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Original question:

LMX2820: SPI Configuration Details

LMX2820: Programming RESET bit after POR

Matthew Huszagh
Prodigy 40 points

Part Number: LMX2820

Both the 2594 and 2820 datasheets say to reset the device after POR by programming RESET=1 followed by RESET=0. I have two questions in regard to this:

1. Why is it necessary to program RESET=0? In the LMX2820 register map document it states that the reset bit is self-clearing.

2. Can all register values be left as their default values (except the reset bit of course) when toggling this reset bit? In particular, FCAL_EN (for VCO calibration) is by default enabled. When I clear the reset bit by programming it to 0, will this trigger a VCO calibration? If that happens, the VCO calibration will be performed for a target frequency I don't want and before the LDOs have sufficient time to power up (10ms). Why would I do this?

In TICS pro, it doesn't appear that the program performs this manual RESET toggle after power up.

  • +1
    TI__Guru** 119790 points

    Hi Matthew,

    1. Right, program RESET=0 is not critical as the reset is self-clearing. That is, after RESET=1 is programmed, the device will reset and return to its silicon default status. In this state, the RESET bit is equal to 0.

    2. when we want to reset the device, simply program R0 with RESET=1, no other registers programming is required. After a reset, if we program RESET=0, you are right, this will trigger a VCO calibration as FCAL_EN is equal to 1 by default. However, since the internal LDOs have not been programmed yet, a calibration will not hurt. Actually I doubt if a calibration will happen, I am not sure, this evaluation was done by my colleague. 

  • 0 in reply to Noel Fung
    Prodigy 40 points

    Separately, I noticed that TICSPro writes to a lot of registers with read-only fields (e.g., R71). What's the action of the LMX2820 in these instances? Does the device simply ignore these writes?

  • 0 in reply to Matthew Huszagh
    TI__Guru** 119790 points

    Hi Matthew,

    This was due to the frame work that is being used to construct TICs Pro, not sure if there is way to fix this, I will look into this. 

    Those are read only registers, the device will ignore these writes.