Mobile Handheld Devices : MEMORY, STORAGE AND BATTERIES

MEMORY AND STORAGE

Desktop PCs or notebooks usually have between a few hundred Mbytes and a few Gbytes of memory available for users, whereas handheld devices typically have only few tens or hundreds of Mbytes. PDAs normally have more storage space than smart cellular phones, with the former commonly having 64 Mbytes, and the latter a memory size that may be as low as a few Mbytes. Four types of storage are usually employed by handheld devices: (i) flash memory, (ii) hard disks, (iii) random access memory (RAM), and (iv) read-only memory (ROM). Table 1 compares these four types of storage; a comprehensive survey of storage options can be found in Scheible (2002). Today's wireless devices demand higher memory throughput for more advanced features, such as Internet browsing, e-mail, data streaming, and text messaging. Brief introduction of the four types of storage is given next:

• Flash memory: This is a kind of non-volatile storage similar to electrically erasable programmable read-only memory (EEPROM), but updating can only be done either in blocks or for the entire chip, making it easy to update. Flash memory is not as versatile as random access memory because RAM can be addressable down to the byte (rather than the block) level, whereas flash memory allows multiple memory locations to be erased or written in a single programming operation. However, unlike RAM chips, flash memory is a form of rewritable memory chip that is capable of preserving its content without the need for a power supply. Flash memory's price per Mbyte fell 56 percent from 2004 to 2005, followed by another 47 percent drop in 2006 and a further 35 percent in 2007. Figure 1 shows a Palm 3234WW flash memory card.

• Hard disks: Cornice, Inc. introduced the largest capacity version of its fifth generation hard drive, the ultra-slim Dragon series . Available in 8 GB and 12 GB versions, each of which is only 1-inch across, the Dragon series is claimed to be 40 percent smaller than both its predecessors and the drives currently available from its competitors, thus allowing hardware vendors to add larger amounts of storage in smaller devices. Cornice also decreased the power consumption of Dragon by half in the 12 GB model, with the result that it should account for only five percent of the total battery drain within a typical MP3 player system. The new Dragon series drives are protected by Cornice's Crash Guard system, so they are expected to be able to withstand excessive shaking, extreme drops, and other abuse without skipping, stopping, or crashing. Figure 2 shows both (a) exterior and (b) interior views of one of the new 1" 12 GB Dragon hard drives from Cornice Inc.

  Figure 1. Palm 3234WW flash memory card © 2008 Palm, Inc.

  

  Figure 2. A 1-in 12 GB hard drive © 2008 Cornice Inc.

  

• RAM (Random Access Memory): There are two basic types of RAM, dynamic RAM (DRAM) and static RAM (SRAM). Dynamic RAM, the more common type, needs to be refreshed thousands of times per second in order to hold data, whereas static RAM does not need to be refreshed, making it faster but also more expensive than dynamic RAM. Both types of RAM are volatile; i.e., they do not retain their contents when the power is switched off.

• ROM (Read-Only Memory): ROM is manufactured with fixed contents and is usually used to store the programs that boot up the device and perform diagnostics. It is inherently non-volatile storage, in contrast to RAM.

Currently, the cost of NAND flash memory per Mbyte is three times as much as for a minidrive and 100 times as much as for a large capacity drive. Hard drives generally hold up to 400 Gbytes in PCs and up to 120 Gbytes in laptops, while compact-flash-sized minidrives hold up to 6 Gbytes. There are two obstacles preventing flash memory replacing the hard drive:

• Flash memory technology's higher price makes it difficult for the technology to compete with hard drives in larger devices.

• Software applications grow even bigger, thereby requiring more storage capacity.

Flash memory will always be more expensive than hard drives, but in small capacity devices such as digital cameras, cell phones, and embedded controllers, flash memory's benefits are likely to could compensate for its higher costs.

BATTERIES

Replaceable, rechargeable lithium-ion batteries are most commonly used in handheld devices. In smartphones using this kind of battery, the talking time, standby time, and full recharging time currently take a couple of hours, a few hundred hours, and a couple of hours, respectively, and the browsing time will be slightly shorter than the talking time. In the future, it should be possible to use handheld devices without the need to recharge them frequently by replacing the lithium-ion batteries with fuel cells, which although they are not yet practicable are likely to represent the best choice in the long-term. Table 3.6 provides a comparison between lithium-ion batteries and fuel cells, and detailed descriptions are given below.

• Lithium-Ion Batteries: Replaceable and rechargeable, lithium-ion batteries are commonly used by handheld devices and their prices have dropped quickly. They generate electricity by chemical reactions and currently provide one of the best energy-per-weight ratios available for rechargeable batteries. However, the life of this kind of battery is short, generally only a few hours of operating time and few hundred hours of standby time. Figure 3 shows a BlackBerry C-S2 Lithium-Ion battery, which is used for the BlackBerry 8700 and 7100 series.

• Fuel cells: Battery technology will not significantly improve unless and until manufacturers begin to switch to fuel cells, which is unlikely in the near future. A fuel cell operates like a battery, but unlike a battery does not run down or require recharging and will continue to produce energy in the form of electricity and heat as long as fuel is supplied. A fuel cell turns fuel such as hydrogen into electricity using the oxygen in air and a catalyst. The fuel cell harnesses chemical energy trapped in the fuel and converts it into kinetic energy, in this case electricity, without fossil fuels, combustion, or polluting emissions. It offers the prospect of serving as a remarkably efficient and clean source of renewable energy, replacing both batteries and engines in handheld devices. Since the fuel cell relies on chemical energy rather than combustion, its emissions are much lower than emissions from the cleanest existing fuel combustion processes.

  Table 2. A comparison between lithium-ion batteries and fuel cells

  	Contents	Output	Type & Method
  Lithium-Ion Battery	Lithium ions	Electricity	Rechargeable using a power outlet
  Fuel Cell	Natural gas	Electricity and water	Refuelable using fuel such as natural gas

  
  

  Figure 3. A BlackBerry C-S2 Lithium-Ion battery © 2008 Research In Motion Limited