How can I see the bootloader log via uart?

[bigbearishappy]

Hello.
I am working with my nrf52840 chip.
In order to debug the bootloader.I need to see the log of bootloader.
I tried to enable the uart in /src/nrfx_config.h with the following code:
`
#define NRFX_UART_ENABLED 1

#define NRFX_UART_DEFAULT_CONFIG_IRQ_PRIORITY 7

#define NRFX_UART_DEFAULT_CONFIG_HWFC NRF_UART_HWFC_DISABLED

#define NRFX_UART_DEFAULT_CONFIG_PARITY NRF_UART_PARITY_EXCLUDED

#define NRFX_UART_DEFAULT_CONFIG_BAUDRATE NRF_UART_BAUDRATE_115200

#define NRFX_UART_CONFIG_LOG_LEVEL 4
`
however I can't see any log in my serial assistant.
I connect the 52840 chip with my PC via a usb interface.
Is there any suggestion for me?
Thank you.