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TWL6032: RTC accuracy

Takashi Yamaguchi
Genius 11090 points
Part Number: TWL6032
Other Parts Discussed in Thread: TPS65950

Hi team,

I received a question about TWL6032 RTC accuracy from my customer

Customer is using TWL6032 for their products, but it seems that there is a large time lag on RTC. Therefore, customer wants to know the accuracy of the RTC.

 

The datasheet doesn’t describe the RTC accuracy, so could you tell me it?

 

If possible, could you show the accuracy as following example?

 For example : The RTC will shift XX seconds for YY months under the ZZ conditions.

 Customer requested me to show it with several conditions.

Regarding temp range, the application would not be used at too high temp/too low temp.

Therefore, it is helpful that the temp range is under use environment(e.g. 0degC ~ 45degC).

 

 Customer mentioned that input clock is 32.768kHz, and the frequency deviation is +-20ppm including temp deviation.

Customer thinks that there is no problem for input clock.

 

 

Regards,

Yamaguchi

  • 0
    TI__Genius 17380 points
    Hi Yamaguchi-san,

    Thank you for posting your question to E2E.

    The accuracy of the RTC is mostly impacted by the accuracy of the crystal. The impact of the TWL6032 is typically +- 0.5 ppm/Celsius as shown in pg 33 of the specification. The 20 ppm of the crystal is much larger.

    If their crystal is +- 20 ppm, this is the same as +-0.002%. Over the course of a 30 day period (2592000 s), the RTC will be off by up to .002% = 51.84 seconds.

    The TWL6032 has compensation registers which can be used by the processor to correct for crystal inaccuracy. This compensation can reduce the RTC mismatch of 30 days down to 30 ms. It is the same compensation as used in the TPS655950 which has an application note regarding this:
    www.ti.com/.../swca024.pdf

    If you have any questions on how this works after reading the application note, please let me know.
  • 0 in reply to Kevin LaRosa
    TI__Genius 11090 points

    Hi Kevin-san,

     

    Thank you for the answer. 

    Please let me check my understanding.

     

    1. The TWL6032 has internal crystal which tolerance is +-20ppm as shown in pg33. If customer wants to have higher accuracy, they can use external high accuracy crystal. However, customer is using +-20ppm of external crystal, so you said it is large. Is my understanding correct?

     /cfs-file/__key/communityserver-discussions-components-files/196/TWL6032_5F00_P33.pdf

     

    2. You mentioned that "the accuracy of the RTC is mostly impacted by the accuracy of the crystal". In this customer’s case, the +-20ppm of external crystal accuracy is mostly impacted. The impact (time lag) of TWL6032 itself is only +-0.5 ppm / Celsius. Is my understanding correct?

     

    3. Regarding the +-0.5ppm/Celsius, when the Ta is 25degC, will it be 0.5ppm x 25degC =12.5ppm? Therefore, total accuracy would be +-20ppm(external crystal) +-12.5ppm(TWL6032 internal) = +-32.5ppm at 25degC. Is my understanding correct?

    Regards,

    Yamaguchi

  • 0 in reply to Takashi Yamaguchi
    TI__Genius 17380 points

    Hi Yamaguchi-san,

    You are mostly correct with a few small changes.

    1. The TWL6032 does not have an internal crystal, the +-20 ppm is the requirement we have for the external crystal. As long as their crystal is +- 20 ppm, we can support it. In fact we can support roughly +- 100 ppm accuracy with the compensation, but +-20 ppm is better for margin.

    2. Correct.

    3. The baseline is where they calibrate their RTC. Drift will be based on where they are relative to that point. For example, if they do compensation at 25 C, then operate at 50 C, then the TWL6032 will contribute .5 ppm * (50 C- 25 C) = +-12.5 ppm. If they redo compensation at 50 C, this can be eliminated. The ppm error from the crystal will depend on its ppm / C, not it's accuracy (20 ppm) because the accuracy was adjusted for in the compensation step.

  • 0 in reply to Kevin LaRosa
    TI__Genius 11090 points

    Hi Kevin-san,

    Thank you for the answer.

    I received feedback form customer. 

    Customer said that they may not do the compensation. The engineer said he will need to check it to software team.

    If they don’t do compensation, what temperature will be based of the +-0.5ppm/ Celsius ? For example, 25degC becomes default value or it is based on a temperature when RTC value is set. 

     

    In addition, where can I find the description about this compensation in the datasheet?

     

    Regards,

    Yamaguchi

  • 0 in reply to Takashi Yamaguchi
    TI__Genius 17380 points

    Hi Yamaguchi-san,

    The specification does not detail a specific base temperature, but room temperature is most likely the case given this is a typical value.

    I don't see information regarding the compensation in the datasheet for TWL6032. The registers for it are in the register map (SWCU095B) and the functionality is defined in the TPS65950 RTC Compensation app note: www.ti.com/.../swca024.pdf