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LMX2485E / Order of programming registers

S.Satoshi
Guru 21675 points
Other Parts Discussed in Thread: LMX2485E

Hi,

I found the note about order of programming registers as below.

"Note that it is best to program the N counter last, since doing so initializes the digital lock detector and Fastlock circuitry."

Then which should be set RF-N or IF-N at last?

Best Regards,

Sonoki / Japan Disty

  • 0
    Guru 21675 points
    Hi,

    I'm waiting for your comment. Your response will be appreciated.

    Best Regards,
    Sonoki
  • 0
    TI__Expert 5370 points

    Dear Satoshi,

    fast lock is available on the RF PLL only. So using the RF PLL, the fast lock is triggered when N is written. So it is better to write it last.

    As for the digital lock detect, They are combined on the Ftest/LD output. One or the other will trigger a reset on DLD so it does not really matter which one is written last.

    Hope this helps,

    Regards, Simon.

  • 0 in reply to Simon Damphousse
    Guru 21675 points

    Hi Simon,

    Thank you for your comment. You commented that it is better to write RF-N last, then are there any concern if RF-N will not be written last?

    Best Regards,

    Sonoki

  • 0 in reply to S.Satoshi
    TI__Expert 5370 points

    Satoshi-san,

    If you plan on using fast lock, you should program RF-N last. If not, it's not critical.

    Regards, Simon.

  • 0 in reply to Simon Damphousse
    Guru 21675 points

    Hi Simon,

    Let me apologize for asking you over and over, but I need your comment for question below.

    • Can you comment the difference of lock time of RF PLL between using FastLock mode or not using it?
    • You commented that RF-N should be programmed last at using FastLock in previous post, then what do concern if RF-N will not be programmed last at using FastLock? Please let me know the rough delay time if the delaying lock time will be concerning.

    Your kind response will be appreciated.

    Best Regards,

    Sonoki 

  • 0 in reply to S.Satoshi
    TI__Mastermind 27581 points

    Sonoki-san,

    The fastlock mode can be used to improve lock times for the RF PLL by increasing the loop bandwidth for a short time during frequency switching.  Typically, you can get about a 50% reduction in the lock time using it.    However, if you do choose to use it, then you have to use a lower charge pump current in the steady state.  Fully optimizing the delay might take more effort, but I would start by making the timeout length equal to about 1/5th of the lock time you get without using it.

    If you were to use fastlock and not program the RF-N register, the PLL would still lock fine, but you would use lose the benefits.

     

    Regards,

    Dean

  • 0 in reply to Dean Banerjee
    Guru 21675 points
    Hi Dean,

    Thank you for your comment.
    Now I'd like to the case of using fastlock mode but RF-N bit is not set last and IF-N bit is set first. Simon commented that it's better to set RF-N bit last, then I'd like to know what is concern if RF-N bit is not set last.

    Your comment will be helpful.

    Best Regards,
    Sonoki
  • 0 in reply to S.Satoshi
    TI__Mastermind 27581 points

    Sonoki-san,

    Let me clarify some things:

    1.  You can program the registers in any order for this device and the part will still lock.   Note that this is NOT the case for other devices that have an integrated VCO.

    2.  You could introduce spurs if you program the fractional order after you program the RF_N register and they might not be consistent.

    3.  If you are using fastlock, but not programming the RF-N last, then the fastlock will start when you program the RF_N based on the settings you do have programmed.  The fastlock is impacted by information in other registers such as the fastlock current, fastlock timeout counter, OSCin doubler, and especially the RF_R counter value.  If the RF_R counter value is wrong, then the fastlock will be engaged, but the the device will be going to the wrong frequency because the R divider is wrong.  When the correct RF_R value is programmed, the frequency will go to the right frequency, but without the aid of fastlock.

    There actually are registers that you could program after the RF_N value (like IF_N), but there is no benefit to doing so.  If we discuss multiplie options to do something that do not provide any value, this only creates confusion.    Is there some compelling reason you have to not program the RF_N value last?

     

    Regards,

    Dean

  • 0 in reply to Dean Banerjee
    Guru 21675 points
    Hi Dean,

    I apologize for my lack of background. My customer just starts to learn LMX2485E and review datasheet. They have not evaluated it by using EVM. Now I'm asking about the order of setting RF-N because there is state of "Note that it is best to program the N counter last" in datasheet but LMX2485E has RF-N and IF-N, so I have asked which should be program RF-N or IF-N.

    I have received from Simon that it's better to program RF-N last, so I'm asking the concern at the case of programming IF-N last.

    Best Regards,
    Sonoki
  • 0 in reply to Dean Banerjee
    Prodigy 25 points
    Hi Dean,
    I too am confused about the order of programming these registers. You said above that the registers could be programmed in any order, but I wondered about the ACCESS bits on R3 which dictate whether R5, R6, and R7 should get written to. Can I successfully write to these three registers before the ACCESS bits have been written? For example, can I simply write R7, R6, R5, R4, R3, R2, R1, R0?

    Second question - to save time in a system where I program the part multiple times for a frequency sweep, can I only write to the registers that have changed since I last programmed the part or must I always write all 8 registers? (or 6 if I skip R7, R6 & R5).

    Thanks,
    George