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DS280MB810: Signal Detect Questions

Daniel Jackson
Intellectual 575 points
Part Number: DS280MB810


Looking in the DS280MB810 Programmers' Guide, October 2016 (which is assumed still the current version), I see in Section 6.3, Adjust Signal Detect Threshold on p.14-15, that each voltage range cited (which are in fact just typical range values) has a spread of 3:1 or more. Is such a wide range due to variation of the threshold under various operating conditions, such as per the range of serial datarates supported, as well as temperature and Vcc? Which influence(s) exert the most effect on causing the threshold to vary significantly? What sort of spread could be expected over the full supported range of operating conditions (Vcc and temperature)?

Further, please provide insight into where in each of the six ranges typical thresholds would occur for very low frequency signaling, such as in the range of 10-50MHz. Please also confirm that the IC should have no difficulty in passing such low frequency signaling through, either as continuous square waves, or as an individual pulse as short as two cycles, or such a pulse repeated as infrequently as every 250 milliseconds (with squelch (0V differential signal) occurring between successive pulses).

Related to the above and looking further in the Programmers' Guide and searching for all text matches to "SD_" (thus related to Signal Detect functionality), there were found the SD_REF_HIGH and SD_GAIN register bits (p.31), with however no explanation anywhere in the document regarding what they do and what effect they have (except for the note that SD normal operation results when both bits are true). Since understanding all SD-related capabilities of the part is critical in the application here, and particularly since the threshold ranges cited (p.14-15) may all be too high for the application, please provide detailed information regarding these currently-mysterious control bits.

  • 0
    TI__Mastermind 43795 points

    Hi Daniel,

    The DS280MB810 signal detect circuit is designed to work with Gbps signals.  Signals with long periods of squelch will definately shut off the device datapath.  Lower frequency (10-100MHz) signals can keep the datapath alive, but they must be continuous.

    SD_REF_HIGH is a control to raise or lower the signal detect threshold.  It is in the higher level (less sensitive) state by default.

    SD_GAIN is a bandwidth control.  It is in the lower bandwidth setting by default.

    The wide spread is mostly due to the overall range of datarates and encoding/scrambling schemes supported.

    Regards,

    Lee

     

  • 0 in reply to Lee Sledjeski
    Intellectual 575 points

    Lee,

    The IC would indeed be utilized to process Gbps signaling, with the addition that the protocol also involves low-frequency signaling before and after periods of high-speed signalling, not unlike the well-known lower-speed SATA and USB protocols.

    It is understood that any squelch lasting long enough would shut off the datapath (that is, would result in squelch being signaled out), which is not only fine but absolutely necessary.

    Thanks for outlining the general functionality of the SD_REF_HIGH and SD_GAIN control bits, although I was hoping for something just a bit more quantitative (pun possibly intended).

    And, as already requested, what sort of [SD-threshold] spread could be expected over the full supported range of operating conditions (Vcc and temperature)?

  • 0 in reply to Daniel Jackson
    TI__Mastermind 43795 points

    Daniel,

    The turn-on and turn-off period would result in lost data and may swallow your low speed signaling.  In order to pass OOB signaling in SAS, this device needs to have its SD circuitry overridden and forced into an active state 100% of the time.

    Regards,

    Lee