DM0,DM1,DM2,一起用于驱动模式
Bits 7 to 0: Drive Mode x[7:0]. In the PRTxDMx registers
there are eight possible drive modes for each port pin. Three
mode bits are required to select one of these modes, and
these three bits are spread into three different registers
(PRTxDM0, PRTxDM1, and PRTxDM2). The bit position of
the effected port pin (for example, Pin[2] in Port 0) is the
same as the bit position of each of the three drive mode register
bits that control the Drive mode for that pin (for example,
bit[2] in PRT0DM0, bit[2] in PRT0DM1, and bit[2] in
PRT0DM2). The three bits from the three registers are
treated as a group. These are referred to as DM2, DM1, and
DM0, or together as DM[2:0].