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MSP430 VLO -vs- External Oscillator Clock

Harlan Zamora
Prodigy 60 points 


Hello,
 
I am working a project that involves porting some HW and SW that is based on an MSP430.  
The project that I am working on is porting an MSP430 hardware design and MSP430 code from one project to another. 
The existing code and hardware on the othe project uses an external 32Khz clock as ACLK.  I was told that the reason for 
using this was that the external clock is because the 32Khz clock will give more more precise timing when keeping track 
of time as a Real Time Counter.  The project that I am working on is considering not using an external 32Khz clock oscillator 
but instead using the internal VLO as the ACLK to keep time of time as a Real Time Counter.  Will using the internal VLO on 
the MSP430 that is running at 12Khz compared to an External 32Khz oscillator cause a big inacurracy in time due to drift?
 
Thank you for your help.
 
regards,
 
 
  • 0
    Expert 2740 points

    Harlan Zamora said:
     Will using the internal VLO on 
    the MSP430 that is running at 12Khz compared to an External 32Khz oscillator cause a big inacurracy in time due to drift?

    Have you looked at the spec sheet for VLO specifications ?

    Peter

  • 0
    Intellectual 870 points

    Look if your need precision you can choose the DCO, which has a margin of error of 2%, and if you have low consumption you choose the SMCLK that feeds the DCO and is also quite accurate because the VLO is quite vague in count

  • 0
    Guru 46710 points

    Harlan Zamora said:
    Will using the internal VLO on the MSP430 that is running at 12Khz compared to an External 32Khz oscillator cause a big inacurracy in time due to drift?

    Yes. You shall start your question specifying application requirements.

    VLO frequency over -40 +85 oC range is: 4 to 20 kHz, nominal 12kHz (around -300%/+200% inaccuracy).

    Typical 32kHz oscillator initial precision is around 10ppm (0.001%), over same temp range -160ppm to +160ppm (0.016%).

    So VLO accuracy is 18750 times worse than typical 32kHz fork oscillator. If this is ok for your application - use VLO then!

  • 0
    Prodigy 60 points

    Hello everyone,

    Thank you so much for the feedback and information that you all have provided regarding my question, it has been extremely

    useful.

    Below is some additional information that I received from TI that might be helpful. I figured I share this information with the forum.

    The references that need to be looked at to compare using an external (higher accuracy crystal) versus the internal VLO is shown

    in the datasheet. Only part of this will come from us directly the other part will need to come from the specific crystal manufacture's

    documentation.  Please see both tables on page 41 showing the DCO (VLO) calibrated frequencies tolerance and over temperature:

    http://www.ti.com/lit/gpn/msp430f2619s-ht

    Then, please see the bottom of page 46 for the VLO frequency temperature drift and voltage drift (4%/V). Please note how this is

    calculated by the footnotes under that table (we are asked often how this is derived, so the equation is helpful to know).  Then, compare

    this to the specific crystal that you are using, against their datasheet. They will specify their testing of drift over temperature and voltage.

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