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MSP430FR5969: Set VLO as RTC Source in LPM3.5

Mason08
Prodigy 120 points
Part Number: MSP430FR5969


Hello, 

I'm a bit of a newbie when it comes to the MSP430 family, and I have a hopefully easy question to answer. I'm using an MSP430FR5969 launchpad kit, and I need to configure the RTC to work in LPM3.5 using the VLO instead of a watch crystal. I can see in the datasheet that this is supposed to be possible, but I can't see how to configure the device that way. My first guess would be to assign the VLO as the source for ACLK, but that gets shutdown in LMP3.5. 

Could someone point me in the right direction? I'd really appreciate it. 

  • 0
    Guru 100785 points

    Per data sheet (SLAS704G) Fig 1-1 Note A (and User Guide (SLAU367O) Fig 28-1), RTC_B can only be used with a crystal, not VLOCLK.

  • 0 in reply to Bruce McKenney
    Prodigy 120 points

    3.2.4 Internal Very-Low-Power Low-Frequency Oscillator (VLO)

    The internal VLO provides a typical frequency of 10 kHz (see the device-specific data sheet for
    parameters) without requiring a crystal. The VLO provides for a low-cost ultra-low-power clock source applications that do not require an accurate time base. To conserve power, VLO is powered down when
    not needed and enabled only when required.
    VLO is enabled under any of the following conditions:
    • VLO is a source for ACLK (SELAx = 1) and in active mode (AM) through LPM3 (OSCOFF = 0)
    • VLO is a source for MCLK (SELMx = 1) and in active mode (AM) (CPUOFF = 0)
    • VLO is a source for SMCLK (SELSx = 1) and in active mode (AM) through LPM1 (SMCLKOFF = 0)
    • VLOOFF = 0. VLO enabled in active mode (AM) through LPM4.
    • VLO is selected as the source for RTC, RTC is enabled (RTCHOLD = 0), and LPMx.5 is entered.

     

    This was pulled from the User's Guide (SLAU367O). Would the highlighted statement not imply that we can use the VLO for RTC? 

     

  • 0 in reply to Mason08
    TI__Mastermind 23305 points

    Hi Mason

    I agree with Bruce that VLO can't be used as RTC_B clock source according to RTC_B block diagram on User Guide (SLAU367O). for VLO description in the same UG, I think it means that VLO can be used in some other MSP RTC IP.

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