Host Interfaces

To transfer the data between the CPU and the mass storage device, a host interface for mass data transportation is defined. In general, there are several kinds of host interface defined based on the target applications and the technology level. In Table 2-I, we have listed the major features of several host interface standards. The most popular interface for mass storage device, AT Attachment (ATA) specification [13], is specifically defined for PC mass storage devices, such as Hard Disk Drives, Optical Disk Drives, Solid-State Drives, etc. Serial ATA interface is the advanced high speed serialized ATA interface to support higher data rate up to 3.0 Gbit/s¹ for SATA II standard [14] [15]. The SCSI interface is another popular storage interface for more applications not only specific for PC [16]. The high speed PCI and PCI Express interface are defined for PC-based local bus application internally [17] [18]. Some internally embedded storage devices are using the PCI interface bus, such as RAID (Redundant Array of Inexpensive Disk Drives) devices [35]. The IEEE 1394 interface is a standard interface developed and targeted for multimedia application [19]. The peer-to-peer connectivity of IEEE 1394 creates an inter-operate ability among many Audio-Video devices. Some storage devices for consumer electronics are using IEEE 1394 interface as well. The USB (Universal Serial Bus) is another very popular I/O interface in PC and notebook computer [20]. The mass storage class of USB defines the mass storage device interface on USB bus. The hot-plug capability and plug-and-play function supporting make some removable storage devices use USB interface. Such as:

UFD (USB Flash Disks), UMD (USB Mobile Hard Disk Drives), etc.

A very important application field for the flash memory storage system is the flash memory cards. The flash memory card has become an inevitable peripheral for many

1 Gbits/s stands for Giga (10⁹) bits per second.

consumer electronic devices, such as: DSC (Digital Still Camera), cellular phones, handsets, MP3 player, DV (Digital Video recorder), etc. On the different application consideration, there are also many kinds of the host interfaces defined by the corresponding associations. The CompactFlash card interface is the first successful standard for small-form factor flash memory cards [21]. The CompactFlash interface is a PCMCIA-ATA interface flash memory card, but it is with a smaller mechanical size.

The CompactFlash card interface was successfully adopted by DSC as a storage media to store the pictures. Then, the demand of much smaller devices in consumer electronics requires the size of the flash memory card need be simpler and smaller. The MMC (MultiMediaCard) card interface is developed by MMC Association [22]. The SD (Secure Digital) card defined another flash memory card interface with the security support with content protection [23] [24]. The Memory Stick interface is developed by SONY corporation and applied to the SONY branded devices [29]. Although there are so many different host interfaces for flash memory cards, they are quite similar at the functional and technical point of view. They are all in a kind of the flash memory storage systems with different host interface and mechanical form-factors.

The host interface is a necessary part of the flash memory storage systems. It needs to be defined by the target market and technology consideration. The major factors of the host interface can be divided into 4 categories: the general features, the electrical specification, the interface protocol, and physical specification.

In the general features of Host interface, the target applications, power consumption, hot-plug, plug-and-play are included. The target applications are the most important factor to define where the interface is adopted, and where is the device used.

The power consumption defined based on how much power need for the transmitter and receiver. It’s depends on how long and how fast of the data transfer is required in the application. The hot-plug supportability can support the storage device with the function of removable. The plug-and-play feature defines the support level in the host operating

systems.

In the electrical specification of host interface, the bus bandwidth, interface signaling, the functional block of the hardware interface controller are included. The bus bandwidth of the host interface defines the maximum data transfer between the host and the mass storage device. The general metric for the bandwidth of data transfer rate is MB/sec. The signaling of the interface bus includes the DC and AC specification and signal assignment of the interface signals. In general, there are analog or digital types for interface signaling for data transportation. The functional block description of the interface controller defines how many the functional units and the connectivity and the signal and data flow among the functional units.

In the interface protocol of host interface, the data bus type, the protocol handshaking and operational state machines, and bus topology are included. There are generally two types of data bus of the interface: the serial or parallel bus. The serial interface can save space for the connectors and cables of the interface. While the parallel interface provides the simple logic to increase the data transfer rate in the comparatively lower clock rate. The protocol of the interface defines the data transfer and command handshaking mechanism of the interface. There are Master-Slave or Peer-to-Peer types. In general, the protocol of the interface defines the communication mechanism between the devices in the interface bus. The bus topology defines the interface bus using by the host system and the storage device. There are 4 types of bus topology using now: Master-slave, Point-to-Point, Tree, and Common Bus. The Master-Slave type was developed for the ATA standard. The Point-to-Point can have the independent bus bandwidth; thus, it can maintain a highest bandwidth of the data transfer. The new generation Serial ATA interface adopts the point-to-point connection.

The tree-type bus topology is easier to manage at the host point of view, but the bus bandwidth will be shared to all the devices attached on the bus. The USB interface uses the tree-type bus topology. The common bus was developed to have multiple interface

type in a bus, and can switch the interface mode automatically [109].

In the physical specification of host interface, the mechanical form-factor and dimensions, operating conditions, and testing criteria are included. The mechanical form-factor and dimensions of the interface cables, connectors, and the storage device are defined. The mechanical form-factors and dimensions show the physical outlines of all the components defined in the interface specification. The operating conditions define the operating environment factors of the storage devices. The testing criteria define how to qualify the storage devices to be compliant with the host interface.

In Table 2-I, the major factors of some typical host interface specifications are listed. The SATA II interface shows the highest bandwidth at 300 MB/s². The interfaces of flash memory cards are lower speed, smaller size, and support hot-plug for hot insertion and removal. The analog serial signaling connection is for higher speed interface. From the target application point of view, the flash memory storage systems have three major application fields: the Solid-State Drive (SSD) for PC/Notebook, the flash memory cards for consumer electronics (e.g., DSC, Handsets, etc), the embedded flash modules for IA (Information Appliance or Industrial Application) devices (e.g., IPC, POS, Medical Instrument, etc).

2 MB/s stands for Mega (10⁶) Bytes per second.

Table 2-I. The major factors of some typical host storage interfaces

Interfaces Applications Max. BW Signaling Bus Topology Hot-Plug

Parallel ATA

Digital Parallel Master-slave

No: fixed

Analog Serial Point-to-Point Yes

SCSI Storage Devices

Ultra2: 40MB/s Ultra2W⁴: 80MB/s Ultra3: 160MB/s

Digital Parallel Common Bus No

Serial Attached SCSI

Storage Devices 300 MB/s Analog Serial Point-to-Point No

PCI ^{Embedded 266 MB/s} Digital Parallel Common Bus No

Analog Serial Point-to-Point Yes

USB Mass Storage

Mobile Storage

1.1: 12Mbps 2.0: 480Mbps

Analog Serial Root-and-Tree Yes

CompactFlash Card

Flash Cards

CF 3.0: 66 CF 4.0: 133

Digital Parallel Point-to-Point Yes

MMC Card Flash Cards

3.3: 20Mbps 4.0: 52MB/s

Digital Parallel Common Bus Yes

SD Card Flash Cards

1.01: 12.5MB/s 1.1: 25MB/s

Digital Parallel Point-to-Point Yes

Memory Stick Flash Cards

1.4: 2.5MB/s PRO: 20MB/s HG: 60MB/s

Digital Parallel Point-to-Point Yes

Note: 1. MB/s: Mega Bytes per second.

2. Gbps: Giga bits per second, same as Gb/s . 3. Mbps: Mega bits per second, same as Mb/s.

4. Ultra2W: SCSI Ultra 2 Wide.