LMX2820: SPI Configuration Details

Part Number: LMX2820

I have several questions about configuring the LMX2820 via SPI:

  1. During the initial power on programming sequence, do all registers need to be programmed or only those whose programmed value differs from the default? (i.e., reset value).
  2. For reprogramming, the datasheet makes it sound like apart from register 0 (which is used to trigger calibration), only the registers whose value has changed need to be programmed and they can be programmed in any order. e.g., i could program R38 then r41 then r0. is this correct?
  3. During the initial power up sequence do I actually need to program the registers in descending order, or is it simply that register 0 needs to be programmed last?
  4. What triggers the LDO power up sequence? Do the LDOs automatically start powering up after CE is brought high? Or, does the power up sequence result from programming a register (e.g., programming register 0 with POWERDOWN=0)? Something else?


  • Hi Matthew,

    After Vcc power up, the chip will perform a Power On Reset. In order to ensure the chip is reset properly, we recommend perform a manual reset by writing R0 with RESET = 1.

    Then, we recommend program all registers in descending order. You may skip those registers whose value is same as the reset value. 

    After it is locked and you want to change the output frequency or other setting, program those related registers only, programming sequence is not matter.

    The internal LDOs will be up and running after a Vcc power up with CE pin = HIGH. However, we need to program the LDOs to their desired values, some of the fixed value registers are used to trim the LDOs.

  • Thanks Noel. That clears up some of my questions. However, I'm still confused on two points.

    Why does it matter (apart from programming R0 last) that the registers are programmed in descending order at startup? If, for example, I wanted to configure the PLL using the reset values, I would only program R0. This would suggest to me that the PLL only takes an action when R0 is programmed, at which point it uses the values in the other registers to perform various actions, such as trimming the LDOs. If this is correct, why would the sequence matter? If it's not correct, does that mean I need to program more registers than just the ones who's value I'll change from the reset value? Or, is my logic flawed?

    When you say

    "After it is locked and you want to change the output frequency or other setting, program those related registers only, programming sequence is not matter."

    Does that mean that I do not need to retrigger calibration (by programming R0) after programming the other register values?

  • Hi Matthew,

    Whenever you change the VCO frequency, you need to program R0 last to trigger VCO calibration. If you change the output frequency without the need of changing VCO frequency, you don't need to program R0. 

    Initial power on programming sequence is especially important for a robust initialization of the device in order to maintain consistent RF performance. Those registers having the same reset default values are usually less important or function specific, for example, SYSREF. So the programming sequence of these registers are not matter.