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CDCM6208V2 EVM external crystal SEC_REF

Simon Guignard
Prodigy 180 points
Other Parts Discussed in Thread: CDCM6208

Hi,

I want to use my CDCM6208 board in order to output a clock signal from an external crystal input.

First, I tried to output a clock from the internal 30.72 MHz crystal and it worked as expected. Then, I removed the R72 and R73 resistors as explained in the EVM User's Guide and populated R87 with a connection.

This time, my board outputs different frequencies which have nothing to do with my input while my registers configuration should disable 7 of the 8 outputs.

Instead, these are the frequencies I measure in outputs : ~128 MHz on Y0/Y1, ~102 MHz on Y2/Y3, ~51 MHz on Y4/Y5 and ~26 MHz on Y6/Y7.

I also made sure my input signal was delivered by measuring it on the corresponding pin of the chip. My signal is a waveform of 10 MHz between -0.6 and 0.6 V (JMP11 on GND).

Is my signal voltage not high enough?

Does this correspond to the basic frequency configuration provided when there is a Loss Of Reference? I noticed it was the same frequencies we can find on the first two lines at page 38 of the CDCM6208 chip datasheet with a 1.2 factor (~26 x 1.2 = 30.72 ; ~52 x 1.2 = 61.44 ...).

Thanks you for your answers,

Simon

  • 0
    TI__Expert 4400 points
    The PLL is operating in open loop and what you get is the open loop VCO frequency. This indicates that the input swing isn't high enough to be recognized as a valid input.

    What is R4[7-6] set to? Is it "10" which is LVCMOS input?

    If so, the input should have a Vil of < 0.2*VDD_SEC and Vih of > 0.8*VDD_SEC (min)
  • 0 in reply to Madhu Balasubramanian
    Prodigy 180 points

    Hello and thank you for the reply.

    My input is crystal type so the signal is waveform therefore I set "11" on R4[7-6].

    I would like to know a few other things.

    What VDD_IN should I use for VDD_SEC?

    What is the output voltage level of the on-board crystal (the NX3225GA 30.72 Mhz)? It used to work with it but I removed it from the board according to the User's Guide.

    What is the minimum input voltage to be recognized as a valid input? I can get the Vil < 0.2*VDD_SEC and Vih > 0.8*VDD_SEC for LVCMOS as it comes from the '7 - Specifications' of the datasheet, but there is no indication about voltage levels with crystal inputs, or I can't find it.

    Kind regards,

    Simon

  • 0 in reply to Simon Guignard
    Prodigy 180 points

    Still waiting for an answer...

  • 0 in reply to Simon Guignard
    TI__Expert 4400 points

    Sorry for the delay as I was out. When you say input is crystal type, what exactly is it? Did you replace the original on-board crystal with your crystal? Or is the input a sine wave?

    The recommended input buffer setting will depend on what exactly is your input. In general, the input supply can be set to 1.8V. 

  • 0 in reply to Madhu Balasubramanian
    Prodigy 180 points
    Hi Madhu,

    I plug my input on the "SECp" SMA connector after correctly having removed R72, R73 and having connected R87.
    The signal comes from an external crystal which is a waveform 10 MHz, -0.6 V to 0.6 V.

    Thanks.
  • 0 in reply to Simon Guignard
    TI__Expert 4400 points

    Hi Simon,

    I'm still unclear on your input type. While it is not recommended to have a separate PCB that houses the crystal and connect to the CDCM6208 via SMAs, you won't even get correct operation of the crystal doing what you are doing. You can't just connect SECp to crystal and leave SECn hanging. In this case, the crystal won't even oscillate. How are you measuring -0.6V to 0.6V?

    Is your input a crystal or a crystal oscillator?

  • 0 in reply to Madhu Balasubramanian
    Prodigy 180 points
    Hi Madhu,

    My input comes from the ouput of a Rubidium SRO-100. So this is a single-cable output coaxial.
    I plug this Rubidium output onto the SEC_REF SMA input of the CDCM6208V2 board.
    I can measure my signal directly on the CDCM6208V2 input pin of the chip : it is a sine wave 10 MHz -0.6 V to 0.6 V.

    Thanks,
    Simon
  • 0 in reply to Simon Guignard
    TI__Expert 4400 points
    Hi Simon,

    In this case, please place zero ohm on R87, R89 and remove R72, R73. Then tie mid pin of JPM11 to pin labeled DVDD. Then set Register 4, bits 7:6 = 00. Please make sure C29, C30 are present. Then apply the sine wave from Rubidium source to SEC_REFP.
  • 0 in reply to Madhu Balasubramanian
    Prodigy 180 points

    I've already done this.

    But the problem finally seems not to be the voltage level.

    Actually it works when I set my configuration with the GUI and the USB connected to the board. But when I try by setting the configuration with the MSP430 as a bootloader (I've already validated the programming when I used the on-board crystal), what I've described earlier is happening. So this might be a program issue, even if I am sure the program is running when I power up the board (no USB connection) as I added an LED and a blink output on the MSP430.

    Got to look at it.