Is it possible that DFU without 32.768k crystal on custom board?

Thanks in advanced. I want to implement OTA on sdk17.0.2 with nrf52832, but the example secure_bootloader and ble_app_buttonless_dfu are both configed with NRF_CLOCK_LF_SRC_XTAL. As I know, it should be NRF_CLOCK_LF_SRC_RC when there is no 32.768k crystal on board, I try it and get failed. 

So I wondered to know if 32.768k crystal must be existed then OTA can be implement? If not, how can I set params.

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