I design a board with the same hardware configuration as PYBOARD V1.1 but I meet a strange problem. After my board powering on, the LED Green and Yellow blink no matter I put any python code in the flash. Meanwhile, the program in the flash won't execute. After 60s, the board can connect to the computer and I notice the boot.py doesn't execute either( for I put 'import pyb' in it but I cannot use pyb in REPL). When I connect to the computer, I can use REPL without any error and all the functions are normal. I have no idea with this for a long time and I wonder if someone can help me. I will appreciate anyone who offers me help.