打开COMP时钟
Internal reference voltage output can be routed to any I/O of group 3 following this procedure:
1. Set the bit VREFOUTEN in COMP_CSR3
2. Close the I/O switch of any I/O of group 3 by setting CH7E in RI_IOSR1 or CH8E in RI_IOSR2 or CH9E in RI_IOSR3.