What is the 32KHz clock actually used for? The chip seems to work OK without it (I am not using the RTC)

I have been experimenting using an external 32KHz clock source with the 100k pull-up as per the data sheet. As long as I have 100K pull-up resistor on Pin 52, it always boots and WiFi’s OK. It does not matter what frequency I drive on the 32KHz input (pin 51). In fact even if I do not have anything connect to the 32KHz input (pin 51) – i.e. no clock source – the processor still boots and connects to my WiFi OK.

I do not use the real time clock, and it would be convenient in my volume production to no use a 32Khz crystal at all (save money and PCB space). However I see in the data sheet that it has a tight spec (no explanation of what it actually does though, other than RTC). I am assuming it is also used in the low power sleep modes (which I am also not using).

I also read on other posts that it is recommended.

Is the 32KHz (crystal, or external input) really needed in all situations? 

There is an internal RC oscillator which works before the 32Khz stabilizes, but its accuracy is very poor. Hence the device is able to boot up without 32KHz clock.  In sleep modes, the power consumption would be higher in case of poor clock quality. Also some of the modes like LPDS with always connected mode will not work reliably without RTC clock. Also once the device enters hibernate mode, it would not be able to wake-up without the 32Khz clock.

You may also see my other similar posts...

So As I read this .

We can use the internal RC oscillator only (no RTC crystal and no external 32khz source)

if:

1) We are not concerned about the extra power consumption

2) We are not using LPDS with always connect mode.

Obviously the 40MHz Oscillator is still required.

Is this correct (it is OK to use internal RC oscillator only)?

This clock will be used when you use Hibernate. Where Slow clock can be used as a wake up source and also RTC uses this clock

Also from datasheet you can see

32.768KHz crystal oscillator:
– Generates precision 32.768KHz for RTC and WLAN power-save protocol timing.
– Supports feeding an external square wave 32.768KHz clock in lieu of XTAL .
• 32KHz RC oscillator for chip startup: The 32.768KHz XTAL oscillator requires 1.1 sec to become
stable after first time power-up or chip reset (ie. nRESET). Until the slow xtal clock is stable, the
alternate RC slow-clock is used by the system.