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Switch from DCO to VLO clock not working on MSP430G2553

Richard DuBose
Prodigy 20 points
Other Parts Discussed in Thread: MSP430G2553

I have been trying to switch my MCLK signal from the DCO to the VLO for the MSP430G2553.  I have a small loop that is toggling a port bit to watch the toggle frequency to verify the clock has indeed changed.  Below is the subroutine code that is setting up the clock switch.

set_12khz
SET     bis.b        #LFXT1S_2,&BCSCTL3    ;select VLOCLK for MCLK source
            bis.w        #SCG1+SCG0,SR              ;turn the DCO off
            bis.b        #SELM_3,&BCSCTL2        ;select VLOCLK for MCLK source
            ret

This code matches example code provided by TI, but although I believe all the register bits are set correctly, the toggle frequency on the port does not change.  The order of the instructions does not matter either (DCO can be turned off last, first or as shown).

Any ideas?

  • 0
    Prodigy 90 points
    Hi Richard,

    I am also using the same mcu MSP430G2553. I am also facing the same problem which you mentioned above (Switching MCLK from DCO to VLO).

    Did you find any work around for this ?

    Regards,
    Ashok
  • 0
    Guru 300740 points

    The oscillator fault flag (OFIFG) is always set at startup.

    As long as it is set, MCLK is sourced from the DCO regardless of configuration, so you must clear it:

    BCSCTL3 |= LFXT1S_2;            /* LFXT1 = VLO */
IFG1 &= ~OFIFG;                 /* clear oscillator fault */
BCSCTL2 |= SELM_3;              /* MCLK = LFXT1 = VLO */
__bis_SR_register(SCG1 + SCG0); /* stop DCO */
  • 0 in reply to AshokAG
    Prodigy 90 points
    Hi Richard,

    Here, is the configuration which may work for you.


    void switchClockDCOToVLO(void)
    {
    BCSCTL3 = LFXT1S_2; // Select VLO as clock
    IFG1 &= ~OFIFG; // Clear oscillator fault interrupt flag
    _NOP(); // Wait for atleast 50 usec or execute 1 instruction(83.3 usec) at 12 KHz.
    _NOP();
    BCSCTL2 = SELM_3; // Select MCLK source as VLO Clock with divider 1
    BCSCTL1 = (XT2OFF | DIVA_0); // ACLK = VLO/1 = 12 KHz
    }


    Regards,
    Ashok
  • 0 in reply to AshokAG
    Guru 300740 points
    The comment on the NOPs is wrong because the CPU clock has not yet been switched to VLO.

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