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Backup power supply for MSP430F6779A

Jesse Frey
Prodigy 220 points

Other Parts Discussed in Thread: MSP430F6779A, MSP430F6638

The datasheet for the MSP430F6779A mentions the backup subsystem and provides a link to the user guide. The device also has the AUXVCC3 pin which is seems to be the pin that is used by the backup subsystem.

When I try to write code to use the backup subsystem I find that the BAKCTL and BAKCHCTL registers are not defined in the header file. Am I missing something? If so where is this in the user guide/datasheet? Also if not for the backup subsystem, what is AUXVCC3 for?

  • 0
    Guru 23000 points
    BAKCTL and BAKCHCTL applies to the RTC_B module, your device has a RTC_C module and the charger is controlled trough the AUX3CHCTL register.
  • 0 in reply to Leo Bosch
    Prodigy 220 points

    That makes sense looking at the datasheet and the user guide on the AUX system. It is still not clear to me why the datasheet references the backup subsystem (the end of page 75) and links to the module that is not present on the chip.

  • 0 in reply to Jesse Frey
    Guru 12050 points
    yes it should link to this one instead: www.ti.com/.../slau420d.pdf
  • 0 in reply to Jesse Frey
    Guru 23000 points

    You are correct, this is in error. The linked document also states that is meant for the RTC_B and not for RTC_C.

    I think this needs the attention of .

  • 0 in reply to Leo Bosch
    TI__Guru* 83901 points

    Hi all,

    This is a common confusion, and we need to find a way to make it clearer since these modules are available on different sub-families of F6xx parts that share the same user's guide www.ti.com/.../slau208. My understanding is that there are two different things - the Battery Backup System (BATBAK and its registers), but then there is also the "backup subsystem" referred to in both the Battery Backup System chapter and Auxiliary Supply System (AUX) chapter.

    The Battery Backup System module/registers are on parts with RTC_B like MSP430F6638, and on these parts the backup subsystem includes things like the RTC_B and XT1 and backup RAM that are powered by the Battery Backup System. For these parts you'd refer to the Battery Backup System chapter in the user's guide.

    The AUX is on parts with RTC_C like MSP430F6779A, and on these parts the backup subsystem includes things like the RTC_C and XT1 and backup RAM that are powered by the AUXVCC3. So for these parts you would refer to the AUX chapter.

    What is included in the backup subsystem is device specific, so the best place to find this is in the device datasheet - then you'll know which user's guide chapters apply to your device. For MSP430F6779A in the datasheet you can see section 6.11.4 Backup Subsystem, where it says the backup subsystem runs on AUXVCC3 and includes the XT1, RTC_C, and Backup RAM. Now, I see here the "link to user's guide" points to the Battery Backup System chapter, which it should not I don't think, because my understanding is that the Battery Backup system module is a different thing than the backup subsystem. I will file this as a bug to be fixed and further clarified in our documentation.

    Regards,
    Katie

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