ok, I copyt he reply from expert team to you :
"
I confirmed with SOC design and this is what they said. He was answering the question I asked: Please confirm for the customer this is the external clock mux input and be clear what happens if the 24 MHz crystal is not enabled while the SysTick timer is using it.
The 24MHz XTALOSC is the external clock source of the systick. Note that it is divided down to 100KHz before it goes to systick. Also note that in case XTALOSC24M_LOWPWR_CTRLn[OSC_SEL] is set the whole chip’s 24Mhz clock will be from RCOSC.
The 24MHz crystal must exist, otherwise the chip won’t power up. In low power modes user may disable the 24MHz crystal but before that they must enable the 24Mhz RCOSC otherwise the system fails.
"
