Hi @BryceWang
I hope that you are doing great!
To my understanding fig 9-18 is the boot flow when the MCU is set to boot in serial downloader. The ROM will wait for any commands from the Host, over LPUART1 or USB1 interfaces. This command could be as simple as the following example.
sdphost -u 0x1FC9,0x0130 -j -- error-status
To which the boot ROM will simply reply as shown in the below figure. The error command works as a simple ping command, that the host sends to discover if the MCU is active.
diego_charles_0-1706141183722.png
Below I provide an example, where the command write-file is used. Each command issued from the host receives a response.
diego_charles_1-1706141732646.png
If the MCU is not in serial downloader mode, there won't be any responses from the host.
I hope this could help,
Diego