Articles: Storage

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Compact Flash Format

The Compact Flash format was developed by SanDisk in 1994. October 11, 1995, the Compact Flash Association was established by twelve companies (Apple Computer, Canon, Eastman Kodak, Hewlett-Packard, LG Semicon, Matsushita, Motorola, NEC, Polaroid, SanDisk, Seagate and Seiko Epson) to standardize and promote it. The first revision of the specification was ratified then. The goal of the new standard was to preserve all the advantages of ATA Flash cards while eliminating their main drawback, which was their too big size. Compact Flash got a PCMCIA-compliant 50-pin parallel interface and dimensions of 36 x 43 x 3.3 millimeters. You can install a Compact Flash card into a PCMCIA slot via a simple adapter. All CF cards support two voltages, 3.3V and 5V.

The CD card is based on flash memory of the EEPROM type (Electronically Erased Programmable Read-Only Memory) whose characteristic features are:

  • Lack of moving parts
  • Non-volatile (no additional power is necessary to store the data)
  • High reliability of the chip, tolerance to magnetic fields

Another feature of this memory type is that memory cells are accessed in blocks. A block of several cells is read or written to at once even if only some of the block data are actually required. If data has to be written into a partially free block, the existing information is read and merged with the new information, and then the resulting block is written in full instead of the old one. This method has a rather poor random access time, but also high sequential read speeds.

Memory cells are getting destroyed from being rewritten and their service life is about 100 thousand rewrite cycles. The controllers of modern cards feature special tracking algorithms for distributing data evenly among all the card cells, which makes the service life of the whole card longer. The controllers also keep track of the status of particular cells. When a cell is destroyed, the entire block with that cell is marked as destroyed and is substituted for a reserve block. Each card has reserve blocks for that purpose. And when there are no more reserve blocks, the card capacity begins to shrink as more blocks get destroyed from use. It is a very unlikely event for your flash card to get destroyed fully. More probably, its capacity won’t suit you anymore and you will replace it with a larger card before the described situation.

Interesting to note, early Compact Flash cards used to come in capacities of 2, 4, 10 and 15MB and the standard described a maximum data-transfer rate of 8MB/s.

The standard was evolving steadily towards larger capacities, higher speeds, and broader functionality. It quickly became the most widespread among flash card formats.

In March 1998 the Type II card specification was added (and ordinary cards began to be called Type I). The new cards were thicker (5 millimeters as opposed to 3.3 millimeters) to accommodate more storage space. Type I cards were compatible with Type II connectors and such connectors came to be used everywhere although the cards themselves were getting less and less popular.

In the fall of the same year the CF+ specs (CF 1.4) were written to describe input/output functions for devices designed in Compact Flash format. Various fax-modems, Ethernet adapters, barcode readers and, eventually, TV-tuners, Bluetooth adapters, GPS receivers and Wi-Fi became available in Compact Flash format, making it even more popular. In the spring of 2001 the standard is expanded by adding security features (Secure Compact Flash).

The version 2.0 specification was released on the 16th of June, 2003. The maximum allowable interface speed was increased from 8MB/s to 16MB/s (the then-available chips could only yield 5-7MB/s, so the authors of the specification tried to make some reserve of speed for the future). The support for a DMA interface with UltraDMA-33 mode was introduced for new devices. Considering the growth of card capacities, the makers of new devices were advised to provide support for FAT32 besides FAT12 and FAT16. This should have eliminated the possible problem with 2GB and lager cards.

These future-proof measures proved to be exhausted in just a year and a half. On the 6th of January, 2005, the version 3.0 specification appeared. It increased the speed to 66MB/s while maintaining compatibility with earlier released cards and added support for UltraDMA-66. The maximum storage capacity was increased to 8GB.

And finally, in March 2007 the version 4.1 specification was released to increase the speed to 133MB/s and add UltraDMA-133 mode. You can download the specification from the CFA website after passing a simple registration procedure.

Today, there are thousands of various devices using Compact Flash cards. The maximum storage capacity is 16 gigabytes and Lexar, one of the leaders in this field, has announced a card with a speed of 300x (or 45MB/s).

Now let’s get back to the devices we’ll test today.

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