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Reduce memory subsystem power consumption in handsets

Posted: 16 Mar 2007 ?? ?Print Version ?Bookmark and Share

Keywords:DRAM? MobileRAM? DDR MobileRAM? reduce power? handset battery?

By Odilio Vargas
Infineon Technologies AG

The continued evolution of mobile phones from simple voice-communication devices into multi-featured multimedia marvels is powered by innovation across different IC-component categories. A great deal of attention has been focused on developing more powerful graphics and application processors, and highly-integrated, flexible RF blocks, with an emphasis on controlling power budgets to allow for day-long talk times and multi-day standby performance.

However, less attention has been given to memory components despite the fact that the power demands of memory!at least 20 percent of total power budget!are equal to the demands of the application processor. Reducing the power used by memory can significantly extend handsets battery life.

Use of such low-power memories as CellularRAM and MobileRAM is a key to minimizing power usage. Also, a new architecture derived from commodity DRAM, known as double-data-rate (DDR) MobileRAM, combines the low-power design techniques pioneered in CellularRAM and MobileRAM with DDR performance, thus achieving higher performance with low power drain.

DRAM has become ubiquitous in PCs as an inexpensive but fast IC memory component used in conjunction with an HDD. DRAMs have advanced circuit-design technologies to address market requirements for higher densities and faster interfaces.

Mobile phones are demanding
New developments in mobile phones place significant performance demands on memory and are driving developments in DRAM that will create architectures that break away from the PC-centric past. For DRAM to succeed in wireless applications, it is necessary to meet requirements the best possible battery life and minimize system "real estate" occupied by the memory.

Figure 1: The majority of mobile phones continue to use an XIP platform, while new high-processing-intensive platforms are moving to a new memory architecture called code shadowing.

CellularRAM and MobileRAM align with the mobile-phone performance expectations of either eXecution-In-Place (XIP) or code-shadowing memory architectures, respectively (Figure 1). CellularRAM is a pseudo-SRAM that mimics the SRAM and NOR flash interfaces used for XIP memory architectures common on voice-centric platforms.

CellularRAM includes a hidden logic circuit to automatically manage refresh and pre-charge operations inherent in DRAM technology, without user commands. By copying familiar interfaces used in wireless platforms.

OEMs were able to quickly exploit the potential of DRAM-based technology as a low-cost alternative memory with increased performance and densities from 32MB to128MB. That also allowed manufacturers of legacy wireless platforms to extend the traditional six-to-nine-month product life cycle of voice-centric phones by making minor modifications to use CellularRAM and achieve increased performance and density compared to SRAM, while providing lower standby and operating currents compared to traditional DRAMs.

The development of CellularRAM involved new design innovations by DRAM manufacturers, including low-power features such temperature- compensated self-refresh (TCSR), partial-array self-refresh (PASR) and deep power-down (DPD), while also conforming to a new multichip-package (MCP) requirement. By mirroring the parallel address/ data signaling scheme and performance of NOR flash devices, CellularRAM became a popular companion RAM for easy creation of stacked-die solutions for space-constrained cellphone platforms.

Manufacturer experience with CellularRAM also led to the development of a commodity DRAM-type named MobileRAM. This component reduces the core voltage of DRAM cells from 3.3V to 2.5V and 1.8V, while also introducing a new key low-power feature!on-chip temperature sensor (OCTS).

Refresh rates are key
The physics of DRAM requires periodic refreshing of data to compensate for the discharge effect in the capacitance associated with a DRAM memory cell. The rate of refresh was discovered to be a direct function of temperature. As the DRAM was exposed to higher temperatures, the refresh rate would need to increase, leading to more power being consumed.

A general rule of thumb is that there is a doubling of refresh rate for every incremental 15?C increase in temperature. To minimize this effect, an OCTS can be used to automatically sense the temperature and program the most efficient refresh rate to restore memory contents while minimizing standby current.

MobileRAM's higher performance and low-power management features was well received by PDA OEMs attempting to provide optimal mobile computing without jeopardizing battery life. This led to a direct impact on smart phone development, since many OEMs have chosen to use PDA-type platforms as the basis for these powerful "converged" personal communicators.

With the addition of an RF unit and telephony functions, these platforms are evolving into full-featured devices. To address the growing processing power of new application and graphic units while maintaining the low cost required of a consumer product, a code-shadowing memory technique was developed. In a code-shadowed memory, the MobileRAM's performance benefit of 133MHz is able to exceed NOR flash operating at a top frequency of 80MHz in XIP.

Offsetting redundant memory
To offset the redundant memory needed to execute program code from a volatile memory, a NAND flash was selected due to its lower cost per bit and smaller cell structure than NOR flash.

The key software feature to manage code between NAND and MobileRAM is demand paging, which swaps pages of code/data between memories according to processor expectations. To lay the groundwork for selecting the right memory components and memory architecture, engineers must understand power-consumption differences between XIP and code-shadowing.

Figure 2: Wireless platforms allocate more power to a memory subsystem than to the entire platform.

A power analysis was conducted to determine the impact of a memory subsystem on the overall wireless platform consumption. The results demonstrate that a memory subsystem is the third most power-hungry structure in a mobile phone (Figure 2).

With a significant amount of power dedicated to the memory subsystem, knowing the proper use of the memory regarding application requirements is important. Two key market segments are voice-centric phone and emerging smart phone, each of which has its own unique breakout between standby-, talk-, and application-mode usage (Figure 3).

Figure 3: With the migration toward smart phones, the importance of an optimized operating current increases.

The trend is toward a platform capable of doing multiple tasks, in addition to communicating as a voice device, such as serving as an MP3 player, Internet browser and video/still camera. These new applications demand more usage from a memory subsystem than before, which MobileRAM is capable of satisfying with its code-shadowing approach.

With CellularRAM, a significant emphasis was placed on lowering standby current at the expense of performance (Figure 4). While smart-phone platforms place large demands on the memory subsystem, sacrificing a fast interface is not an option. Reducing operating current by any amount had greater impact on total power consumption than decreasing standby current.

Figure 4: The most significant reduction in total memory power consumption is achieved by decreasing operating current instead of standby current.

That's because operating currents are measured in milliamps rather than microamps of standby current. Thus, focus more on reducing the operating current because the memory subsystem of a smart phone spends more time in the active mode than voice-centric phone platforms. Besides lowering operating currents, the DDR MobileRAM interface reduces the amount of time required to achieve identical read/write commands compared to a MobileRAM using an SDRAM interface.

The slight increase in operating current of DDR MobileRAM - attributable to the doubling of sense amps -is offset by the lower duration of command execution time (Figure 5).

Figure 5: Random READ (eight words) command/performance benefit adds to 12 percent power savings for DDR MobileRAM due to shorter duration in active mode even with a slightly higher operating current.

That allows for further possibilities on increasing system performance of future wireless platforms without decreasing battery life.The introduction of CellularRAM and MobileRAM products into the wireless world brings the cost benefits of DRAM-based memory into the mobile communications environment. The flexibility of using DRAM-based memory in legacy or next-generation wireless platforms while addressing low-power management and space-constraint requirements has validated the effectiveness of DRAM in these new application markets.

About the author
Odilio Vargas
is product marketing manager of mobile business unit, Infineon Technologies AG.

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