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Other possible reasons why the CC1101 might lose PLL Lock

Thaddeus Meizelis
Prodigy 10 points
Other Parts Discussed in Thread: CC1101

A little background:

I have a CC1101 that has been working well. Occasionally though, it would stop receiving, and not reboot until the device was reset. I traced it down to the fact that occasionally, the CC1101 would lose PLL Lock, indicated by a 0x3F at the FSCAL1 register. I am using auto-calibration (FSCAL=0x01) which means I know at some point soon I will have to implement a handler that will detect the condition and cause the firmware to recalibrate the CC1101.

The strange part is that I have earlier versions of our firmware that I have never (to my knowledge) caused this PLL Lock issue. Nothing in the old firmware sets the registers of the CC1101 any differently then I do now, and there are no significant changes to the code that deals with the CC1101.

Here is my question: Is a change in temperature and voltage the *only* thing that can cause the PLL not to lock? For instance, can the frequency at which I send commands to the radio cause the PLL not to lock more often?

  • 0
    TI__Mastermind 39860 points

    Our testing show that if you calibrate at a certain temperature the PLL will stay in lock if the temperature changes by less than +/-40C. 

    Since the VCO uses an internal voltage regulator, the frequency drift over supply voltage is negligible, at least above ca. 1.9 V unregulated supply voltage. The charge pump runs on un-regulated voltage and the charge pump current might change the PLL BW with significant changes in supply voltage resulting in a non-optimum PLL BW (i.e the phase margin might get low). We have not measured the maximum drop, so to play it safe I would say the maximum drop as 1.0 V.

    I have also come across a case where the CC1101 was calibrated and the PLL in lock. However, when strobing CSn there was a coupling of the CSn signal to the crystal oscillator resulting in the PLL going out of lock for a certain period of time (don't have details on timing - sorry).  Simple test is to cut the CSn trace at both ends and running a jumper wire to check if the problem goes away completely.