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LMK03328: Timing estimate to switch configuration

Part Number: LMK03328
Other Parts Discussed in Thread: USB2ANY,


In my application, I have 2 input clocks, only one of which is active at any given time, and 2 different output clocks which are always active.

I need to switch between input clocks as fast as possible.

- Jitter requirements are assumed to be relaxed. PLL bandwidth can thus be higher to reduce lock time. Can you comment on an order of magnitude for the lock time? I know this depends on many parameters, but I need some input for a feasibility analysis.

- I can either store 2 configurations in ROM and switch between those or do the minimum necessary adjustments over I2C. Can you comment on time budget of both solutions?

- The PLL will need some reset to calibrate the internal VCO when switching configuration Can you comment on time budget of this reset?

Thank you very much for input!

  • Hello,

    We will take a look on the bench and get back to you by EOD Wednesday.



  • Hello,

    If your two input frequencies are the same [within 2000ppm] then you can implement the following:

    To switch the reference used, R50 must be be written to:

    For using the minimal amount of I2C transactions, then the time for locking can be around 1-2us, subject to communication speed. USB2ANY was used for this test which takes up a large amount of time.

    If the two input frequencies are NOT the same, then you must rewrite a new config to the device and the VCOs must recalibrate:

    For rewriting all of the registers, then the time for locking can be on the order of ms, again subject to communication speed and how different your two configurations are. Most of this time is spent recalibrating the PLL [see image below from page 141]. Assuming you are using both PLLs in the LMK03328, you may also need to include time for PLL2 to calibrate.

    VCO calibration time is listed in the datasheet [see image below from page 141]. Assuming you are using both PLLs in the LMK03328, you must also include time for VCO2 to calibrate.

  • Thank you very much!