Kinetis Software Development KitのトピックDiscovering LPTMR frequency

Discovering LPTMR frequency

robertpoor — Sat, 25 Mar 2017 00:39:22 GMT

On my KL27Z, I have configured LPTMR0 to source its clock from kLPTMR_PrescalerClock_0 (which is MCGIRCLK, right?) like this:

  lptmr_config_t lptmrConfig;
  LPTMR_GetDefaultConfig(&lptmrConfig);
  lptmrConfig.prescalerClockSource = kLPTMR_PrescalerClock_0;  // MCGIRCLK?
  LPTMR_Init(LPTMR0, &lptmrConfig);

What is the correct way to find its frequency? I could do something like this:

uint64_t clock_freq_in_hz = CLOCK_GetFreq(kCLOCK_McgInternalRefClk);

... but I'm not sure that kCLOCK_McgInternalRefClk is the right constant (though empirically it appears to be correct). Is there a more dependable way to get the LPTMR's clock frequency, perhaps one that uses the actual state of its configuration registers?

Re: Discovering LPTMR frequency

xiangjun_rong — Mon, 02 Nov 2020 13:52:29 GMT

Hi,

This is the LPTME clock source from the 5.7.5 LPTMR clocking in RM of Kl27:

Hope it can help you

void LPTMR_Init(LPTMR_Type *base, const lptmr_config_t *config)
{
assert(config);

#if !(defined(FSL_SDK_DISABLE_DRIVER_CLOCK_CONTROL) && FSL_SDK_DISABLE_DRIVER_CLOCK_CONTROL)
/* Ungate the LPTMR clock*/
CLOCK_EnableClock(s_lptmrClocks[LPTMR_GetInstance(base)]);
#endif /* FSL_SDK_DISABLE_DRIVER_CLOCK_CONTROL */

    /* Configure the timers operation mode and input pin setup */
    base->CSR = (LPTMR_CSR_TMS(config->timerMode) | LPTMR_CSR_TFC(config->enableFreeRunning) |
                 LPTMR_CSR_TPP(config->pinPolarity) | LPTMR_CSR_TPS(config->pinSelect));

    /* Configure the prescale value and clock source */
    base->PSR = (LPTMR_PSR_PRESCALE(config->value) | LPTMR_PSR_PBYP(config->bypassPrescaler) |
                 LPTMR_PSR_PCS(config->prescalerClockSource));
}

lptmrConfig.prescalerClockSource is used to write PCS bits in LPTMRx_PSR register.

Hope it can help you

XiangJun Rong

Re: Discovering LPTMR frequency

robertpoor — Thu, 13 Apr 2017 01:21:50 GMT

I understand the hardware that generates the clock signals. What I don't understand is what compiler constants correspond to those signals.

For example, in figure 5-5 that you show above, which signal corresponds to kCLOCK_McgInternalRefClk? And does kLPTMR_PrescalerClock_0 correspond to MCGIRCLK in that diagram? (I agree that the _0 suffix suggests that it's the mux 00 input, but I'm not sure.) Is lptmrConfig.prescalerClockSource responsible for setting the LPTMRx_PSR[PCS] source?

Etc...

The relationship between these constants and the register settings must be documented somewhere, or is that "documentation" the source code itself?