--- /dev/null
+.. _sata-index:
+
+========================
+SATA
+========================
+
+.. note::
+ This chapter is a lightly modified version of the excellent SATA summerization found in Chapter 2 of Erik Landström's Thesis_.
+
+Serial Advanced Technology Attachment (SATA) is a serial link replacement of
+Parallel ATA (PATA), both standards for communication with mass storage devices.
+This high-speed serial link is a differential layer that utilizes Gigabit technology
+and 8b/10b encoding. The link supports full duplex but the protocol only permits frames
+in one direction at a time. The other non-data direction is used for flow control of the
+data stream
+
+.. image:: sata_layers.png
+ :scale: 50 %
+ :align: center
+
+SATA’s architecture consists of four layers, Application, Transport, Link, and Physical.
+The Application layer is responsible for overall ATA commands and of controlling SATA
+register accesses. The transport layer places control information and data to be transferred between
+the host and corresponding SATA device in a data packets. One such packet is called a frame
+information structure (FIS). The Link layer is responsible for taking data from a FIS and
+encode/decode it using 8b/10b. It also inserts control characters for flow control and calculates
+the cyclic redundancy check (CRC) for error detection. Finally the Phy layer’s task is to deliver
+and receive the encoded serial data stream on the wire.
+
+Dword - Data Representation
+===========================
+In the SATA standard the smallest allowed data is a Dword, its 32 bits are divided
+into four bytes. Where each pair of bytes represent a word and a pair of words
+represent a Dword. In this way it’s easy to see that odd number of bytes is not
+allowed in SATA communication.
+
+.. image:: byte_word_dword.png
+ :scale: 50 %
+ :align: center
+
+The Dwords can be represented by either a data Dword or a so called primitive. A
+primitive is a predefined Dword like for example start of frame (SOF) and end
+of frame (EOF).
+
+Primitives
+==========
+Primitives are Dwords with a purpose to enable and control the serial communication.
+They all begin with a control character followed by three other characters to
+fill up the Dword. The control character makes it easy to recognize a primitive from
+a ordinary Dword of a frame. There is 18 different primitives, all with a dedicated
+task like for example mark a frame with a SOF or to provide synchronization
+with the SYNC.
+
+8b/10b - Encoding
+=================
+8b/10b encoding is rather common in high speed applications, it’s used to provide
+bounded disparity but still provide enough toggling to make clock recovery possible
+(synchronize internal clock with the data stream). The bounded disparity means
+that in a string of twenty bits the difference between zeros and ones shall be -2, 0,
+or 2 and with a maximum runlength of five. The drawback is the created overhead
+of two bits per byte making the actual transfer speed of for example 1.5 Gbps link
+to 1.2 Gbps, a loss of 20 %. Since the 8b/10b extends the possible alphabet from
+256 symbols to 1024 it can provide detection and encoding of special characters
+(also called k-characters) in an easy and effective way. This is used in the SATA
+standard by encoding every primitive as such special characters.
+
+
+Out of Band Signaling
+======================
+Since SATA devices and hosts always sends junk over its differential channels,
+when it is idle (otherwise the link is considered lost), there has to be a way of
+recognizing a signal before a link has been initialized. For this SATA uses so
+called out of band signaling (OOB) to initialize a connection between a host and a
+device. The OOB mechanism supports low speed transmission over a high speed
+connection, such as a SATA link. The OOB signals are non-differential but are sent
+over a differential channel. This is possible by letting the differential transmitters
+drive their output pins to the same voltage, resulting in a reduced difference and
+when a preset threshold limit is reached the receiver can recognize the signal as
+OOB.
+
+.. image:: oob_signals.png
+ :scale: 50 %
+ :align: center
+
+As can be seen in the figure there are three types of (actually two
+since COMINIT and COMRESET are equal) valid OOB signals where bursts of
+six ALIGN are sent with different timing. The importance in the signaling lies
+in the timing, it does not really matter if an ALIGN or something else are sent
+because the receiver only detects the drop of voltage difference between rx+ and
+rx-. In the next figure the complete startup sequence is visualized and
+the calibration steps in it are optional to implement. The host sends COMRESET
+until the device is powered on and can respond with a COMINIT. Upon reception
+of the COMINIT the host sends a COMWAKE to the device which shall send a
+COMWAKE back. If this procedure is finished within a correct time the OOB signaling
+ends and the differential communication can proceed with determining the link speed
+(right part of the figure).
+
+.. image:: oob_sequence.png
+ :scale: 50 %
+ :align: center
+
+Physical Layer
+==============
+This section describes the physical interface towards the actual SATA link.
+The features of the phy can be summarized to:
+ - Transmit/Receive a 1.5 Gbps, 3.0 or 6.0 Gbps differential signal
+ - Speed negotiation
+ - OOB detection and transmission
+ - Serialize a 10, 20, or other width parallel data from the link layer
+ - Extract data from the serial data stream
+ - Parallelize the data stream and send it to the link layer
+ - Handle spread spectrum clocking (SSC), a clock modulation technique used
+ to reduce unintentional interference to radio signals
+
+At startup the physical layer is in its OOB state and after a link has been initiated
+it changes to Idle Bus condition and normal SATA communication is now
+supported. Since the SATA connection is noisy the physical layer detects a frame
+when it receives a SOF primitive and it will keep on listening to the incoming
+signal until an EOF primitive is received. Except from FIS the SATA traffic
+also consists of single primitives which all are easy for the PHY to recognize because
+of their starting control character.
+
+Link Layer
+==========
+This section describes the SATA link layer.
+The link layer’s major tasks are:
+ - Flow control
+ - Encapsulate FISes received from transport layer
+ - CRC generation and CRC check
+ - FIS scrambling and de-scrambling
+ - 8b/10b encoding/decoding
+
+A FIS is framed between a SOF and a EOF creating the boundaries of a frame.
+The last Dword before a EOF is the CRC value for the FIS. The CRC is calculated
+by applying the 32-bits generator polynomial G(x) in Equation on every bit in
+every non-primitive Dword in a FIS and then summarize (modulo 2) all these terms
+together with the Initial Value. The CRC is fixed to value of 0x52325032.
+
+.. image:: crc.png
+ :scale: 50 %
+ :align: center
+
+Scrambling a FIS reduces EMI by spreading the noise over a broader frequency
+spectrum. The scrambling algorithm can be expressed as a polynomial or as a linear
+feedback shift register. The scrambling creates a pseudorandom bit pattern of the
+data that reduces EMI. The algorithm resets to a of value of 0xFFFF every time a SOF
+is encountered at the scrambler. The de-scrambler uses the same algorithm on scrambled
+data so it retakes its original form.
+
+.. image:: scrambler.png
+ :scale: 50 %
+ :align: center
+
+It is important that the CRC calculations are made at original data and that
+the scrambling/de-scrambling are made between the CRC and the 8b/10b encoding/decoding.
+The flow control between host and device is managed by sending
+primitives to one another telling its status (which originates from the transport
+layer). Some of these primitives can be inserted into FIS. Primitives are not
+supposed to be scrambled or added to the CRC sum. Internally the flow control
+are regulated by signaling between the layers.
+
+Transport Layer
+===============
+The main task for the SATA transport layer is to handle FISes and a brief description
+of the layer’s features follows:
+ - Flow control
+ - Error control
+ - Error reporting
+ - FIS construction
+ - FIS decomposition
+ - FIS buffering for retransmission
+
+There are eight types of FISes each with its specific 8-bit ID and unique header.
+FISes vary in size from 1 Dword up to 2049 Dwords. The number of bytes in a
+FIS are always a multiple of four so the transport layer has to fill up with zeros if
+there are bytes or bits missing for an entire Dword.
+The flow control in this case is only to report to the link layer that the data buffers
+are close to over- or underflow. Errors detected are supposed to be reported to
+the application layer and the detectable errors are:
+ - Errors from lower layers like 8b/10b disparity error or CRC errors.
+ - SATA state or protocol errors caused by standard violation.
+ - Frame errors like malformed header.
+ - Internal transport layer errors like buffer overflow.
+
+Errors are handled in different ways, for example are resending of complete FISes
+supported for all kind of FISes besides the data FISes (and the BIST FIS which
+is used typically during testing), because that would need buffers in size of 8192
+bytes (maximum supported FIS size). The max sized non-data FIS is 28 bytes so
+the costs of a large buffer can be spared.
+
+Command Layer
+=================
+The command layer tells the transport layer what kind of FISes to send and receive
+for each specific command and in which order those FISes are expexted to be delivered.
+
+
+.. note::
+ This chapter is a lightly modified version of the excellent SATA summerization found in Chapter 2 of Erik Landström's Thesis_.
+
+.. _Thesis: http://www.diva-portal.org/smash/get/diva2:207798/FULLTEXT01.pdf
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