FTM Frequency not being set appropriately

[stevenneves]

ftm_pwm_param_t flexTimer1_ChnConfig0 = {

  .mode = kFtmEdgeAlignedPWM,

  .edgeMode = kFtmLowTrue,

  .uFrequencyHZ = 100U,

  .uDutyCyclePercent = 50U,

  .uFirstEdgeDelayPercent = 0U,

};

Above is my initial FTM structure (KSDK 1.2 w/MQX).  I'm driving a series of stepper motors and BLDC motors that require a fixed duty cycle but a variable frequency (0-1000 Hz).  In the above I'm looking to come up driving my motor at 10% speed (the duty cycle is always fixed at 50% as long as I want it on).  When I measure the FTM generated signal however the 100 Hz I think I'm sending is actually 1250 Hz. 

[Hui_Ma]

Hi Steven,

Could you let us know the Kinetis product part number you are using?

I could do a same test on Kinetis related tower board.

Thank you for the attention.

Have a great day,
Ma Hui

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[stevenneves]

I actually figured out the problem.  There is a bug in your driver.  In ftm_status_t FTM_DRV_PwmStart(uint32_t instance, ftm_pwm_param_t *param, uint8_t channel)

uint32_t uFTMhz;

uint16_t uMod, uCnv, uCnvFirstEdge = 0;

uFTMhz = FTM_DRV_GetClock(instance);

    switch(param->mode)

    {

        case kFtmEdgeAlignedPWM:

            uMod = uFTMhz / (param->uFrequencyHZ) - 1;

            uCnv = uMod * param->uDutyCyclePercent / 100;

            /* For 100% duty cycle */

            if(uCnv >= uMod)

            {

                uCnv = uMod + 1;

            }

            FTM_HAL_SetMod(ftmBase, uMod);

            FTM_HAL_SetChnCountVal(ftmBase, channel, uCnv);

            break;

In the block of code above (my part is the KV31, 512 KB Flash, 100 pin package).  I run at 120 MHz, my bus clock ends up being 60 MHz.  uMod is stored as a unsigned 16 bit variable.  Using simple math virtually any value under 900 Hz will overflow.  (60,000,000/ 900) - 1 = 66665.  As the Bus speed increases we will lose more and more frequency values on the lower end with this bug.

[Hui_Ma]

Hi Steven,

For the FTM module counter is 16bit, the Max. counter value is 65535(65536), while you could change the FTM pre-scaler factor of the clock source with FTM_DRV_SetClock() function.

For example:

FTM_DRV_SetClock(BOARD_FTM3_IDX, kClock_source_FTM_SystemClk, kFtmDividedBy1);

The FTM clock pre-scaler factor is 1, you could change the pre-scaler factor to 2 or 4:

FTM_DRV_SetClock(BOARD_FTM3_IDX, kClock_source_FTM_SystemClk, kFtmDividedBy2);

There doesn't need to change the bus clock, it need to modify the FTM pre-scaler factor value.

Wish it helps.

Have a great day,
Ma Hui

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