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LMK04832: OSCout default function after power on

Alexander Sobko
Prodigy 30 points
Part Number: LMK04832

Hi,

I wanted to use the OSCout/CLKin2 as auxiliary ref clock input. According to datasheet, the default function for OSCout/CLKin2 after power On and before register programming is an output - buffered copy of OSCin. If I will connect an external clock oscillator to these pins, there may be a signal contention here until CPU will load correct configuration to LMK04832's registers.

I'd prefer to avoid such situation.

Is there a way to change the default power on function of OSCout/CLKin2 to be ref clock input?

Any other suggestions to avoid signal contention after power on?

Thanks and best regards,

Alex

  • 0
    TI__Genius 10995 points

    Hello Alex,

    Unfortunately, the LMK04832 has no way to control pin 40 via another pin, therefore, this cannot be done directly from the LMK04832. Why does this matter to you (is it to not damage the OSC used for CLKin2)? What system are you using that has this requirement? With more information I could potentially help you find a solution external to the LMK04832.

    Best,

    Andrea

  • 0 in reply to Andrea Vallenilla
    Prodigy 30 points

    Hello Andrea,

    Thanks for your replay.

    My design has two reference clock sources (main and auxiliary) and also SYNC/SYSREF input signal. The latter one is the differential signal coming from another LMK04832 located on different PCB ("master").

    As I can see in block diagram, the CLKin0 input may be routed to SYNC/SYSREF Control block in LMK04832. So, I wanted to benefit from advantages of differential signaling and connect my incoming differential SYNC/SYSREF signal to CLKin0. CLKin1 and CLKin2 would be utilized as main and aux ref clocks inputs.

    Unfortunately, CLKin2 pins are shared with OSCout function which is the default setting. In such case the signal contention will occur for sure between OSCout and aux clock source. This will lead to signal corruption and possible circuit damage.

    If the OSCout/CLKin2 default setting cannot be changed, I see two possible solutions:

    1. Use single ended SYNC/SYREF input.

    2. Disable external ref clock oscillator until correct configuration will be loaded into LMK04832.

    If you have any other ideas, please share.

    Thanks,

    Alex

  • 0 in reply to Alexander Sobko
    TI__Genius 10995 points

    Hello Alex,

    I'll look more into this and get back to you by this week.

    Best,

    Andrea

  • 0 in reply to Andrea Vallenilla
    TI__Genius 10995 points

    Hello Alex,

    Thank you for the detailed explanation. Are you using the device in dual-loop mode/are you sing an external VCXO?

    Alexander Sobko said:
    1. Use single ended SYNC/SYREF input.

    When you say this, are you assuming to use the positive pin for the SYNC/SYSREF signal and the negative pin as the auxiliary clock and not use CLKin2 at all? If so, the LMK04832 should not be able to route those signals accordingly.

    Alexander Sobko said:
    2. Disable external ref clock oscillator until correct configuration will be loaded into LMK04832.

    This is what I was going to suggest. The best suggestion I would say would be a switch that disconnects the auxiliary OSC from OSCout/CLKin2 pin (pin 40). If this is not possible, I would suggest doing a power-up sequence that first powers up the LMK04832, then changes the registers of the device to change the function of pin 40, power up the OSC, and then power up the VCXO.

    Let me know if you have any other questions.

    Good luck,

    Andrea

  • 0
    Prodigy 30 points

    Hi Andrea,

    Yes, I'm using dual loop nested 0-delay mode and I'm using external VCXO for PLL1 and embedded VCO0 for PLL2.

    1. When I wrote "Use single ended SYNC/SYREF input" I meant connecting the positive line of my incoming differential SYSREF signal to dedicated single-ended input on LMK04832 - pin 6. In this case the auxiliary ref clock signal (also differential) may be connected to CLKin0 (pins 37,38). OSCout/CLKin2 (pins 40,41) are not used in this setup and signal contention will not occur.

    2. "Disable external ref clock oscillator" - I mean "to put oscillator's output to Hi-Z state by pulling Low its OE input". The best way to do this would be if LMK04832 would has GPIO available for programming - I will program it to active value at the end of LMK's configuration. Thus, auxiliary ref clock oscillator will be enabled only after LMK is configured properly. Unfortunately, it seems there is no GPIO available in LMK04832. 

  • 0 in reply to Alexander Sobko
    TI__Expert 8421 points

    Alex,

    Andrea is out of office so I will be taking over your question.  I will get back to you by Thursday.

    Regards,

    Will

  • 0 in reply to William Pfennigwerth
    TI__Expert 8421 points

    Alexander,

    Both of your options are valid options.  

    • Using the Single Ended is the simple solution, the only tradeoff being the performance difference from using the single ended input vs. a differential signal.  
    • This would also work, although it would require additional SPI register writes as you are correct the LMK04832 does not have GPIO for toggling these features. 

    Let me know if you have any further questions,

    Will

  • 0 in reply to William Pfennigwerth
    Prodigy 30 points

    Hi Will,

    Thank you. I will look on this more to decide what solution is better for aplication.

    Regards,

    Alex