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Stellaris LM4F : when PLL is set to 491.52MHz, what will happen?

Junchuan Tan
Prodigy 150 points

Hi,

We set PLL up to 491.52MHz, and the system clcok is 96MHz accordingly. ---- the target is to output same PWM frequency as external crystal.

I know TI provides a simple way to set PLL, user can just select what crystal to use. PLL will be set to 400MHz automatically. In our design, we set the crystal to one frequency, but we don't use it, we use a higher frequency crystal. So the PLL can exceed 400MHz.

The launchpad is running well with the high frequency.

But in the datasheet, it tells me PLL is 400MHz and system clock is up to 80MHz. But it does not tell me the PLL can not exceed 400MHz. I want to make sure if the 491.52Mhz and 96MHz are safe for mass production.

 

Thanks,

Jackie

 

  • 0
    TI__Guru 58650 points

    Hi Jackie,

    This configuration is absolutely not safe for mass production.  TI only guarantees correct operation of the device when it is operated according to our published specifications.  If you operate the part outside of these specifications, we do not guarantee correct operation.

    Regards,

    Sue

  • 0 in reply to Sue Cozart
    Guru 117855 points

    Ms. Cozart has provided the official, factory view/guideline.  As humble - long-term ARM MCU buyer/user (both this vendor and others) - we agree.

    We have done just as you describe - and for some population of MCUs - over some (limited) environmental & electrical conditions - operation may appear to be ok.  However there is no guarantee that future "lots" of that MCU (or other devices w/in that same lot) will perform in that same manner.  In addition - it is unlikely that you deeply/properly scrutinized "all" of the MCU's performance measures - when operated beyond spec.  Such "spec violation" will likely add extra stress (especially thermal, among others) to the MCU - often leading to unwanted, MCU behavior & likelihood of shortened device life...  

    Bottom line - "Risk-Reward" should enter your decision making - and the down-side risk far exceeds benefit - thus arguing against such use.  (misuse!)

  • 0 in reply to cb1_mobile
    Prodigy 150 points

    Thanks Sue and cb. We won't take the risk. The qty could be so big, it is not 10s or 100s. It is 10Ks. After efforts, we have solved the issue, we can output accurate frq by a special way, not beyond the spec. Now the egineering samples work well. Thank you for you kindly reminder and suggestions.

    BR,

    Jackie

  • 0 in reply to Junchuan Tan
    TI__Guru 58650 points

    I'm so glad to hear it, Jackie!  Thank you for posting your resolution here.

    Regards,

    Sue