I have a teensy 3.2demo board, which can be programmed via usb, and mk20 can run the program normally. But when I replaced the MK20 on the board with a brand new blank MK20, and pressed the programming button, the teensy software was able to recognize it. After downloading the program, it prompts that the programming is complete, but it cannot be reset, and it cannot be aligned for programming at this time. To program it, click the button again. But it still shows that the programming is complete and cannot be reset. Disconnect and re-power the MK20 without running the downloaded program, as if the program was not successfully downloaded. The schematic diagram is as follows.
Click image for larger version. 

Name:	t32.PNG 
Views:	22 
Size:	89.3 KB 
ID:	21186

13:26:40.476 (loader): Device came online, code_size = 262144
13:26:40.477 (loader): Board is: Teensy 3.2 (MK20DX256), version 1.03
13:26:40.486 (loader): File "k20.hex". 1668 bytes, 1% used
13:26:40.491 (loader): set background IMG_ONLINE
13:26:40.725 (loader): HID/win32: HidD_GetPreparsedData ok, device still online :-)

13:30:11.960 (loader): Program event
13:30:11.969 (loader): File "k20.hex". 1668 bytes, 1% used
13:30:11.973 (loader): can't open file 'C:\Users\JP\Desktop\k20\k20\Objects\k20.elf' (error 2: 系统找不到指定的文件。)
13:30:11.976 (loader): elf file is for Unknown Board
13:30:11.979 (loader): begin operation
13:30:12.005 (loader): flash, block=0, bs=1024, auto=0
13:30:12.007 (loader): gauge old value = 0
13:30:12.008 (loader): flash, block=1, bs=1024, auto=0
13:30:12.010 (loader): gauge old value = 1
13:30:12.019 (loader): end operation, total time = 0.039 seconds
13:30:12.023 (loader): set background IMG_DOWNLOAD_OK
13:30:12.027 (loader): redraw timer set, image 12 to show for 1500 ms
13:30:13.526 (loader): redraw, image 10
13:30:13.529 (loader): set background IMG_ONLINE
Attachment 21184

13:30:30.922 (loader): Reboot event
13:30:30.922 (loader): begin operation
13:30:30.928 (loader): sending reboot
13:30:30.930 (loader): begin wait_until_offline
13:30:33.403 (loader): offline failed, timeout after 50
13:30:33.403 (loader): reboot error
13:30:33.404 (loader): end operation, total time = 2.482 seconds
13:30:33.405 (loader): set background IMG_REBOOT_ERROR
13:30:33.408 (loader): redraw timer set, image 13 to show for 4000 ms
13:30:37.397 (loader): redraw, image 10
13:30:37.398 (loader): set background IMG_ONLINE
Attachment 21183