what is "NRF_SYSTICK_CSR_CLKSOURCE_REF" for systick ?

[MCB]

Hello , 

There is two kind of clock source as i can see from the sdk files;

one of them is cpu that is ok but  for the first one i couldn't figure out it's source "NRF_SYSTICK_CSR_CLKSOURCE_REF" What is this source to the systick ?

[WestCoastDaz]

MCB

Systick is part of the ARM core.  It's not normally used as a clock source because it's stopped when the processor is put to sleep.

[MCB]

Thank you for reply but arm specification tell us ;

CLKSOURCE

Indicates the clock source: 0 = external clock 1 - processor clock.

But i can not see any information what is "external " refers for nrf52832 .

So as i understand external is not defined by arm ,it only says if we will set to 0 ,systick will use external clock and  i believe the "external" implemented by nordic in this case. Isnt it ?