topic Re: MCXN547: how to enable iRTC subSecond function ? in MCX Microcontrollers

MCXN547: how to enable iRTC subSecond function ?

jimmyli — Thu, 09 Oct 2025 06:16:20 GMT

Hi NXP,

I find that MCXN547 has subsecond function register for iRTC, and I consider using this function to record millisecond.

According to official SDK (SDK_2.x_FRDM-MCXN947), I import the iRTC driver example and add the code to test this function, but failed.

Can you help to check and provide some tips ?

Thanks;

1>subSecond register in MCXN reference manual

2>test code

based on example: frdmmcxn947_irtc_cm33_core0

hardware: FRDM-MCXN947

IRTC_EnableSubsecondCounter(RTC,true);

uint32_t subSecond_cnt = 0;

while(1){

subSecond_cnt = IRTC_GetSubsecondCount(RTC);

PRINTF("subSecond Counter is %d.\r\n",subSecond_cnt);

//intervals time

uint32_t i,j;

for(i=0;i<6534;i++)

for(j=0;j<6232;j++){

__NOP();

}

3>test result

the parameter "subSecond_cnt" is still 0.

Re: MCXN547: how to enable iRTC subSecond function ?

Harry_Zhang — Fri, 10 Oct 2025 02:50:20 GMT

Hi @jimmyli

The Subsecond counter use these register, not RTC_TEST2.

If you want to use is,

Please refer to chapter 56.3.2 Subsecond counter

You can try.

Harry

Re: MCXN547: how to enable iRTC subSecond function ?

jimmyli — Fri, 10 Oct 2025 03:06:10 GMT

Hi Harry,

Yes, you are right, the subsecond function has its own register.

But, I called the static inline void IRTC_EnableSubsecondCounter(RTC_Type *base, bool enable) and static inline uint32_t IRTC_GetSubsecondCount(RTC_Type *base) APIs in fsl_irtc.h to enable and read the subsecond counter, but failed.

Do you have any tips to help to enable this function ?

Thanks very much.

Re: MCXN547: how to enable iRTC subSecond function ?

Harry_Zhang — Fri, 10 Oct 2025 07:46:50 GMT

Hi @jimmyli

I just tested it on my end.

It can work.

According to the RM.

There are two important points to note.

1. You must independently enable the field by writing 1 to SUBSECOND_CTRL[SUB_SECOND_CNT_EN] after enabling RTC.CTRL[CLKOUT] (see chip-specific RTC_SUBSYSTEM for more information) to select the 1 Hz clock output.

2.In RTC.CTRL[CLKO_DIS] you should select The selected clock is output to other peripherals. 0b

This is my test code.

IRTC_GetDefaultConfig(&irtcConfig); irtcConfig.disableClockOutput = false; if (IRTC_Init(RTC, &irtcConfig) == kStatus_Fail) { return 1; } PRINTF("RTC Example START:\r\n"); RTC->CTRL &= (uint16_t)(~RTC_CTRL_CLKOUT_MASK); RTC->CTRL |= RTC_CTRL_CLKOUT((uint16_t)kIRTC_ClkoutFine1Hz); IRTC_EnableSubsecondCounter(RTC, true); uint32_t subSecond_cnt = 0; while(1){ subSecond_cnt = IRTC_GetSubsecondCount(RTC); PRINTF("subSecond Counter is %d.\r\n",subSecond_cnt); //intervals time uint32_t i,j; for(i=0;i<6534;i++) for(j=0;j<6232;j++){ __NOP(); } } for (;;) { }

Harry

Re: MCXN547: how to enable iRTC subSecond function ?

jimmyli — Fri, 10 Oct 2025 09:02:24 GMT

Hi Harry,

Thanks very much for your help!

I have checked with your code, and subsecond can work now.

But I have a question that when I convert subsecond counter to minisecond, the value seems wrong.

Eg: 1>default clock source for subsecond counter is 32KHz;

2>Minisecond Value = ( 1 / 32KHz ) * subsecond counter = 1/32000 x 53479 = 1.67 (s) > 1(s)

Do I calculate it wrongly ?

Re: MCXN547: how to enable iRTC subSecond function ?

Harry_Zhang — Tue, 14 Oct 2025 10:10:13 GMT

Hi @jimmyli

I think your calculation method is correct,

So i need to confirm with the internal team.

Harry

Re: MCXN547: how to enable iRTC subSecond function ?

Harry_Zhang — Wed, 15 Oct 2025 08:07:20 GMT

Hi @jimmyli

I have tested again.

By default, the RTC Clock select

/* 16.384kHz clock is selected */
config->clockSelect = kIRTC_Clk16K;

So the count should less than 16384

You just need to change

RTC->CTRL |= RTC_CTRL_CLKOUT((uint16_t)kIRTC_ClkoutFine1Hz);

IRTC_ConfigClockOut(RTC, kIRTC_ClkoutCoarse1Hz);

It can work.

Harry

Re: MCXN547: how to enable iRTC subSecond function ?

jimmyli — Wed, 15 Oct 2025 14:07:22 GMT

Hi Harry,

Got it.

Thanks very much.

Re: MCXN547: how to enable iRTC subSecond function ?

lucindaschatz — Wed, 15 Oct 2025 17:50:12 GMT

yes thanks