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Configuration to set MCLK to 3MHz

Sowmya BR
Intellectual 530 points
Other Parts Discussed in Thread: MSP430F4152

Hi,

I am working on MSP430F4152 controller and need to get 8MHz MCLK frequency from DCO.

Configuration is like this:

SCFI0 = 0x01 ;
    SCFI0 &= ~FN_8;
    SCFI0 |= FN_4;

    FLL_CTL0 |= DCOPLUS;
    FLL_CTL0 &= ~XTS_FLL;
    FLL_CTL1 |= SELM0 + SELM_DCO;

    _BIC_SR(OSCOFF);

    do {

        int i;

        /* Clear oscillator fault interrupt flag */
        IFG1 &= ~OFIFG;

        for (i = 0; i < 0xFF; i++);             /* Delay for oscillator to stabilize */

    } while (IFG1 & OFIFG);           /* Wait until oscillator interrupt flag and LF fault occurs */
    /* LF fault pin sets when oscillator is turned ON */

I am able to get 3.1Mhz on MCLK pin. And I am using MCLK as source for SPI but SPI clock is not getting exact square pulses.

Can anyone guide me in frequency settings or the issue I am facing?

Regards,

Sowmya

  • 0
    Guru 46710 points

    Sowmya BR said:
    I am using MCLK as source for SPI but SPI clock is not getting exact square pulses.

    To change SPI clock frequency, you don't need to change frequency of CPU clock! Please refer to User's Guide, SPI peripheral baudrate generation chapter. Look for clock prescaler/divider function

  • 0
    Mastermind 7575 points

    slau056l  chapter 18.2.5

    baud rate = ( BRCLK / UxBR) 

    in your case

    BRCLK= 3.1 MHz ( FN_4) , and baud rate = 3 MHz

    so

    UCBRx =  BRCLK /  baud rate

    UCBRx = 3.1MHz / 3 MHz = 1

    but you mistake is HERE:

    "The maximum baud rate that can be generated in master mode is BRCLK/2."

    so using 3.1 MHz BRCLK you could have maximum 1.55 MHz SPI clock  baudrate !

    you should set higher MCLK=DCOCLK=BRCLK clock frequency (for example 6 MHz)

    and after that you should configure SPI boud rate generator:

      UCA0BR0 = 0x02;                           //  6 MHz / 2 = 3 MHz
      UCA0BR1 = 0;                                 //
      UCA0MCTL = 0;                               // No modulation

    b.r.

    Lukasz, Poland

  • 0 in reply to Lukasz
    Intellectual 530 points

    Thank you Lukash and llmars for your reply.

    It is working now. I changed SPI baud rate to 1.5Mhz and checked.

    Regards,

    Sowmya

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