It believe it might be related to the security bits of your KL25, I remember reading some discussion about it that the check on it was too stringent, I thought they changed it, but maybe not. Martin probably knows more about it.

@Yuting, can you share your binary? How are bits configured in the flash protection area (0x400-F) ?

Hi, Erik. Martin, Thanks for the swift replies, i checked the area(0x400f), it was the same as official firmware mbed_if_v2.1_frdm_kl25z.s19, the data in this area is 0xffffffff, 0xffffffff, 0xffffffff, 0xffffFDFE. i think the problem is caused because i connected the SWD_IO_OUT and SWD_IO_IN together (i was trying not to use 74vc125), this may interfere swd timing.it's ok, we'll design our board referring to the FRDM-KL25Z schematic.

now i have another problem. i wanna confirm the interface firmware compiled by the project from https://github.com/mbedmicro/CMSIS-DAP works fine on FRDM-KL25Z board. so i downloaded the project and compiled k20dx128_kl25z_mbed_bootloader_if project, but i can only get a binary file, we know that the offical file is a s19 file. and the bootloader inside K20 doesn't recognize bin file. is there some tools to help convert the bin file to s19 file?

best regards yuting you