Description
At first glance BW15-kit board looks a a very convinient device to play with libretiny. Everything is on board, many LEDs, the switch for Serail ports etc.
Unfortunately, ltchiptool can't flash it. As any other tool from either recommended or not-recommended tool from the list.
After some experiments I realised, that I still can upload firmware if USB port is used for power supply only, external USB->RS232 adapter is connected directly to RXD0, TXD0 pins and a default ceremeny of entering in download mode is used (hold PROG, click RST, release PROG).
So the problem is around embedded USB->RS232 serial adapter (CH340 chip).
According to datasheet, on this board, pin RTS of the CH340 chip is connected to CHIP_EN and pin DTR is controlling state of IO_0.
I don't not specifically how pins RTS/DTR are managed in CH340, but looks like first chunk of data from ltchiptool just reset the board and turn it out of download mode.
A bit later I found out, that AmebaZII_PGTool works seamlessly with that board and switch ony flash mode in a fully automatic way. But I didn't how it is implemented.