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SpaceWire Physical Layer Issues

SpaceWire Physical Layer Issues. Shaune Allen NASA Goddard Space Flight Center. 2006 MAPLD International Conference Washington, D.C. September 25, 2006. Problem Statement . Chapter (Clause) 5 Of the SpaceWire Standard (ECSS-50-E-12A) provides a good basis for the SpaceWire Physical Layer

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SpaceWire Physical Layer Issues

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  1. SpaceWirePhysical Layer Issues Shaune Allen NASA Goddard Space Flight Center 2006 MAPLD International Conference Washington, D.C. September 25, 2006

  2. Problem Statement • Chapter (Clause) 5 Of the SpaceWire Standard (ECSS-50-E-12A) provides a good basis for the SpaceWire Physical Layer • SpW issues should be carefully considered by the designer to insure requirements are understood and met

  3. What are the Requirements? • Jitter • not specified in the SpW Standard • Many use the recommended maximum 20% tui • National Semiconductor AN-1088 • Actual application may require a different requirement • Eye Pattern measurement • Skew • Pair to Pair: <0.15ns/m • Within Pair: <0.1ns/m • Bit Error Rate • not specified in the SpW Standard • often <1.0E-12 using worst case pseudo-random sequence • Actual application may require a different requirement

  4. Skew and Jitter

  5. Contributors to Skew Connector Skew Encoder Driver PCB Skew Cable Skew Connector Skew Decoder Driver PCB Skew

  6. What are the Requirements? • LVDS Threshold Levels • ± 100mV or minimum 200mV eye height • LVDS Common Mode Range • not specified in the SpW Standard • Often ±1V • Actual application may require a different requirement

  7. LVDS Threshold Levels

  8. 2.4V 200mV VOS = 1.2V 200mV 0V Common-Mode Range • LVDS signal swing should remain within the 0-2.4V range • Check your with LVDS manufacturer for specific range for your application • Common-mode range dependent upon length of cable assembly and data rate

  9. What are the Requirements? • Cable Length • 10m suggested (should vs. shall) • Up to 20m shown to operate at applicable rates • Actual application may require a different requirement • Wire Gauge of signal wires • AWG 28 consisting of seven strands of 36 AWG wire • Current petition to add AWG 26 version to the Standard • Differential Impedance • 100 ± 6  • Standard requirement • Make sure design, analysis, and test supports this requirement

  10. Cable length vs. data signaling rate • ANSI/TIA/EIA-644-A offers the following guidelines • The maximum permissible length of cable separating the generator and the load is a function of • data signaling rate • influenced by the tolerable signal distortion • the amount of longitudinally coupled noise • common potential differences introduced between the generator and the load circuit commons • cable balance

  11. Cable length vs. data signaling rate • ANSI/TIA/EIA-644-A offers the following guidelines • Eye patterns are recommended to determine the amount of jitter at the load at the application data signaling and comparing that to system requirements • Typically, maximum allowable jitters tolerances range from 5% to 20% depending upon actual system requirements • Testing should be done in the actual application if possible, or in a test system that models the actual application as close possible • Parameters that should be taken in account include • balanced interconnect media characteristics • Termination • protocol and coding scheme • worst case data patterns (I.e. pseudo random). • The generator / receiver manufacturers and also the media manufacturers should provide additional guidance in predicting data signaling rate versus cable length curves for a particular generator / receiver and a particular media as this relationship is very dependent upon the actual characteristics of the selected devices and media.

  12. Eye Patterns

  13. Eye Patterns

  14. Eye Patterns

  15. What are the Requirements? • Insertion Loss (or Attenuation) • not specified in the SpW Standard • Consider the requirements of the application • Return Loss • not specified in the SpW Standard • Consider the requirements of the application • Shielding Effectiveness • not specified in the SpW Standard • Consider the requirements of the application • Near End Crosstalk (NEXT) • not specified in the SpW Standard • Consider the requirements of the application

  16. What are the Requirements? • SpaceWire Standard specifies MDM-9 connector • Mismatched to 100 differential impedance of SpaceWire cable • Mechanical issues (mate/demate) • Inner shields (signal ground) pass through a single pin • Unable to pass inner shields through bulkheads • Near-end Crosstalk (NEXT)

  17. Suggested Changes to Standard • Add 26 AWG version of SpW cable to detail spec • Allow use of twinaxial connectors

  18. SpaceWire Testing • Bit Error Rate Measurement • Shielding Effectiveness • Near End Crosstalk (NEXT) • Ground Bounce Susceptibility • LVDS Common Mode Range • Common Mode Rejection

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