1. 12006 MAPLD International ConferenceSpaceWire 101 Seminar Time-code Enhancements for SpaceWire Barry M Cook Presented by Paul Walker Both of 4Links 2006 MAPLD International Conference Washington, D.C. September 25, 2006

2. 22006 MAPLD International ConferenceSpaceWire 101 Seminar Introduction Time codes can be inserted into a SpaceWire data stream

3. 32006 MAPLD International ConferenceSpaceWire 101 Seminar Introduction Time codes can be inserted into a SpaceWire data stream but there is some uncertainty in their time of arrival – jitter – of some 10 bit times per link

4. 42006 MAPLD International ConferenceSpaceWire 101 Seminar Introduction Time codes can be inserted into a SpaceWire data stream but there is some uncertainty in their time of arrival – jitter – of some 10 bit times per link –1µs per link at 10Mb/s –100ns per link at 100Mb/s

5. 52006 MAPLD International ConferenceSpaceWire 101 Seminar Introduction Time codes can be inserted into a SpaceWire data stream but there is some uncertainty in their time of arrival – jitter – of some 10 bit times per link –1µs per link at 10Mb/s –100ns per link at 100Mb/s We show how this can be reduced to 10ns (or better) per link.

6. 62006 MAPLD International ConferenceSpaceWire 101 Seminar Content SpaceWire links, tokens and time-codes The causes of jitter Experimental verification of jitter Reducing jitter Experimental verification of improvement Removing delay differences Limitations Conclusions

7. 72006 MAPLD International ConferenceSpaceWire 101 Seminar SpaceWire links, tokens and time-codes A SpaceWire link sends sequences of bits, in units of tokens: Nulls are sent if there is no higher priority token, to prevent disconnect detection. 8-bits EOP FCT ESC-Data Data ESC-FCT ESC EEP End-of-packet marker4-bits End-of-packet marker4-bits Flow control token Null (only as part of compound) 4-bits Byte-wide data Time code 10-bits 14-bits

8. 82006 MAPLD International ConferenceSpaceWire 101 Seminar The causes of jitter 2: Receive – synchronization to local clock FCT 4-bits Data 10-bits Null 8-bits Time-code 14-bits Request 1: Transmit – link is busy when time code requested The time between the request and the time-code being sent varies.

9. 92006 MAPLD International ConferenceSpaceWire 101 Seminar Experimental verification – test set

10. 102006 MAPLD International ConferenceSpaceWire 101 Seminar Experimental verification – normal link Link speed 12.5Mb/s – bit period = 80ns, Link idling Shows both random jitter and different delays

11. 112006 MAPLD International ConferenceSpaceWire 101 Seminar Reducing Jitter (1) Normal waveforms Regular spacing of bits (80ns/bit at 12.5Mb/s)

12. 122006 MAPLD International ConferenceSpaceWire 101 Seminar Reducing Jitter (1) Normal waveforms Regular spacing of bits (80ns/bit at 12.5Mb/s) BUT: clock is encoded in this stream (XOR of D and S)

13. 132006 MAPLD International ConferenceSpaceWire 101 Seminar Reducing Jitter (1) Normal waveforms Regular spacing of bits (80ns/bit at 12.5Mb/s) BUT: clock is encoded in this stream (XOR of D and S) SO: bit period does not need to be always the same

14. 142006 MAPLD International ConferenceSpaceWire 101 Seminar Reducing Jitter (2) One, or more, bits can be longer than normal

15. 152006 MAPLD International ConferenceSpaceWire 101 Seminar Reducing Jitter (3) Fix the time between request and time code transmission (to >10-bits)

16. 162006 MAPLD International ConferenceSpaceWire 101 Seminar Reducing Jitter (3) Fix the time between request and time code transmission (to >10-bits)

17. 172006 MAPLD International ConferenceSpaceWire 101 Seminar Experimental verification – normal link Link speed 12.5Mb/s – bit period = 80ns, Link idling Shows both random jitter and different delays

18. 182006 MAPLD International ConferenceSpaceWire 101 Seminar Experimental verification – modified link Link speed 12.5Mb/s – bit period = 80ns, Link idling Shows much reduced jitter, still different delays

19. 192006 MAPLD International ConferenceSpaceWire 101 Seminar Removing delay differences Link 1 delayed by 3970ns Link 2 delayed 1990ns Link 3 delayed by 0ns Early arriving time ticks are delayed at the receivers so that all ticks in the system are synchronized.

20. 202006 MAPLD International ConferenceSpaceWire 101 Seminar Limitations Disconnect detection limits the length of any single bit –If all the delay is placed before the time code, the minimum link speed is 12.5Mb/s; –Distributing delays through the bits of the time- code reduces this to below 2.5Mb/s. Each time-code is 0 to 10 bits longer, so a time-code grows from 14 bits to an average of 19 bits.

21. 212006 MAPLD International ConferenceSpaceWire 101 Seminar Conclusions A small change in the link transmitter can give a very significant reduction in time-code jitter. The receiver need not be altered, so long as its behaviour is deterministic. It is possible to produce accurately synchronized time codes at all nodes in a system. This makes time codes easier to use and enables their use in more applications. And it is all completely compliant with the ECSS standard and interoperable with devices that do not have reduced jitter.