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CDCE925: Can not adjust frequency if clock input is from buffer

user5883945
Expert 1526 points
Part Number: CDCE925

Hi

  When the external clock is provided using the clock buffer, the frequency cannot be adjusted; It can be adjusted when using the crystal directly as an external clock source.

REN_8L30110_DST_20180411.pdf

  • 0
    TI__Genius 16580 points

    Hello,

    Are you trying to use the part in VCXO mode, in XTAL mode, or in LVCMOS clock mode for the input buffer? If in VCXO mode, this will not work without an XTAL input.

    Can you please provide the configuration that is being used for both conditions?
    Thanks,

    Kadeem

  • 0 in reply to Kadeem Samuel
    Expert 1526 points

    Hi Kadeem,

       They can not adjusted output frequency when input is clock buffer in LVCMOS mode. Is there any other reason.

  • 0 in reply to user5883945
    TI__Genius 16580 points

    Hello,

    Is the series output resistor required by the 8L30110 clock buffer present?
    Is the clock present and register changes are not changing the output frequency? If they write and read back, are the registers the same as the written values (in other words, is the device recognizing the I2C writes)?
    Is the pass-through path for Y1 able to output a 24 MHz LVCMOS clock when Pdiv1 is set to 1?

    Thanks,

    Kadeem

  • 0 in reply to Kadeem Samuel
    Expert 1526 points

    Hi,

       The input is LVCMOS signal, dose it can adjust by VCXO. Customer's issue is that they can not use VCXO adjust output frequency if the input come from clock buffer LVCMOS signal. 

  • 0 in reply to Kadeem Samuel
    Expert 1526 points

    Do you have a solution to solve problem if can not use VCXO at LVCMOS input.

    The problem as below，

    The input 24Mhz clock is also mcu clock input, so the VCXO need have no influence for mcu clock. MCU need clock first power on for CDCE I2C set, so mcu can not use the CDCE925 output for clock input.

  • 0 in reply to user5883945
    TI__Genius 16580 points

    Hello,

    If using an LVCMOS clock input, the VCXO input adjustment is not valid. The VCXO works by adjusting the internal capacitive loading on the external XTAL to tune the XTAL. If there is no XTAL, no tuning can occur. Modification of the output frequency with an LVCMOS clock is only achievable by adjusting the PLL and divider settings.

    Thanks,

    Kadeem

  • 0 in reply to Kadeem Samuel
    Expert 1526 points

    Hi，

      Do you have advice if customer need XTAL to VCXO and the MCU clock is not affected.

    The input 24Mhz clock is also mcu clock input, so the VCXO need have no influence for mcu clock. MCU need clock first power on for CDCE I2C set, so mcu can not use the CDCE925 output for clock input.

  • 0 in reply to user5883945
    TI__Genius 16580 points

    Hello,

    If the VCXO mode is being used, then changing the XTAL frequency will affect all clocks output by the device, including the clock used for the MCU.

    A dedicated oscillator, such as the LKM6C, would be required for the MCU, with the CDCE925 generating the remaining clocks.

    Thanks,

    Kadeem