What is 32kHz used for on iMX8XL/DXL ?

[chirag_patel1]

Hello Folks, I'm working on an iMX8XL based product and wondering if we can get rid of external 32kHz crystal on RTC_XTAL pins. For instance we don't support any of the low power modes, ie iMX will either be Fully up Or Completely powered down.

According to ref manual, the 32kHz clock supplies the following blocks

PWM, GPT, KPP, CAAM, SYSCTR, WDOG, FlexCAN

If needed, can 32kHz clock be derived off of high frequency clock that's derived from external 24MHz crystal across XTAL pins (assume 150ppm for crystal tolerance across temperature)? If yes, can you provide details? What is the accuracy if derived this way?

Will iMX power up and run off of internal RC in absence of external 32kHz clock/crystal? Is 32kHz clock needed for powerup?

[chirag_patel1]

Hi Gusarambula, can 32kHz be generated off of high freq 24MHz crystal?

[gusarambula]

Hello Chirags Patel,

Since you would like to avoid extra crystals on your design, I’m assuming you mean to drive the 32Khz from the 24Mhz crystal either trough external circuitry or internally.

Internally this is not possible. Externally it would not be ideal as adding an additional load to the crystal may affect its signal integrity.

Also, ideally you would use a 32.768 kHz crystal or external oscillator, but a 32 kHz oscillator may work if keeping the RTC on time is not critical to your application.

My apologies for the inconvenience but using an external 32.768 kHz is the recommended configuration to ensure robust operation of the processor.

Regards,

[gusarambula]

Hello Chirags Patel,

The internal ring oscillator would be present anytime there is not an external 32Khz source. The processor can boot and won’t shutdown in the absence of a 32Khz clock, but the internal ring oscillator should not be used as the sole 32Khz provider.

One of the main and more noticeable drawbacks of using the internal ring oscillator is that the RTC would not be very accurate due to frequency variations, but other of the modules that use this clock may be affected by these variations. The usage of this processor without an external 32Khz clock has not been validated and is not meant to be used in customer applications since robust operation cannot be guaranteed without an external 32Khz source.

My apologies for the inconvenience.

Regards,