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Serial ATA - Next Generation Storage Interface |
| Why Do We
Need a New Interface? |
| Limitations of Parallel ATA |
| Serial ATA was designed to overcome a number of limitations of Parallel
ATA. The most significant limitation of Parallel ATA is the difficulty
in increasing the data rate beyond 100 MBytes/s. Parallel ATA uses
a single-ended signaling system that is prone to induced noise. Increasing
the Parallel data rate beyond 100 MBytes/s would require a new signaling
system that would not be backward compatible with existing systems.
Hard disk drives, such as the Deskstar 7K500, 7K250 and 7K80 as well
as the Travelstar 7K100 and 5k100 series currently outrun the 100
MByte/s data transfer rate. As a result, a new interface system has
been defined to accommodate the faster processing capabilities of
next generation high-speed desktop, notebook and entry-server architectures. |
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| Serial ATA has emerged as the industry standard internal storage
interface designed to solve the bandwidth constraints of Parallel ATA, as
well as well as the dependence on 5V signaling lines that are incompatible
with silicon processes used in a wide variety of microprocessors. Serial
ATA overcomes these issues by employing a 250mV differential signaling method.
Differential signaling rejects induced noise. The 250mV differential signal
level is compatible with future microelectronic fabrication processes. |
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| Forecasts indicate ATA dominance |
| ATA is the dominant HDD interface in the industry. The ATA
interface market is expected to be approximately 190 million units in 2003,
accounting for about 90% of all HDDs shipped, according to International
Data Corporation’s (IDC) 2002/03 forecasts. By 2006, IDC projects
ATA unit shipments will increase to beyond 310 million and continue to account
for 90% of all HDDs shipments. It is clear that the market will demand ATA-class
HDDs for the foreseeable future. |
| Serial ATA standards activities |
| The Serial ATA Working Group published the Serial ATA 1.0a
specification in 2003, which was in turn adopted by the ANSI T13 public
standards committee as the ATA/ATAPI-7 V3 specification. In 2004, the Serial
ATA Working Group evolved into a formal organization called the Serial ATA
International Organization (SATA-IO), dedicated to sustain the specification
and create further SATA-based interface solutions. The pioneering work instigated
by the Serial ATA Working Group and carried forward by SATA-IO has resulted
in a number of extensions to the basic Serial ATA 1.0a specification, including
both interface transfer rate solutions of 1.5 Gb/second and 3.0 Gb/second,
as well as optional features such as Native Command Queuing, Asynchronous
Notification, Staggered Spin-Up, Hot Plugability, Link Power Management,
new cabling configurations, Port Multiplier, Port Selector, and Click Connect
to name a few. As of March, 2005, SATA-IO has grown to over 100 members,
and Hitachi Global Storage Technologies was elected to the SATA-IO Board
of Directors. |
| Serial ATA
Features and Specifications |
| The basics |
| Serial ATA is designed to be transparent to the host system
software layer which allows existing operating systems, device drivers and
applications to run without modification. The interface is a 4-wire, point-to-point
configuration — supporting one device per controller connection. Thus,
there are no master/slave configuration jumper issues as there are with
Parallel ATA drives. The SATA interface provides a substantial reduction
in pin count from Parallel ATA, and a smaller cable configuration which
both facilitates air flow as well as improves cable routing. |
| Layering model |
The Serial ATA function is divided into four layers, as shown in
Table 1. The Transport and Link layers control overall operation.
The Application layer is designed to appear identical to Parallel
ATA, thereby maintaining software compatibility. The Physical layer
handles the high speed serial communications between the host and
device.
Serial ATA can transport all ATA and ATAPI protocols, and is designed
to be forward compatible with future ATA and SATA standards. |
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| Physical layer |
| The Serial ATA physical layer (PHY) uses low-voltage (250mV) differential
signaling to enable speeds of 1.5Gb/s and beyond. The roadmap is designed
to carry the interface for 10 years, through 6.0Gb/s. There are 2
differential pairs, one for transmit and one for receive. The PHY
layer incorporates serializer/deserializer, provides out of band (OOB)
signaling, and handles power–on sequencing and speed negotiation.
Transmit Data is serialized from 10-bit characters, and Receive Data
is deserialized to 10-bit characters. Device status feedback is provided
to the to the link layer. |
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| The interface supports both cabled (up to 1 meter) and back-plane
connections. The connectors are designed to blind mate, and staggered
contacts are provided to facilitate hot plugging. There are three
power supply voltages: 12V, 5V and 3.3V. The first generation cables
and connectors are designed to support 3.0Gb/s speed. The connector
location and interface is common for 3.5- and 2.5-inch devices, facilitating
the ability to support multiple form factors within a single bay. |
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| Link layer |
| The Link layer is responsible for sending and receiving frames,
control signal primitives and performing flow control. The Link layer contains
a primitive character encoder/decoder, 8B/10B encoder/decoder, 32-bit CRC
calculator, data scrambler/descrambler and a layer controller. |
| Transport layer |
| The Transport layer handles the packing and unpacking of ATA
and ATAPI information into Frame Information Structures. The Transport layer
also manages the FIFO or buffer memory for controlling data flow. |
| Application layer |
| The Application layer interacts with the Transport layer through
a register interface that is equivalent to that presented by a traditional
Parallel ATA host adapter. A shadow register block is defined that is both
compatible with Parallel ATA and anticipated future extensions. Software
is thus backward compatible with Parallel ATA devices. |
| Serial ATA
Opportunities |
| The opportunity for Serial ATA architectures transcends hard
disk drives and host bus adapters, and encompasses all ATA devices attached
to the system bus. The key to rapid adoption is widespread industry support.
The introduction of Serial ATA support in Intel chipsets is the most significant
step toward the industry adoption of Serial ATA HDDs. |
| At the initial stages, hard disk drives which support the
Serial ATA 3.0 Gb/s interface will be best suited for entry server and the
high-end desktop markets, with 1.5 Gb/s SATA mobile drives appearing in
early notebook PCs. Ultimately Serial ATA should permeate the entire Parallel
ATA market. This will require costs dropping enough to replace Parallel
ATA in non-HDD applications where the interface performance demands are
not as critical, such as from optical and DVD drives. Eventually SATA performance
features and benefits could encroach on markets currently served by other
interface solutions like parallel SCSI. For example, Parallel ATA implementations
are already seen in single user low-end workstation products. Integrators
in these entry enterprise markets including low-end workstations, entry
server and NAS may find Serial ATA an attractive architecture solution. |
| The Serial ATA interface is an important technology upgrade
to the Parallel ATA interface. Serial ATA advantages include the following: |
- Point to point connection which eliminates Master/Slave configurations
- Thinner and longer cables
- Continuity in software drivers — SATA hard disk drives are compatible
- Low differential voltage signals
- Robust migration path for interface performance — bandwidth
today defined at 150, 300 and 600 MBytes/s
- Improved connection design to accommodate hot-plug and blind mate
applications
- 32 bits CRC error checking on all data and control information
|
| References
and specifications |
For more information and the latest specifications, please
refer to the Serial ATA website at www.SATA-IO.org.
For more information on Hitachi Serial-ATA product offerings, please visit
our website at www.hitachigst.com/products |
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Hitachi Global Storage Technologies
© 2005 Hitachi Global Storage Technologies
Hitachi Global Storage Technologies
5600 Cottle Road
San Jose, CA 95193
Produced in the United States 6/03, revised 3/05
All rights reserved.
References in this publication to Hitachi Global Storage Technologies
products, programs, or services, do not imply that Hitachi Global Storage
Technologies intends to make these available in all countries in which
it operates.
Product information is provided for information purposes only, and does
not constitute a warranty. Information is true as of the date of publication
and is subject to change. Actual results may vary. This publication is
for general guidance only. Photographs may show design models.
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