Comprehensive analysis of mobile storage

It seems like a new mass storage standard is introduced every day. CompactFlash (CF) once dominated the market, but today, new device manufacturers have to choose between CF, Secure Data (SD), SDIO, MultiMediaCard (MMC), RS-MMC, MMC Plus, MemoryStick, XD-Picture (XD), CE-ATA, and more. Sometimes a new standard has a clear advantage; other times, the new and existing standards seem to be a repeat of the battle between the Betamax and VHS formats, and it is difficult to see who will win in the early stages.

　　The CF standard is the ancestor of all small mass storages and was developed by SanDisk more than 10 years ago. It has an 8 or 16-bit parallel data bus with a transfer rate between 3 and 66MBPS. The CF standard is still needed in many high-transfer rate and high-capacity devices that require a 16-bit wide UDMA bus (66Mbps). Currently, the available capacity of CF+ format hard drives can reach up to 10GB, while the maximum capacity available for CF flash memory cards is 8GB.

　　Because the first CF cards used NOR flash memory, the CF standard requires that the CF card hide the interface with NAND flash memory. When the host communicates with the CF card, it will see it as an IDE hard disk, and the controller on the CF card is responsible for bad block management, wear leveling algorithms, and logical-physical mapping required to work with NAND flash memory.

　　Although CF cards have never become the mainstream removable storage for mobile phones, they have maintained a very high market share in the field of digital cameras, especially in the high-end market where large capacity and high speed are extremely important. Currently, several other peripherals can also be connected to the CF+ version of the interface, including Ethernet, RS-232, fax/modem, USB, Bluetooth and 802.11b WLAN.

　　Multimedia Memory Card (MMC)

　　In 1997, Siemens and SanDisk introduced MultiMediaCards, which are smaller than CF cards, allowing for smaller portable devices. In basic applications, MMC can be controlled via a standard three-wire SPI interface plus a chip select line. The SPI interface has a clock frequency of up to 20MHz. For applications that require higher bandwidth, the specification provides expanded 4- and 8-bit bandwidths. Version 4.0 of the MMC specification adds a 52MHz frequency, supporting a transfer rate of 50MBPS.

　　Unlike CF, the MMC specification does not waive licensing fees. According to information provided on www.MMCA.org: If you are not an MMC manufacturer, you can order MMC version 3.1 or 4.1 (MMCmobile and MMCplus) specifications for $500 or $1,000 respectively, and your company does not need to become a member of MMCA.

　　There are currently three types of memory cards based on the MMC framework: MMC Plus, MMC Mobile, and MMC Micro. MMCplus is a nominal-size MMC card that operates at 2.7 to 3.6 V; has a bus bandwidth of 1, 4, or 8 bits; a minimum read and write performance of 2.4 MBPS and a frequency of 26 MHz (52 MHz is optional). MMCmobile is smaller in size and supports lower voltages: 1.65 to 1.95 V and 2.7 to 3.6 V. MMCmobile must also support the performance that MMCplus needs to provide. MicroSD is the latest addition to the series. MicroSD is less than 1/3 the size of miniSD and is the smallest memory card currently available (Table 1).

　　Table 1: Comparison of the main parameters of various memory cards.

　　MMC and SD Cards: What's the Difference?

　　The MMC standard and the SD standard are often confused, but in fact, they are two different standards. The SD card specification is owned by an organization led by Panasonic, Toshiba and SanDisk, while the MMC specification is controlled by the MMCA (MultiMedia Card Association), which is led by a wide-ranging industry organization.

　　Somewhat surprisingly, the impetus behind the SD card has never been widely recognized by the industry. The SD card has encryption hardware similar to Sony's MagicGate, which is used in Sony's MemoryStick products. It took more than eight years for the SD card to gain industry acceptance before the music industry accepted the digital distribution of music, and now the SD card has become a sideline in this field. Early last year, the MMC Association accepted the competing security card standard, Secure MMC version 1.1 specification. An overview of Secure MMC can be found on the Samsung website www.samsung.com.

　　MMC cards are inserted into a physical slot designed for SD cards, which come in two flavors: thin and standard. Thin SD cards can be inserted into an MMC slot, but standard SD cards cannot due to their thickness. The protocols used by MMC and SD cards were fully compatible in SD card specification rev 2.11, but since then the two specifications have diverged somewhat.

　　Figure 1: The difference between a 7-pin MMC card and a 9-pin SD card is clearly visible.

　　The pinouts of MMC and SD cards are compatible (Figure 1). SD cards have a maximum of 9 pins, while MMC cards have a maximum of 13 pins (Figure 2). The only function of the extra pins on the MMC card is to increase the bus width (Table 2). Because the bus width can be programmed, the controller can easily find the common characteristics and set up accordingly. All microprocessors with built-in MMC support also support SD cards.

　　Smaller size: MMCmicro vs. MicroSD

　　The two different standards created by the MMC and SD organizations for small flash memory cards have caused confusion in the industry. Through the use of mechanical adapters, both MMCmicro and MicroSD (also known as TransFlash) are backward compatible with existing SD/MMC slots (Figure 3). Both memory cards are small, but MMCmicro is faster than MicroSD. MMCmicro uses the higher 52MHz clock rate defined by the MMC specification, while MicroSD continues to use 25MHz. In addition, MMCmicro cards have a 4-bit data bus, while MicroSD only supports serial data transmission. In addition, MMCmicro supports 1.8V voltage, while MicroSD can only operate at 2.7 to 3.6V voltage.

　　Figure 2: The 13-pin MMC card is backward compatible with the 7-pin version.

　　XD-Picture Card

　　The XD-Picture card (hereinafter referred to as the "XD" card) was launched in July 2002. Like Sony's MemoryStick, it is a proprietary format, so it is difficult to find out more information from the XD card official website (www.xd-picture.com). If you want to know how much your company will have to pay to license the use of XD cards, you must sign a non-disclosure agreement with the XD card licensor.

　　XD cards are similar to the SmartMedia standard in that they are both packaging technologies for raw NAND flash memory. There is no embedded controller in the XD card, so the control CPU is responsible for maintaining the logical-physical table, managing bad blocks, and performing error correction. The advantage of this architecture is that it reduces silicon area and allows the management CPU to have more interface control capabilities, thereby reducing write time. The disadvantage of this architecture is that the management CPU must perform all SmartMedia control functions.

　　Figure 3: Pin arrangement of MMCmicro, MMC and SD.

　　SDIO

　　SDIO defines a peripheral interface on top of the SD standard. Currently, there are two main types of SDIO applications - removable and non-removable. Current removable devices are used as expansion devices for Palm and Windows Mobile to add Bluetooth, camera, GPS and 802.11b functions. Non-removable devices follow the same electrical standards, but are not required to meet the physical standards. Some mobile phones contain 802.11 chips that connect to the CPU via SDIO. This move frees up "precious" I/O pin resources for more important functions.

　　Bluetooth, camera, GPS, and 802.11b devices have application profiles defined for them. These application profiles are very similar to the class profiles defined for PCI and USB devices. They allow any host device to "talk" to any peripheral device, as long as they both support the application profile.

　　An important difference between the SDIO and SD Card specifications is the addition of the low-speed standard. SDIO cards only require SPI and 1-bit SD transfer mode. Low-speed cards target applications that support low-speed I/O capabilities with minimal hardware overhead. Low-speed cards support applications such as modems, barcode scanners, and GPS receivers. For "combo" cards (memory + SDIO), full-speed and 4-bit operation is mandatory for both the memory and SDIO portions of the card.

　　CE-ATA

　　CE-ATA helps simplify the combination of consumer electronics (CE) and ATA hard disks. There is a growing trend of using hard disks in consumer electronics, but in compact handheld devices, the 40-pin ATA connector and the 50-pin CF connector really take up too many I/O resources. In the CE-ATA specification, the ATA command structure is overlaid on top of the MMC physical layer. This process allows the reuse of existing MMC controllers built into today's commonly used application processors. The CE-ATA connector uses 12 pins to implement a 9-pin SD/MMC interface. The extra 3 pins provide reserved pins and an extra pair of power grounds so that the hard disk motor can operate at a different voltage from the signal line. The CE-ATA interface performance is the same as that of 4-bit SD/MMC. When using 25MHz and 52MHz clocks, its maximum transfer rate can reach 12.5MBPS and 26MBPS respectively.