Using the CMU on the MPC5744 for IRCOSC Measurement

[ahmedsoubky]

Hi,

According to the MPC5744 reference manual ( Rev. 5, 04/2014 ):

I am using CMU_0 for the IRCOSC measurement and the reference clock is the XOSC.

When I write the value 0x64 in CMU_MDR[MD] register, I get the correct frequency which is, as per the above formula, 16 MHz given that the XOSC frequency is 40MHz.

When I write any other value in CMU_MDR[MD] register, I get incorrect values of the IRCOSC frequency. Do you have any explanation for this ?


Note: Also according to the reference manual,

MDR field is R/W and from my understanding we write into this register the number of clock cycles during which IROSC frequency measurement is done. Is that correct ?

[PetrS]

Hi,

If you changed the MD, what is then a FD value you read after measurement is finished?

The MD specifies a number of IRC clock cycles to be measured.

The FD is loaded with number of XOSC clock cycles counted during measurement period.

BR, Petr

[ahmedsoubky]

Hi Petr,

Thanks for your response.

The FD value that I read after measurement was a normal value in the debugger.

I guess the problem was not waiting enough time for the measurement to be finished successfully and it was resolved.

I have another question regarding the setting CMU_CSR register, as per the manual:

CMU_CSR[CKSEL1]  values 00 and 11, do both values select CLKMT0_RMN ?

Thanks in advance,

Best Regards,

[PetrS]

As per my testing the IRC is always measured regardless of CKSEL1 setting.

I was using this code

float IRC_measure(void)

{

    float IRC;

    uint32_t measDur = 50000;   /* 0 – 1 048 575 */

    CMU_0.CSR.B.CKSEL1 = 0;   /* Measure IRC */

    CMU_0.MDR.R = measDur;

    CMU_0.CSR.B.SFM = 1;

    while(CMU_0.CSR.B.SFM==1){};

    IRC = (float)((40.0 * measDur) / CMU_0.FDR.R);

    return IRC;

}

BR, Petr