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Mobile memories advance to lower voltage use

Posted: 01 Aug 2008 ?? ?Print Version ?Bookmark and Share

Keywords:embedded applications? DDR? SDRAM? mobile memories?

Low-power DDR2 (LPDDR2), a next-generation low-power memory technology for mobile and embedded designs being defined by companies participating in JEDEC standards, offers higher speed, lower-voltage operation, larger capacities and lower pin count than the current generation. This lets non-volatile memory (NVM) share the same bus as SDRAM.

Whether you call them mobile memories or use the formal JEDEC naming system, low-power memory can be found in all kinds of battery-operated and embedded applications. Such memories differ from those popularly used in desktop computers, laptops and servers (collectively known as PC memory) such as DDR1, DDR2 and DDR3.

The current-generation low-power SDRAM (LPDDR1) still uses relatively high-voltage (1.8V) I/Os and is limited to low speed (200MHz). While the PC memory market has been shipping current-generation DDR3 parts in volume for more than a year, LPDDR1 memories have far more in common with two-generation DDR1 PC memory, sharing similar densities and operational modes.

While the target application for LPDDR1 memory in 2003 was often 2.5G mobile phones, LPDDR1 devices are now being used in other portable computing applications, such as GPS, personal media players and the latest-generation 3G data-connected phones.

Upgrading memory capacity
The newer and more demanding applications are putting a strain on the capabilities of LPDDR1 memory. A large concern is the limited speed range of LPDDR1: A clock rate of 200MHz (DDR400) has been reached, but only by a few manufacturers and is having trouble interfacing to these devices using unterminated I/Os.

The normal operation voltage of standard DDR3, at 1.5V, is lower than the operating voltage of most LPDDR1 devices. LPDDR1 has seen even more pressure from the PC memory market space with the introduction of some 1.5V-operation DDR2 and 1.35V-operation DDR3 memories. With power being proportional to the square of the voltage, these low-voltage PC memories use less power than their full-voltage PC equivalents. However, with terminated I/O schemes and power-consuming timing circuits, PC memories can't compete with LPDDR memories for low power at a given voltage.

The good news is that next-generation low-power memory, LPDDR2, is now available. With strong participation from memory manufacturers and users, the specification is close to being complete. LPDDR2 promises to be up to 2.5 times faster than LPDDR1 and to jump down to a 1.2V operating voltage, which is lower than low-voltage DDR3.

There have already been two product launchings for LPDDR2 memory: one from Elpida Memory Inc. (October 2007) and another from Hynix Semiconductor Inc. (April 2008). The memories offer up to 400MHz (DDR800) at 1.2V. Expect to see LPDDR2 product launchings at even higher data rates in the future.

PC memory has been designed so that many memories can share the same address bus. There can be up to 18 memories on a single unbuffered DIMM, necessitating a single-data-rate address bus even with DDR data pins. Since LPDDR memories are not intended for this kind of high-fanout application, the LPDDR2 specification may take the opportunity to reduce pin count, which can reduce the package size for both the memory and the host processor, and lessen system cost.

Road map to better specs
Some applications will use LPDDR2 SDRAM in combination with NAND flash memory on different buses. LPDDR2, however, has the unique property of being defined from the beginning to support both SDRAM and NVM on the same external memory bus. This allows systems using LPDDR2 memory to tap NVM to store the OS and applications and to execute code directly out of the NVM device without downloading it into SDRAM first.

Such an "execute in place" architecture can mean faster boot times, faster application start times, lower standby current, lower power consumption and maybe even lower cost than other architecturesall without increasing pin count.

Operating voltages, LPDDR vs. DDR specs: LPDDR2 may come in under low-volt DDR3.

A rarely discussed aspect of the PC memory road map is that each successive generation of PC memory improves the PC memory performance in PC systems but may inadvertently hamper the ability of the PC memory to operate in an embedded system.

For example, in the quest to get more speed out of the DDR3 memory, the minimum frequency of operation was raised to 300MHz (DDR600), and DDR3 moved to a burst access size that is incompatible with the caches of some commonly used embedded CPUs.

PC memory density is also growing at a rate that is beyond the needs of some embedded systems. For these reasons, with each successive generation of PC memory, we expect more of the embedded systems to move over from PC memory to LPDDR memory, even if those embedded systems do not necessarily need the low-power features offered by LPDDR.

- Mark Greenberg
Technical Marketing Manager
IP Products Group
Denali Software Inc.





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