Types of Mobile Terminal Memory

Mobile handsets have always required elements of memory and storage as somewhere
for the operating system code to reside and execute. In early
phones, the user may have had access to a very limited area of memory to store contacts,
as an alternative to storing these on the subscriber identity module (SIM).

The increase in volume of applications such as messaging and the ability for
modern phones to support multimedia has increased the demands for storage
capacity within the handset. A typical handset contains a range of memory or storage
elements, which typically include the SIM card, hardware memory elements,
removable cards or memory sticks, and, most recently, hard disk drives.
With users now storing pictures, videos, music, and games on their handsets,
the total storage requirement has risen very rapidly; combined with this is the
increased complexity of multimedia signal processing, placing additional demands
on memory requirements.

Handset Hardware Memory
In common with any processing platform, a mobile handset requires a number of
different memory types to support its processing sequences. The two broad categories
of memory are Random Access Memory (RAM), which is as a working area
and as memory for user and application data storage; the other type of memory is
used to store the main program code and other applications. RAM is volatile and
will lose its contents unless power is maintained, whereas the E2PROM or Flash
memory used for the code storage is nonvolatile.

The RAM types found in mobile handsets include Static RAM (SRAM) and
Dynamic RAM (DRAM). SRAM will hold its contents as long as power is maintained
and, unlike DRAM, does not require refreshment on a regular basis. DRAM
memory stores bits of data as a charge on a capacitor; this charge must be regularly
topped up to avoid it leaking away.

Electronically Erasable Programmable Read-Only Memory (EEPROM or
E2PROM) is a nonvolatile memory type that can be programmed and reprogrammed
many times by means of electrical signals. E2PROMs have life cycles of
between 100,000 and 10 million write operations, although they may be read any
number of times. A limitation of E2PROM devices is that only one memory location
can be written to at any one time, this can be overcome by using Flash memory
devices, which can have a number of locations written in one operation.

The amount of hardware memory in handsets has increased considerably in
recent years as phones have included evermore complex operating systems and
applications, and also as users have required more memory for storing personal
data and multimedia files.

At the low end, the amount of E2PROM or Flash memory required might be
16 MB with an additional 8 MB of SRAM or DRAM. In addition, even low-end
phones might offer users memory of between 5 and 20 MB.
The manufacturing requirement to get more and more memory in a handset,
while at the same time decreasing the size of the device, has led to new techniques
such as multi-chip packaging (MCP) where one physical chip package houses both
SRAM and Flash.

Memory Growth
Driven by multimedia applications, the amount of hardware memory in handsets
has increased dramatically from the relatively small amount in voice-centric handsets
to a total of between 500 MB and 1 GB in 3G devices. The memory is typically
a mixture of RAM and Flash.

Until recently there were two main types of Flash memory in production, and
both of these are encountered in handsets; they differ in regard to the type of logic gate
deployed in the memory matrix — either NOR or NAND. However, hybrid Flash
memory devices are now appearing in the marketplace that combine the best features
of the two memory types, and these may soon find their way into mobile products.
NOR Flash was developed first; and while it is a true random access memory
(any location can be addressed when necessary), it suffers from relatively slow write
and erase times and a more limited lifespan than NAND Flash, which is both faster
and cheaper.

Memory Cards
Driven by appliances such as digital cameras, a number of removable data storage
formats have been invented, some of which are also deployed in mobile phones.
These memory cards or sticks can be used to store music, pictures, videos, and
games, which might have been downloaded or, in the case of pictures and videos,
captured by the user with the handset camera.

These memory devices are intentionally small, often a few tens of millimeters
square, and weigh only a few grams. Their storage capability however might be up
to several gigabytes. Typically, these devices are based on Flash technology and are
housed within the handset in a manner similar to the SIM.