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LMX2820: LD pin vs Reg. 74 Readback with No Assist and in InstantCal Mode

Turker Dolapci
Prodigy 30 points
Part Number: LMX2820

Tool/software:

Hi,

I am performing an automated test in which I set the frequency output successively by SPI. I am using no assist for now.

I check the LD by SPI, not the LD pin.

Sometimes, LD pin (LED) shows detection fault, as it needs to be, however SPI readback shows successful LD.

Is this possible, or is it caused by a programming mistake I made? Is it necessary to add a delay before reading Register 74 after setting the frequency? Do I need to set any other register at initialization and/or before reading Register 74 to see correct LD status?

How about Instant Calibration LD status check, what is the correct way with SPI and with LD pin? 

Regards,

Turker

  • +1
    TI__Guru** 107450 points

    Hi Turker,

    The LD pin gives you a real time monitoring to the lock status.  

    For example, after the PLL is locked, if the input clock becomes not stable, we may see LD pin is toggling. If the duration of unlock is very short, we may still read back lock from rb_LD. Even if the unlock duration is longer, if we read back the rb_LD not at the unlock period, we will read back lock.