I didn't find the specific instruction on how to use external channels of ADC.
I've known that there are three decode signals MA0，MA1 and MA2 for a multiplexer, but will the decode signals be generated automatically? How should I configure this module if I want to use ADC external channels with a multiplexer?
MA signals are generated automatically when particular PINs are configured for this function
and your software will convert ADC channels 64-95. Understandably you will have to have use external multiplexers (up to four 8-to-1 MUXes).