DS1820_1_MASK  EQU  01h  ;P2.0
DS1820_2_MASK  EQU  02h  ;P2.1
DS1820_3_MASK  EQU  04h  ;P2.2
[...]
   MOV   R2,#DS1820_2_MASK
   CALL  DS1820_RCV_BYTE
[...]
DS1820_RCV_BYTE:
    MOV	R5,#8
DS1820_RCV1:
    MOV A,R2   ;CLR BIT
    CPL A
    ANL P2,A
    NOP
    NOP
    CPL A      ;SET BIT
    ORL P2,A  
[...]