ABSTRACT—SpaceWire is a real-time communications network for use onboard spacecraft. It has been designed to connect together sensors, mass-memories, processing units, and downlink telemetry sub-systems onboard a spacecraft into a lightweight, high-performance data-handling network. This paper provides an introduction to SpaceWire and the SpaceWire standard. It describes the key features of SpaceWire. The many current and planned space missions that are using or plan to use SpaceWire for onboard data-handling are also introduced

I. INTRODUCTION

SpaceWire is a communications network for use onboard spacecraft. It is designed to connect high data-rate sensors, large solid-state memories, processing units and the downlink telemetry subsystem providing an integrated onboard, data-handling network. SpaceWire links are serial, high-speed (2 Mbits/sec to more than 200 Mbits/sec), bi-directional, full-duplex, point-to-point data links which connect together SpaceWire equipment. Application information is sent along a SpaceWire link in discrete packets. Control and time information can also be sent along SpaceWire links. SpaceWire is defined in the European Cooperation for Space Standardization ECSS-E50-12A standard.

SpaceWire is based on the “DS-DE” part of the IEEE-1355 standard combined with the TIA/EIA-644 and IEEE-1596.3 Low Voltage Differential Signaling (LVDS) standards. Several problems with IEEE-1355 have been solved in the SpaceWire standard and connectors and cables suitable for space application are defined.

The SpaceWire standard is divided into several protocol levels:

· Physical Level which provides connectors, cables and EMC specifications.
· Signal Level which defines signal encoding, voltage levels, noise margins and data rates.
· Character Level which specifies the data and control characters used to manage the flow of data across a link.
· Exchange Level which covers the protocol for link initialization, flow control, fault detection and link restart.
· Packet Level which details how a message is delivered from a source node to a destination node.

The purpose of the SpaceWire standard is to facilitate the construction of high-performance onboard data handling systems, to help reduce system integration costs, to promote compatibility between data handling equipment and subsystems, and to encourage re-use of data handling equipment across several different missions. Use of the SpaceWire standard ensures that equipment is compatible at both the component and sub-system levels. Processing units, mass-memory units and down-link telemetry systems using SpaceWire interfaces developed for one mission can be readily used on another mission reducing the cost of development, improving reliability and most importantly increasing the amount of scientific work that can be achieved within a limited budget.

II. KEY FEATURES OF SPACEWIRE

Ø SpaceWire is simple and can be implemented in ASICs or FPGAs:

SpaceWire uses data-strobe encoding, where a serial data signal and a strobe signal are sent on two differential pairs. The strobe signal is defined so that clock recovery is achieved by simply XORing together the data and strobe signal. No phase-locked loop is required making it easy to implement a SpaceWire interface in any digital ASIC or FPGA device. The use of LVDS for the physical level, allows complete SpaceWire devices including LVDS drivers and receivers to be implemented on a single chip. Ø SpaceWire is small using few logic gates

SpaceWire interfaces can be implemented in around 5000 to 8000 logic gates which is a small enough number to make it possible to include one or more SpaceWire interfaces together with application logic or a micro-computer on a single chip.

Ø SpaceWire devices are radiation tolerant

Several SpaceWire devices are available or are currently being designed in radiation tolerant ASIC technologies. SpaceWire interfaces have also been implemented in radiation tolerant FPGA devices.

Ø SpaceWire is low power

SpaceWire uses Low Voltage Differential Signalling (LVDS) to help reduce power consumption at high-speed. It also uses relatively few logic gates again helping to reduce power consumption.

Ø SpaceWire can be used as a simple point to point link

SpaceWire is bi-directional using two twisted pairs in each direction. SpaceWire can be used to implement simple point-to-point links or can be used to implement networks using routing switches connected by point-to-point links.

Ø SpaceWire can be used in an arbitrary topology network

There is no restriction on the topology of a SpaceWire network. A network is constructed from point to point links and routing switches. When more than one link connects a pair of routing switches, group adaptive routing can be used to share the bandwidth of the links or to provide for fault tolerance, with rapid recovery from a link failure.

Ø SpaceWire routing switches use wormhole routing

SpaceWire is a packet switching network that uses worm-hole routing switches for routing packets across the network. Worm-hole routing was adopted because it minimizes the amount of buffer memory needed in the routing switches, an important consideration for implementation in radiation tolerant chips where memory is at a premium.

Ø SpaceWire has fault isolation properties

SpaceWire uses Low Voltage Differential Signalling (LVDS) as its physical layer. LVDS uses low voltage and low currents which provide good fault isolation capabilities. For example a short to ground or a short between two SpaceWire signals will not cause the transmitter to burn out.

Ø SpaceWire can recover rapidly from a link failure

SpaceWire routers support group adaptive routing where traffic can be shared across two or more links connecting a pair of routing switches or a routing switch to a destination node. If one link in a group fails the packet currently being transmitted on that link is terminated by an Error End of Packet (EEP) and remainder of the packet not yet sent is split. The next packet to be sent will be automatically routed to a working link in the group.

Ø SpaceWire supports time distribution

SpaceWire provides support for the distribution of time information to all nodes on a SpaceWire network. This can be done with a resolution of a few microseconds. A limited amount of event information can be sent along with time-codes.

III. INTRODUCTION TO SPACEWIRE:

Ø SPACEWIRE PACKETS

Information is transferred across a SpaceWire network in distinct packets. The format of a packet is:

The “Destination Address” is the first part of the packet to be sent and is a list of zero or more data characters that defines the node on the network for which the packet is intended. This list of data characters represents either the identity code of the destination node or the path that the packet will take to get to the destination node. The “Cargo” is the data to be transferred from source to destination. The “End_of_Packet” is used to indicate the end of a packet. The data character following an End_of_Packet is the start of the next packet.

There is no limit on the size of a SpaceWire packet.

Ø SPACEWIRE ROUTING SWITCHES

A SpaceWire router connects together many nodes and provides a means of routing packets from one node to one of many other possible nodes. A SpaceWire router comprises a number of SpaceWire link interfaces and a switch matrix. The switch matrix enables packets arriving at one link interface to be transferred to and sent out of another link interface on the router. Each link interface may be considered as comprising an input port (the link interface receiver) and an output port (the link interface transmitter).

A SpaceWire router transfers packets from the input port of the switch where the packet arrives, to a particular output port determined by the packet destination address. A router uses the leading data character of a packet (one of the destination identifier characters) to determine the output port of the router to which the packet is to be routed. If there are two input ports waiting to use a particular output port when the previous packet has finished being sent then an arbitration mechanism in the output port decides which input port is to be served.

Ø PACKET ADDRESSING

There are two ways of addressing SpaceWire packets: path addressing or logical addressing.

Path addressing is used to specify the path through a network directly. The leading data character of a packet (the destination identifier) contains the required output port number of the router. For path addressing the leading data character is removed after a router has used it to determine the output port for a packet. If a packet has to pass through several routers to reach its destination then a separate data character is used to specify the output port for each router. Hence if there are three routers to be traversed, three data characters will be needed to specify the destination.

Logical Addressing is used to specify the path through a network indirectly via routing tables held in the routers. The leading data character of a packet holds the logical address, which is used to look up the required output port number in the routing table of the router. With logical addressing the leading data character is not normally deleted by the router since the logical address identifies the destination node and will be used by each router encountered on the path to the required destination.

Ø GROUP ADAPTIVE ROUTING AND FAULT TOLERANCE

Two of the key features of SpaceWire networks are group adaptive routing and fault tolerance.

A. Group Adaptive Routing :

SpaceWire routers can implement group adaptive routing. When two or more output ports lead to the same destination (another router or a node) then these output ports may be configured as a group. When a packet arrives at an input port for routing out of an output port that is busy, any other output port in the same group as the addressed output port may be used to forward the packet. Group adaptive routing gives rise to one of the most important features of a SpaceWire network: bandwidth sharing. Links in a group share the total data flow between them. If there are two equivalent links between a pair of routers (and the routing tables are configured appropriately) then data may pass over either of these links. There is twice the bandwidth of a single link available for transferring data between the two routers. If another equivalent link is added then the bandwidth become three times that of a single link. To use this extra bandwidth all that needs to be done is to configure the routing tables to identify the three links as belonging to a group.

B. Fault Tolerance :

Bandwidth sharing leads to another important feature of a SpaceWire network: fault tolerance. Consider the previous example of three equivalent links organized into a group. If one of these links fails then the information flow will be automatically distributed over the other two equivalent links. There is no need for intervention by network management software to do this. Provided the routing tables identify the three links as belonging to a group, the fault recovery is automatic and immediate. The only information lost is the tail end of the packet that was being transferred when the fault occurred.

Ø PRIORITY PACKET DELIVERY

If there are two input ports in a router waiting to use a particular output port, then an arbitration mechanism is used to select which input port is to be served. The arbitration mechanism can include a priority scheme. There is no priority flag available within the header of a SpaceWire packet to specify its priority level. The SpaceWire header only contains address information, so packet priority must be associated with a logical address (or with the input port number). In the routing tables logical addresses may be assigned high or low priority. High priority logical addresses have preferential access to an output port when arbitration takes place. A logical address that has been assigned high priority, acts as a high priority channel across the network from many possible sources to the one destination. If high and low priority access to a particular destination is required then two logical addresses are required for a particular destination, one assigned high priority and the other low priority. A source can then decide which logical address to use when sending a packet to a destination, depending on the required priority of the packet. There is a compromise between the number of destinations that can be addressed and the number of priority levels. With two priority levels it is possible to have, for example, 128 low priority destinations and 96 high priority destinations within the 224 logical addresses available. Priority is a means of providing quality of service control.

It is possible to ensure real-time, deterministic delivery of commands (packets) using priority addresses provided that there are no possible clashes between routes through the network operating priority addressing. This can be achieved, for example, if there is only one node sending out priority commands. If deterministic delivery is required there must also be a limit to the maximum packet size used on the network.

Alternatively nodes can take turns to transmit their high priority packets possibly using time-codes to determine which node transmits next.

Ø TIME-CODES

SpaceWire time-codes provide a means of distributing time information across a SpaceWire system. Time can be distributed across a large network with relatively low jitter. This time information can be provided as ticks or as an incrementing value which may be synchronized to spacecraft time. Time-codes provide a mechanism for supporting distributed system synchronization. They may also be used to implement isochronous communication channels, complementary to the asynchronous channels provided naturally with SpaceWire.

A time-code comprises the SpaceWire ESC character followed by a single 8-bit data character. The data character contains six-bits of system time (time-field) and two control flags. Each SpaceWire node or router contains a six-bit time counter. A node or router acts as the time-code master and is responsible for distributing time. The time-master interface has a “tick” input, which is asserted periodically (e.g. every millisecond) by its host system. When the time-master link interface receives a tick, it increments its time counter and then immediately sends out a time-code with the 6-bit time field set to the new value of the time counter.

When the node or router at the other end of the link receives the time-code it updates its internal time counter with the new time and asserts a tick output signal. The new time should be one more than the time counter’s previous time-value. This fact can be used for checking on time validity. If a node or router receives a time-code that is equal to its current time value then it does not emit a tick output signal. This prevents repeated time-code propagation in a circular network. Possible error situations are described in.

When a router receives a time-code it checks that it is one more than the router’s current time setting. It then increments the router’s time-count and emits a tick signal. This tick signal propagates to all the output ports of the router so that they all emit the time-code. This time-code is the same value as that received by the router since the router time counter has been incremented. If there is a circular connection then the router will receive a time-code with the same time value as the router time counter. When this happens the time-code is ignored. In this way time flows forward through a network reaching all nodes but is suppressed if it flows back due to a circular connection.

With the provision of this basic time distribution function, application level protocols can be used to distribute specific time values at full resolution (not just 6-bits) and to issue time dependent commands etc. The two control flags that are distributed with the 6-bit time-code can be used to broadcast information to all nodes and routers on the network.

IV. SPACEWIRE DEVICES

Two devices were developed by Astrium GmbH and Atmel, the SMCS332 (Atmel part number TSS901E) and the SMCSLite (Atmel T7906E) devices, which are based on the IEEE 1355 standard. These devices are currently being upgraded to be fully compliant with the SpaceWire standard. Several other devices are also being developed including a SpaceWire router and a micro-controller with SpaceWire interfaces.

A. SMCS332SpW

The SMCS332SpW is a fully SpaceWire compliant version of the Atmel TSS901E device. It has three SpaceWire interfaces operating at up to 200 Mbits/s, and an interface to a host computer which includes six DMA controllers: transmit and receive for each SpaceWire interface. This device is suitable for connecting a radiation tolerant computer system into a SpaceWire network. The SMCS332SpW provides support for time-codes and is largely pin compatible with the previous TSS901E device.

B. SMCSLite SpW

The SMCSLiteSpW is an upgrade to the Atmel T7906E device and is fully compliant to the SpaceWire standard. It is designed to connect peripheral sub-systems to a SpaceWire network. It has one SpaceWire interface and a variety of other input/output facilities including: a host interface for configuring the device, ADC and DAC interfaces, FIFO interface, memory interface, UART interface, and a general purpose digital interface.

C. SpaceWire Router ASIC

A radiation tolerant SpaceWire router ASIC is being developed by University of Dundee, Austrian Aerospace, EADS Astrium GmbH and Atmel. The architecture of this device is illustrated in Fig. 1.

There are eight SpaceWire ports, two external parallel ports and an internal configuration port in the SpaceWire router. A low latency, worm-hole routing, non-blocking, crossbar switch enables packets arriving at any SpaceWire port, external port or generated in the configuration port to be directed out of any other SpaceWire or external port, or to be routed to the configuration port.

The SpaceWire ports are fully compliant with the SpaceWire standard providing high-speed, bi-directional communications. The external ports each comprise an input FIFO and an output FIFO and can receive and send data characters and end of packet markers. A time-code port is also provided along with a time-counter to facilitate the propagation of time-codes. When a valid time-code arrives at a router port it is sent out of all the other SpaceWire ports and a TICK_OUT signal is generated at the time-code port. The router can operate as a time-code master using the TICK_IN provided in the time code port.
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The configuration port is accessible via any of the SpaceWire or external ports. It contains registers which control the operation of the SpaceWire ports, external ports and the crossbar switch. The configuration port holds status registers for the various ports and the switch. These registers can be read using a configuration read command to determine the status of the router and to access error information. Status and error information can also be selected for output on several status pins. The routing table is accessed via the configuration port. The logical address port mappings and priority bits can be set in the routing table. The routing table is used to control group adaptive routing and priority arbitration in the crossbar switch.

The SpaceWire router has been implemented in Xilinx Virtex-E and Virtex-2 devices. The SpaceWire ports operate at a maximum data signaling rate of 200 Mbits/s. The Virtex-2 device is fully compliant with the SpaceWire standard and is functionally representative of the SpaceWire router ASIC.

The SpaceWire Router (SPROUT) ASIC is being implemented in an Atmel MH1RT gate array with maximum gate count of 519 kGates (typical). This technology uses a 0.35 μm CMOS process with a radiation tolerance of up to 300k rad and SEU free cells up to 100 MeV (used for all critical memory cells) as well as latch-up immunity up to 100 MeV. The maximum baud-rate of the SpaceWire link interfaces of the SPROUT will be 200 Mbit/s, where LVDS I/Os integrated onto the chip are used. Its estimated power consumption at the maximum data rate will be about 4 W and it will operate from a single supply voltage of 3.3V (±0.3V). The package is a 196 pin ceramic Metric Quad Flat Package with 25 mil pin spacing.

V. INTELLECTUAL PROPERTY

An important element in the development of SpaceWire based systems is the availability of validated SpaceWire Intellectual Property (IP) in the form of VHDL designs for key subsystem elements: CODEC and router.

A. SpaceWire CODEC IP

The University of Dundee with ESA funding has developed a high-performance SpaceWire coder/decoder (CODEC) in VHDL that operates at a maximum data rate of 200 Mbits/s when implemented in a radiation tolerant ASIC. The interface is fully compliant with the ECSS SpaceWire standard.
The SpaceWire CODEC VHDL IP is available from ESA for European space projects. It is now being widely used for ASIC and FPGA designs where a SpaceWire interface is required.

B. SpaceWire Router IP

The University of Dundee have developed generic SpaceWire router IP that can be implemented in a range of different technologies (FPGA and ASIC). The number of SpaceWire ports and external ports are controlled by VHDL Generics along with various other characteristics, allowing the Router IP to be customised to particular applications. An example of this is the use of the router IP in an image processing chip being developed for an intelligent camera system for a planetary lander. A SpaceWire router with three SpaceWire ports and two external ports is embedded inside the image processing chip to provide control, data and test access to the device.

The SpaceWire router IP is also used in a router FPGA device developed by the University of Dundee, which has eight SpaceWire ports and one external port.

SpaceWire CODEC IP is available from ESA for European space applications. This IP is available from the University of Dundee for other countries and other applications. The SpaceWire router IP is also available from University of Dundee.

A range of SpaceWire development and test equipment and support services are available from STAR-Dundee Ltd, a spin-out company set up to support users of SpaceWire.

VI. MISSIONS USING SPACEWIRE

SpaceWire elements are being used on the following ESA missions: METOP, Rosetta, Mars-Express, Herschel-Planck, Cryosat, GOCE, COROT, ISS EDR, Galileo, JWST, Bepi Colombo and GAIA .

SpaceWire is being used on the following NASA missions: SWIFT (on-orbit), JWST Phase-B, LRO Phase-A, GOES-R Phase-A, Hubble Robotic Repair Mission Phase-A. Several other NASA missions in the formulation stage are proposing SpaceWire. SpaceWire is also being used on several non NASA USA missions.

JAXA are using SpaceWire for the following: Balloon experiments, Bepi Colombo, NeXT and plan to use SpaceWire extensively on future space missions.

REFERENCES:

Ø http://en.wikipedia.org/wiki/Spacewire
Ø http://spacewire.esa.int