Hi,
Please check the following application note, in this appnote a simple Secondary Bootloader (SBL) software is designed. It can optionally load the new image from user UART terminal. The YModem file transfer protocol is used in this design to transfer the image file from PC to the board. In-Application Programming (IAP) feature is used to download the firmware's image to on-chip FLASH memory. The demo project is developed based on LPCXpresso54608 board and MCUXpresso SDK Software Library.
https://www.nxp.com/docs/en/application-note/AN12384.pdf
The following Figure shows the basic board connections used to power the LPC546xx. devices, connect the external crystal and the 32 kHz oscillator for the RTC, and provide debug capabilities via the serial wire port.

Have a great day,
Sol
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