs must be considered, including performance and features vs. price, size and battery life.

Some of those trade-offs take place in the handset's storage subsystem. Here, designers must decide what type of memory, and how much of it, to use; which package type is most appropriate; whether to use internal or external storage; and how to address that memory in a bus design.

"In most cases, removable vs. nonremovable is an operator issue," said Doug Grant, director of business development at Analog Devices Inc. "Some operators only want you to get stuff from their network, so the only interface they want you to have is through their network so they can charge you for the bits. But that's beginning to change as more connectivity is becoming available."

The storage subsystem in the mainstream handset market comprises a combination of a baseband chip with multichip-package (MCP) memory, which has a flash memory die to store the operating system. The typical SRAM in most designs

is now moving toward a pseudo SRAM, or PSRAM--basically DRAM with an SRAM wrapper around it. From an interface perspective, it looks like an SRAM, but from a technology perspective, it's a DRAM.

Designers can choose from NAND or NOR flash memory, SRAM, PSRAM, low-power DRAM and so on. PSDRAM is one of the newer technologies, and Micron Technology Inc. offers a version designed specifically for handsets, particularly the low-end models. Called CellularRAM, it substitutes pseudostatic-RAM technology for standard RAM. The benefit is that the PSRAM is based on a two-transistor cell architecture, which costs less than conventional six-cell SRAM.