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VLO question in F and G series :)

kazola
Guru 18605 points
Other Parts Discussed in Thread: MSP430G2452

Hi,

I'm on my trip to begin knowing the MSP430 :)

I'm reading about the DCO, VLO and all this stuff and some questions have come to me I do not find in the datasheet:

  1. I know the DCO is more precise and it has a <1us time to wake-up :) But I do not find this time value for the VLO. Can anybody give me some light with this wake-up value time for the VLO?
  2. I have read the DCO has more or less a 3% precision... do I remember right? I know the most precise option is employing a crystal, but if not, you can still feed ACLK with VLO but.... which is the precision of VLO? I've found the variance vs. temperature of the DCO but I cannot find an official precision value for the VLO.
  3. Finally, in the MSP430G2452 I'm doing:

BCSCTL3 |= LFXT1S1;

To run the VLO in the MSP430G2452 but I've read for the F series the code is:

BCSCTL3 = LFXT1S_2; // Select ACLK from VLO (no crystal)

Which is quite different. Are both code fragments right?

Let me know your opinions. Have a nice sunday from Catalonia!

  • 0
    Guru 92251 points

     

    kazola said:
    Can anybody give me some light with this wake-up value time for the VLO?
    I agree the MSP430G2452 datasheet doesn't mention the VLO wake-up time. The MSP430x2xx Family User's Guide SLAU144I says:

    VLOCLK source is selected by setting LFXT1Sx = 10 when XTS = 0. The OSCOFF bit disables the VLO for LPM4.


    This implies that when selected the VLO is not turned off during LPM0 to LPM3 and thus doesn't require wake-up time.

    kazola said:
    which is the precision of VLO? I've found the variance vs. temperature of the DCO but I cannot find an official precision value for the VLO

    Does the following from the MSP430G2x52 datasheet SLAS722E answer that question?

    kazola said:
    Are both code fragments right?

    Ignoring the fact that one uses a bi-wise or and the other is an assignment, looking in the CCS5.2 msp430g2452.h the definitions of LFXT1S1 and LFXT1S_2 are the same:

     

    #define LFXT1S0                (0x10)         /* Mode 0 for LFXT1 (XTS = 0) */
#define LFXT1S1                (0x20)         /* Mode 1 for LFXT1 (XTS = 0) */

#define LFXT1S_0               (0x00)         /* Mode 0 for LFXT1 : Normal operation */
#define LFXT1S_1               (0x10)         /* Mode 1 for LFXT1 : Reserved */
#define LFXT1S_2               (0x20)         /* Mode 2 for LFXT1 : VLO */
#define LFXT1S_3               (0x30)         /* Mode 3 for LFXT1 : Digital input signal */

    Not sure why there are multiple definitions for the same values, but both code fragments are selecting the correct bits (the BCSCTL3 |= LFXT1S1 fragment assumes the LFXT1Sx bits initally have their reset value of zero)

  • 0 in reply to Chester Gillon
    Guru 18605 points

    Great answer Chester, thanks :)

    Only just one thing, our MSP is under LPM4. We press a button and then goes to active mode and starts counting, so we need to know the wake-up time of the VLO from LPM4. Do you thik, for example, this is < 100us?

    Let me know what you think!

  • 0 in reply to kazola
    Guru 92251 points

    I can't seem to find any documentation for the VLO wake-up time. Also, after finding this Watchdog settings for low power mode LPM4 or LPM3 thread I not sure that VLO is off in LPM4, the thread suggests the VLO may only be off in LPM4.5. I suspect an experiment is required.

  • 0 in reply to Chester Gillon
    Guru 18605 points

    Great Chester, thanks :)

  • 0 in reply to Chester Gillon
    Guru 227245 points

    Chester Gillon said:
    I not sure that VLO is off in LPM4, the thread suggests the VLO may only be off in LPM4.5.

    The meaning of the VLO is to have an oscillator that requires only very low power but does not provide any noticeable precision. It' spossible that the VLO wake sup instantaneous, but with an uncertain frequency, which stabilizeds to anotehr uncertain frequency after soem time. Since its frequency is so low that it won't exceed the MSPs allowed operating frequency even in teh lowest supply voltage range, any overshooting is also unimportant. (the risk of overshootig is the main reason for the lon DCO wakeup time).

    However, I'm too not sure whether the VLO is ever off except for LPM4.5, which will shut everything off except the port logic.. It is possible that the part that suggests it will eb shut off in LMP4 actually means that the VLO won't act as fallback for XT1 when XT1 is already deactivated in LPM4. But that's just a guess.

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