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SPACEWIRE STANDARD: LOW SPEED SIGNALLING RATES

Chris McClements, Steve Parkes

Space Technology Centre

University of Dundee

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Agenda

Introduction Context Operation at 2 Mbit/s Results Conclusion Future Work

Introduction

SpaceWire Standard indicates link speed from 2-400 Mbit/s– Start-up Link Speed = 10 Mbit/s– Disconnect period = 850 ns

Bepi-Columbo MMO mission data processor requirements– Low power budget– SpaceWire interfaces operate at 2 MBit/s– Start-up rate of 10Mbit/s is not

implemented (non-compliant)

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Context (State Machine)

SpaceWire State Machine

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Context (Timeouts)

Link initialisation– Both ends exchange NULL characters – Both ends exchange FCT characters– NULL: Started -> Connecting

12.8 μs timeout– FCT: Connecting -> Run

12.8 μs timeout– Lower limit for timeout period

11.64 μs (128 * (10 MHz + 10%))

Disconnection Period– No transitions on Data/Strobe for 850 ns– 727 ns - 1 us upper limit (9 * (10 MHz + 10%))

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Operation at 2Mbit/s

Timeout period at 10 Mbit/s– 1 NULL = 800 ns, 1 FCT = 400 ns– Approximately 14 complete NULL characters

can be transmitted in 11.43 μs– Sufficient time to decode NULLs and FCTs

and initiate start-up

Timeout period at 2 Mbit/s– 1 NULL = 4 μs, 1 FCT = 2 µs– Approximately 2 complete NULL characters

transmitted in 11.43 μs– Time to receive and decode NULLs is greater

than 11.43 μs– Link start-up continuously fails

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Operation at 2 Mbit/s

Worst case– EndA is Link-Start, EndB is Auto-Start

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End A EndB

Start End A Ready Ready

EndA Send NULL (2.5 μs) -> Started Ready

EndB Got NULL (8 μs) Started -> Started

EndB Send NULL (2.5 μs) Started -> Connecting

End A Receive NULL (8 μs) ->Connecting Connecting

EndB Send FCT (5 μs) Connecting Connecting

EndA Receive FCT (6 μs) End B Send FCT (5 μs)

-> Run Connecting

EndB Receive FCT (6 μs) Run -> Run

21 us

19 us

Operation at 2Mbit/s

850 ns disconnection period– 727 ns lower limit (8 * (10MHz + 10%))– 1000 ns upper limit (9 * (10MHz - 10%))

500 ns (2MHz) clock period cannot meet the specification– Must adjust the disconnection timeout period

For example – 2 * (2MHz + 10%) = 909.09 ns– 3 * (2MHz – 10%) = 1,666 ns

Device still capable of operating at 2 Mbit/s (clock period = 500 ns)

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Results

For link initialisation to occur– Both ends exchange NULL characters– Then exchange FCT characters– At 2 Mbit/s it takes longer to encode, send,

receive and decode NULLs, FCTs.

Minimum timeout periods become– Started -> Connecting: 21 µs– Connecting -> Run: 19 µs

ErrorReset and ErrorWait do not change– ErrorReset = 6.4 µs, ErrorWait = 12.8 µs

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Conclusion

SpaceWire operates from 2-400 Mbit/s with a start-up rate of 10 Mbit/s

SpaceWire link cannot be compliant at speeds less than 10 Mbit/s without a clock to generate the 10Mbit/s start-up rate

Adjusting the state machine Started and Connecting timeout periods ensure link start-up can occur

Error Recovery time does not change1010

Future Work

Minimum clock speed before:– SpaceWire link fails to start– SpaceWire link cannot meet disconnect

timeout

Power saving on SpaceWire link logic at 2Mbit/s

Support for lower bit rates than 2 Mbit/s

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