Clock Monitoring unit in MPC5746C

[AshokSubramani]

I want to measure a FIRC & SIRC clock monitoring during runtime, if i use while(CMU.CSR.B.SFM == 1U){} my execution has to wait for sometime, to avoid that i am using a logic to come out of the loop if measurement not completed within the time period . so what is the expected time to complete the SIRC and FIRC measurement  by considering ageing factor and lifespan of MCU. Herewith I have added my logic which runs every 10ms. 

uint8 Clock_Monitor_Runtime(void)

{

    static ClockMonitorState state = FIRC_MONITOR_INIT; /* State variable for both monitors */

    static uint32 elapsed_time = 0U;                   /* Time tracker for timeout */

    static uint32 MeasDur = 0U;                        /* Measurement duration */

    static uint8 result = CLK_IN_RANGE;                              /* Default result to indicate progress */

    switch (state)

    {

        case FIRC_MONITOR_INIT:

            MeasDur = 0x8FFFU; /* Set FIRC measurement duration */

            CMU.CSR.B.CKSEL1 = 0x00U; /* Select FIRC clock or CLKMT0_RMN */

            CMU.MDR.B.MD = MeasDur; /* Set measurement duration */

            CMU.CSR.B.SFM = 0x01U; /* Start FIRC measurement */

            elapsed_time = 0U; /* Reset elapsed time */

            state = FIRC_MONITOR_WAIT;

            break;

        case FIRC_MONITOR_WAIT:

            if (CMU.CSR.B.SFM == 0U)

            {

                /* FIRC measurement completed */

                FIRC_MeaFreq = CMU.FDR.B.FD;

                Firc_MonitoredClk = ((float32)FXOC_CLK / (float32)FIRC_MeaFreq) * (float32)MeasDur;

                if ((Firc_MonitoredClk >= (float32)Min_FIRC_Freq) &&

                    (Firc_MonitoredClk <= (float32)Max_FIRC_Freq))

                {

                    result = CLK_IN_RANGE; /* FIRC clock is within range */

                    state = SIRC_MONITOR_INIT; /* Move to SIRC monitoring */

                }

                else

                {

                    result = CLK_OUT_RANGE; /* FIRC clock is out of range */

                    state = FIRC_MONITOR_INIT; /* Move to FIRC monitoring again */

                }

            }

            else

          {

             /* Increment elapsed time (10 ms per call)  */

              elapsed_time += 10U;

                if (elapsed_time >= 100U) /* Timeout after 100 ms */

                {

                    result = CLK_OUT_RANGE; /* Timeout occurred */

                    state = MONITOR_DONE; /* Skip to done */

                }

            }

            break;

}

[petervlna]

Hello,

what is the expected time to complete the SIRC and FIRC measurement 

The CMU uses the Measurement Duration Register (CMU_MDR) to define how long the measurement should take.

This duration is specified in terms of IRCOSC clock cycles (typically 16 MHz).

The register is 20 bits wide, allowing for a maximum duration of up to 1,048,575 cycle

Typical Measurement Time:

If you set CMU_MDR = 0xFFFFF (maximum value), the measurement duration would be:

petervlna_0-1754475170343.png

For shorter durations (e.g., CMU_MDR = 0x10000), the time would be:

petervlna_1-1754475180207.png

Best regards,

Peter

[AshokSubramani]

Is there any limitation on the minimum value that can be configured in the CMU_MDR (Measurement Duration Register) for clock monitoring?

Specifically, I have configured the following measurement durations:

• FIRC: CMU_MDR = 0x8FFF

• SIRC: CMU_MDR = 0x2020

Are these values valid for accurate and reliable frequency measurement?
Or is there a recommended minimum threshold that should be followed to ensure stability and correctness of the measurement?

[petervlna · Accepted solution]

Hello,

There is no strict minimum value documented for CMU_MDR, but:

Too small values (e.g., < 0x1000) may result in unstable or inaccurate measurements, especially for low-frequency clocks like SIRC.

The recommended minimum is typically ≥ 0x2000 for SIRC and ≥ 0x8000 for FIRC to ensure reliable frequency detection .

Please read carefully AN explaining the CMU:

https://www.nxp.com/docs/en/application-note/AN12080.pdf

Best regards,

Peter