12006 MAPLD International ConferenceSpaceWire 101 Seminar SpaceWire Physical Layer Issues 2006 MAPLD International Conference Washington, D.C. September

12006 MAPLD International Conference SpaceWire 101 Seminar

SpaceWirePhysical Layer Issues

2006 MAPLD International Conference

Washington, D.C.

September 25, 2006

Shaune Allen

NASA Goddard Space Flight Center


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


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


Skew and Jitter


Contributors to Skew

Encoder Driver

Driver Decoder

PCBSkew

PCBSkew

ConnectorSkew

Cable Skew

ConnectorSkew



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


LVDS Threshold Levels


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

200mV

200mV

VOS = 1.2V

2.4V

0V



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


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


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.


Eye Patterns


Eye Patterns


Eye Patterns



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



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)


Suggested Changes to Standard

Add 26 AWG version of SpW cable to detail spec

Allow use of twinaxial connectors


SpaceWire Testing

Bit Error Rate Measurement

Shielding Effectiveness

Near End Crosstalk (NEXT)

Ground Bounce Susceptibility

LVDS Common Mode Range

Common Mode Rejection

Documents

12006 MAPLD International ConferenceSpaceWire 101 Seminar SpaceWire Physical Layer Issues 2006 MAPLD International Conference Washington, D.C. September