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CDCUN1208 Startup Issue

MARK FEYDO
Prodigy 20 points

Hi,

   I have designed the CDCUN1208 clock driver onto my board and am having problems getting it to start reliabily on all boards.  Two boards start perfectly every time, three boards sometimes start and sometimes fail to start and one board never starts.  By "start" I mean the output is or is not present.  Note: the input clock is always present at the frequency (125MHz) and voltage (1.8V) expected.

   I am using the "pins" method to configure the part.  Initially I have the pins tied as follows:

Mode = Open  ...  selects "pins" mode of configuration

OE = High (1.8V)  ...  outputs enabled  ///  ERC = Low

INSEL = LOW (GND)  ...  input #1 is selected  ...  I only have one input and it is connected to input #1

ITTP = open  ...  LVCMOS input  (note both the crystal and the CDCUN are running from the same 1.8V supply)

OTTP = LOW (GND)  ...  LVDS outputs

One of my first thoughts was that the open / floating MODE input might not be floating at the correct level and that the part was waiting for I2C or SPI communication to configure the part  ...  so I added a resistive divider and forced the level of the MODE pin to 1.05V (half way between the 0.9 and 1.2V levels specified for 'open').  With this voltage applied I still am not able to get the outputs to reliabily appear.

I'm not sure what to try next.  Any suggestions would be appreciated.

Thanks,

mark

  • 0
    TI__Mastermind 32575 points

    Are you compliant with the power sequencing and ramp rates Per the RECOMMENDED OPERATING CONDITIONS notes (1) and (2) on page 5 of the data sheet?

    Regards,

    Alan

  • 0 in reply to Alan O
    Prodigy 20 points

    Hi Alan,

    I am using the same 1.8V supply for both the core and the IO supplies  ...  thus I am compliant on this point.

    However, note (2) on the slew rate of the supply is new  ...  it was not in the January 2013 version of the data sheet I used in the spring when I designed the board.  I see this note has been added in the July 2013 revision.  A quick check of my board shows my ramp rate on the 1.8V supply to be much slower than the 6000V/s specification.  Guess some changes are in order.

    Thanks,

    mark