Reducing memory subsystem power consumption in your mobile design -

Reducing memory subsystem power consumption in your mobile design

The continued evolution of mobile phones from simplevoice-communication devices into multi-featured multimedia marvels ispowered by innovation across different IC-component categories.

A great deal of attention has been focused on developing morepowerful graphics and application processors, and highly-integrated,flexible RF blocks, with an emphasis on controlling power budgets toallow for day-long talk times and multi-day standby performance.

Less attention, however, has been given to memory components,despite the fact that the power demands of memory – at least 20 percentof total power budget – are equal to the demands of the applicationprocessor. Reducing the power used by memory can significantly extendhandsets battery life.

Use of such low-power memories as CellularRAMandMobileRAM is a key tominimizing power usage. Also, a new architecture derived from commodityDRAM, known as double-data-rate (DDR)MobileRAM, combines the low-power design techniques pioneered inCellularRAM and MobileRAM with DDR performance, thus achieving higherperformance with low power drain.

DRAM has become ubiquitous in PCs as an inexpensive but fast ICmemory component used in conjunction with an HDD. DRAMs have advancedcircuit-design technologies to address market requirements for higherdensities and faster interfaces.

Mobile phones are demanding
New developments in mobile phones place significant performance demandson memory and are driving developments in DRAM that will createarchitectures that break away from the PC-centric past. For DRAM tosucceed in wireless applications, it is necessary to meet requirementsthe best possible battery life and minimize system “real estate”occupied by the memory.

Figure1. The majority of mobile phones continue to use an XIP platform, whilenew high-processing-intensive platforms are moving to a new memoryarchitecture called code shadowing.

CellularRAM and MobileRAM align with the mobile-phone performanceexpectations of either eXecution-In-Place (XIP) or code-shadowingmemory architectures, respectively (Figure1, above ). CellularRAM is a pseudo-SRAM that mimics the SRAMand NOR flash interfaces usedfor XIP memory architectures common on voice-centric platforms.

CellularRAM includes a hidden logic circuit to automatically managerefresh and pre-charge operations inherent in DRAM technology, withoutuser commands. By copying familiar interfaces used in wirelessplatforms,

OEMs were able to quickly exploit the potential of DRAM-basedtechnology as a low-cost alternative memory with increased performanceand densities from 32MB to128MB. That also allowed manufacturers oflegacy wireless platforms to extend the traditional six-to-nine-monthproduct life cycle of voice-centric phones by making minormodifications to use CellularRAM and achieve increased performance anddensity compared to SRAM, while providing lower standby and operatingcurrents compared to traditional DRAMs.

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

Manufacturer experience with CellularRAM also led to the developmentof a commodity DRAM-type named MobileRAM. This component reduces thecore voltage of DRAM cells from 3.3V to 2.5V and 1.8V, while alsointroducing 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 compensatefor the discharge effect in the capacitance associated with a DRAMmemory cell. The rate of refresh was discovered to be a direct functionof temperature. As the DRAM was exposed to higher temperatures, therefresh rate would need to increase, leading to more power beingconsumed.

A general rule of thumb is that there is a doubling of refresh ratefor every incremental 15°C increase in temperature. To minimizethis effect, an OCTS can be used to automatically sense the temperatureand program the most efficient refresh rate to restore memory contentswhile minimizing standby current.

MobileRAM's higher performance and low-power management features waswell received by PDA OEMs attempting to provide optimal mobilecomputing without jeopardizing battery life. This led to a directimpact on smart phone development, since many OEMs have chosen to usePDA-type platforms as the basis for these powerful “converged” personalcommunicators.

With the addition of an RF unit and telephony functions, theseplatforms are evolving into full-featured devices. To address thegrowing processing power of new application and graphic units whilemaintaining the low cost required of a consumer product, acode-shadowing memory technique was developed. In a code-shadowedmemory, the MobileRAM's performance benefit of 133MHz is able to exceedNOR flash operating at a top frequency of 80MHz in XIP.

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

The key software feature to manage code between NAND and MobileRAMis demand paging, which swaps pages of code/data between memoriesaccording to processor expectations. To lay the groundwork forselecting the right memory components and memory architecture,engineers must understand power-consumption differences between XIP andcode-shadowing.

Figure2: Wireless platforms allocate more power to a memory subsystem than tothe entire platform.

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

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

Figure3: With the migration toward smart phones, the importance of anoptimized operating current increases.

The trend is toward a platform capable of doing multiple tasks, inaddition to communicating as a voice device, such as serving as an MP3player, Internet browser and video/still camera. These new applicationsdemand more usage from a memory subsystem than before, which MobileRAMis capable of satisfying with its code-shadowing approach.

With CellularRAM, a significant emphasis was placed on loweringstandby current at the expense of performance (Figure 4, below ). While smart-phoneplatforms place large demands on the memory subsystem, sacrificing afast interface is not an option. Reducing operating current by anyamount had greater impact on total power consumption than decreasingstandby current.

Figure4: The most significant reduction in total memory power consumption isachieved by decreasing operating current instead of standby current.

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

The slight increase in operating current of DDRMobileRAM – attributable to the doubling of sense amps -is offset bythelower duration of command execution time (Figure 5, below ).

Figure5: Random READ (eight words) command/performance benefit adds to 12percent power savings for DDR MobileRAM due to shorter duration inactive mode even with a slightly higher operating current.

That allows for further possibilities on increasing systemperformance of future wireless platforms without decreasing batterylife.The introduction of CellularRAM and MobileRAM products into thewireless world brings the cost benefits of DRAM-based memory into themobile communications environment. The flexibility of using DRAM-basedmemory in legacy or next-generation wireless platforms while addressinglow-power management and space-constraint requirements has validatedthe effectiveness of DRAM in these new application markets.

Odilio Vargas is Product MarketingManager, Mobile Business Unit, InfineonTechnologies AG.

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