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3 The SpaceWire link
The SpaceWire standard [AD1] defines a full duplex bit serial point to point link with a raw transmit rate of up to 400
Mbit/sec. The link consists of 2 signals in each direction, one for strobe and one for data. By coding the strobe that it only
changes level when the data does not, clock recovery and data synchronization can be achieved by XOR-ing of data and
strobe signals without having the need to run the strobe at very high frequencies. The Character level defines the data and
control characters used to manage the flow of data across a link. The exchange level of the protocol is used to implement
flow control which avoids overflow of the front end buffers. Also error detection is provided by implemented parity checks
during transmission and by timeout supervision in case of inter-connect failures.
The SpaceWire standard aims only to define a transport medium between two nodes and covers the protocol layers only up
to the packet level. This has two consequences:
•
packets with address headers allow to use this link standard in networks using routers,
•
since the standard does not define the data payload within the packets, an efficient transaction layer definition is
missing.
To compensate for these deficiencies of the SpaceWire specification, the SMCS332SpW implementations (the
SMCS332SpW and the SMCS116SpW) introduce an (optional) transaction layer extension to the SpaceWire protocol
standard. This high-level protocol extension supports applications in fault tolerant systems, heterogeneous architectures,
feature power saving modes and remote configuration of the communication controller and autonomous command execution.
With this flexible and powerful protocol, the SpaceWire link has many advantages over commonly used interface solutions
such as RS-485 etc.
3.1
Data/Strobe SpaceWire signals
The SpaceWire links use a protocol with two wires in each direction, one for data and one to carry a strobe signal and are
also referred to as data/strobe (DS-Links). Each DS pair carries characters and an encoded clock. The characters can be data
or control characters. The figure below shows the format of data and control characters on the data and strobe wires. Data
characters are 10 bits long and consist of a parity bit, a control flag which is set to 0 to indicate a data character and 8 bits of
data. Control characters are 4 bits long and consist of a parity bit, a control flag which is set to 1 to indicate a control
character, and 2 bits to indicate the type of control character. One of the four possible control characters is the escape code
ESC. The ESC code is used to form control codes. Two control codes are specified, the NULL code and the time code. The
NULL code is transmitted whenever a link is not sending data or control characters. The time code is used to distribute
system time over a SpaceWire network, which comprises a ESC control character followed by a single data character.
The DS-Link protocol ensures that only one of the two wires of the data strobe pair has an edge in each bit time. The levels
on the data wire give the data bits transmitted. The strobe signal changes whenever the data signal does not. These two
signals encode a clock together with the data bits, permitting asynchronous detection of the data at the receiving end. The
data and control characters are of different lengths, for this reason the parity bit in any characters covers the parity of the data
or control bits in the previous characters, and the control flag in the same character, as shown in the above figure. This
allows single bit errors in the character type flag to be detected. Odd parity checking is used. Thus the parity bit is set/unset
to ensure that the bits covered, inclusive of the parity bit (see below figure), always contain an odd number of 1's. The
coding of the characters is shown in the table below To ensure the immediate detection of parity errors and to enable link
disconnection to be detected NULL code are sent in the absence of other tokens.
SMCS332SpW
User Manual
– All Rights Reserved – Copyright per DIN 34 –
EADS Astrium GmbH, ASE2
Doc No: SMCS_ASTD_UM_100
Issue:
1.4
Updated: 9-Sep-2006
Page:
14 of 131
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