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Digital Lock Detect in LMX 2491

Other Parts Discussed in Thread: LMX2492, LMX2491

In LMX2492, in order to get a digital lock detection bit, I have set TRIG1_PIN to Inverted Pullup/Pulldown Output (value 6 on the data sheet) and set TRIG1_MUX to Output DLD (value 11 on the data sheet). However, when I set TRIG1 to Pullup/Pulldown and probe it with an oscilloscope, the pin stays high no matter what I set TRIG1_MUX to (even when it is set to ground). As a result, I cannot confirm whether the "high" observed is indicative of PLL lock detection or just a fluke. Can anyone shed any light on this? For digital lock detect, should TRIG1_PIN be set to some other form of output such as Open Drain Output or Inverted Open Drain Output?

  • I meant LMX2492 in the title, not LMX2491.
  • Eddie,For the lock detect, you need 2 things:
    1. Define the output. You picked DLD. You might want to do DLD + CPMON because the digital lock detect can be fooled into showing a locked condition if there is no reference input, but the charge pump monitor is not fooled by this.
    2. Define the output type.You picked inverted Push-Pull, which is fine.
    I tried this condition in the lab and I got it to work fine. It was lock detect when the PLL was locked, then I unlocked it by flipping the phase detector polarity and it indicated unlock. Open drain should not be necessary and I don't know why this is not working for you unless there is some issue at the power supply pins.
    Regards,Dean