Solid State Removable Storage

Flash memory: An EPROM, or erasable programmable read-only memory, is a type of computer memory chip that retains its data when its power supply is switched off. From this technology we have the modern solid state storage devices known as flash memory or memory cards, including thumb drives and flash drives.

Solid State Disk

The new term being coined in the industry is SSD or Solid State Disk. Other ways it is known includes Solid State Drive, Solid State Storage Drive. Typically when discussing flash memory such as an SD card they are referred to as SD memory. Alternatively, an SSD is not a removable card or USB stick, it is a replacement for the hard drive, where the system operating system typically reside. An SSD uses higher quality flash memory than its portable cousin, the SD.

Development and adoption of SSD technology has been driven by a need for higher input/output (I/O) performance over traditional magnetic media. Random access with low latency makes SSD technology more appealing. In the past it was cost prohibitive to produce SSD storage and technology limited the number of read write operations before deterioration.

Both SD and SSD Storage devices are fast and more durable than magnetic storage. These devices do have a set life expectancy, as they have a finite number of write cycles before performance becomes erratic. The higher quality SSD devices have more write cycles than cheaper SD media, and SSD devices also cost more. To help mitigate deterioration of the transistor switches they utilize wear leveling which is a way to kind of spread out where data is placed on the transistors. Wear leveling is typically managed by the flash controller, which uses an algorithm to arrange data so write/erase cycles are distributed evenly among all the blocks in the device.

Fowler-Nordheim Tunneling

Removable solid state storage devices "Flash Memory" offer an incredible number of options at present and are popular for mall devices, such as digital cameras and PDAs. Flash memory is a type of solid-state technology, which basically means that there are no moving parts. Inside the chip is a grid of columns and rows, with a two-transistor cell at each intersecting point on the grid. The two transistors are separated by a thin oxide layer. One of the transistors is known as the floating gate, and the other one is the control gate. The floating gate's only link to the row, or wordline, is through the control gate. As long as this link is in place, the cell has a value of "1." To change the cell value to a "0" requires a curious process called Fowler-Nordheim tunneling. Tunneling is used to alter the placement of electrons in the floating gate. An electrical charge, usually between 10 and 13 volts, is applied to the floating gate. The charge comes from the column, or bitline, enters the floating gate and flows to a "ground". This charge causes the floating-gate transistor to act like an electron gun. The excited, negatively charged electrons are pushed through and trapped on the other side of the oxide layer, which acquires a negative charge. The electrons act as a barrier between the control gate and the floating gate. A device called a cell sensor monitors the level of the charge passing through the floating gate. If the flow through the gate is greater than fifty percent of the charge, it has a value of "1." If the charge passing through drops below the fifty-percent threshold, the value changes to "0."

Things to Consider

• Capacity: make sure you can meet your storage needs.
• Read Speed: this is typically the advertised speed, such as 50x.
• Write Speed: what really matters, often represented in mb per second.
• Price Per MegaByte: achieve the best bang for your buck.
• Prevalence/Popularity: avoid proprietary types, look for popular open standards.

 

Popular Flash Memory Types

Talk about your lack of industry standard! Some of these are proprietary (Sony is always guilty) while others are becoming more dominant.

Illustrations Below: We took great care to make the card images all relative to each other in size. This will give you a general idea of the actual size of each.

CF Card - Compact Flash

CompactFlash was originally built around Intel's NOR-based flash memory, though it switched over to NAND.

SD Card - Secure Digital

Secure Digital Memory Cards are tiny, easy-to-use memory storage devices made for a wide range of host devices. SD cards provide encryption capabilities for protected content to ensure secure distribution of copyrighted material, such as digital music, video, and eBooks. SD cards are available with storage capacities as high as 4 gigabytes. SD cards are more rugged than traditional storage media. They have an operating shock rating (basically, the height you can drop them from and still have them work) of 2,000 Gs, compared to a 100-200 G rating for the mechanical drive of the typical portable computing device. This translates to a drop to the floor from 10 feet, as compared to a single foot for the mechanical disk drive. SD cards use metal connector contacts, instead of the traditional pins-and-plugs, so they aren't as prone to damage during handling. The SD card was jointly developed by Matsushita, SanDisk, and Toshiba.

Mini SD Card - Secure Digital

Micro SD Card - Secure Digital

SDHC

The SD High Capacity (SDHC) card is the new SD memory card based on the SDA 2.00 specification, introduced by the SD Card Association. This new SDA 2.00 specification enables SD cards to reach higher capacities - 4GB-32GB. Because SDHC works differently then standard SD cards, this new format is not backwards compatible with legacy SD format host devices. More SDHC compatible devices are expected to be available in the second half of 2006. To ensure compatibility, look for the SDHC logo on cards and host devices (cameras, camcorders, etc.) The SD Association has created and defined 3 speed classes to help you identify speed and performance capabilities/minimum requirements of SD/SDHC cards and the host products. The SD Speed Class Ratings specify a minimum sustained write speed for SDHC cards (Class 2: 2 MB/s; Class 4: 4 MB/s; Class 6: 6 MB/s).

SD Duo

These are like regular SD memory cards but have a USB module so they can be directly plugged in to a computer.

 

Popular Flash Memory Size Comparison Illustrations

Illustration A: Panasonic SD Cards

Panasonic offers a wide range of high quality flash memory. From left to right: SD, Mini SD, Micro SD. SDHC (High Capacity) versions are the same size respectively.

Illustration B: Micro SD

As stated previously, the MicroSD and MicroSDHC (SDHC) are the same size. Micro SD cards are about the size of your fingernail. One common use of Micro SD cards is for expansion memory in cell phones. Here we see a Micro SD card next to a paper clip.