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LMX2694-EP: Timing of SPI communication start after power up

David Horowitz
Prodigy 70 points
Part Number: LMX2694-EP


I am working with a design with an LMX2694-EP. I am seeing a problem where if the unit is not powered down and reprogramming is attempted, the synthesizer fails to lock. If the unit is powered down, powered on and then programmed, the synthesizer locks. I suspect we are  programming the LMX2694 too quickly after power up or it is not powering down completely during power cycles due to our board design.

What is the minimum time after powering on the LMX2694 must elapse before programming?

  • 0
    TI__Guru** 110790 points

    Hi David,

    Every time you Vcc power up the device, you have to program all registers, including R0. After it is locked, if you program it to change to a new frequency, in addition to the frequency related registers such as PLL_N, PLL_NUM, you also need to program R0 to initiate a VCO calibration, otherwise, it may unlock. 

    In addition, whenever it is Vcc power up, we suggest make a software reset by programming RESET=1. This will ensure the registers are clear to their default value before programming is stared.

  • 0 in reply to Noel Fung
    Prodigy 70 points

    We do all those steps. In the datasheet for the LMX2694 I didn't see any specification for how long is required after supply voltage is applied before SPI communication can start. I assume there must be some delta required. That's the information I really need.

    Thanks,

    David

  • 0 in reply to David Horowitz
    TI__Guru** 110790 points

    Hi David,

    We don't have a precise data on this, we'd never do this kind of evaluation before.

    A conservative guess would be, after supply voltage is applied, wait for 2ms and then program RESET=1. 

    Did you measure the Vcc ramp up/down time on your board? Maybe this information is useful to determine the wait-before-programming time.