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DS90CR287 no output

Alexander Scheuermann
Genius 14890 points
Other Parts Discussed in Thread: DS90CR288A, DS90CR287, DS90CR285

In a customer's circuit design (Defence Engineering Corp.), they are using the National parts DS90CR288A and DS90CR287.  They are also using the Linear part LTC6995-1 to enable the DS90CR287 through the PWR_DWN(active low) pin.  The DS90CR288A provides the input data and clock to the DS90CR287.  The data and clock output from the 288A National chip looks good, and the enable provided from the 6995-1 looks correct (the delay is in the 30 micro-second range).  The National 288 chip powers up when power is applied to the circuit and it has data immediately for decoding.  The National 287 chip uses the 6995-1 to delay its PLL initialization until a time after the National 288 chip is providing a stable clock, and this has been verified.  However, there is no output from the National 287 chip. 

 

Can you think of any reason that would cause the output to not function? I currently am having them double check their communicaiton with the PWR_DWN pin. Thanks!

  • 0
    TI__Genius 14890 points

    Some clarification:

    Previously, I mentioned that I had the delay of the 6995 chip set to 30 micro seconds, I meant to say 30 milliseconds.  This allows for the 10 ms delay mentioned in the datasheet before the 287 chip from National is enabled.  The timing has been verified using a multi-channel oscilloscope.  So the clock to the 287 chip is powered up and stable long before the chip is enabled by asserting the PWR_DOWN signal.  The problem is something else, but we can't figure out what it is.... just doesn't work.

  • 0 in reply to Alexander Scheuermann
    TI__Expert 3050 points

    Hi,

     

    This is somewhat of an unusual implementation. Why is the customer using a timing device to delay the power on of the receiver? Does their application have special requirements that mandate the use of this timer chip?

     

    Can you provide me with some details about the customer's application?

    • Clock rate?
    • interconnect type?
    • Interconnect length?
    • Scope shot of LVDS clock measured at the termination resistor for the CR288A?
    • Scope shot of LVDS data measured at the termination resistors for the CR288A?
    • Schematic and/or pcb file for these boards?

     

    Mike Wolfe

    DPS APPS / SVA

  • 0 in reply to Mike Wolfe
    TI__Genius 14890 points

    o    Clock rate? 42MHz

    o    interconnect type? LVDS and LVTTL

    o    Interconnect length? around 2 inches

    o    Scope shot of LVDS clock measured at the termination resistor for the CR288A? attached screenshot from single ended probe

    o    Scope shot of LVDS data measured at the termination resistors for the CR288A? attached screenshot from single ended probe

    o    Schematic and/or pcb file for these boards? attached as the file "cl_interface_board.pdf" Note that the LTC6994CS6-1 part has been replaced with the 6995-1 part but is not reflected in the schematic

    I also attached screenshots of the LVTTL signal inputs to the CR287 because that is the part that is not working.  There is also a screenshot from the o-scope of the timer delay.

     

    (this discussion is being held via email due to customer specific information)

  • 0 in reply to Alexander Scheuermann
    TI__Guru* 93670 points

    To close up this issue, please below note the outcome or resolution to this problem. Here is an excerpt from Alexander email:

    Regarding the DS90CR285.  We finally figured on the last issue we were having.  We had already tested the 10 Ohm series termination resistors to reduce the overshoot issue, but that didn't fix the circuit completely.  What we found was that the switching frequency of the DC/DC converter was causing a modulation on the output clock.  I need to correct the problem with how the layout stack-up is defined, then the problem will go away.  I'm confident that this identifies the final issue we were having.

    Regards,,nasser