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Basically, the main difference from PIC16 to PIC18 series is the bits width of the core. PIC16 uses 14 bits core (which represents only 35 instructions) and PIC18 uses 16 bits core (more combinations of op codes for instructions = 75 instructions). Also PIC18 series have linear addressing (do not have the memory banks as PIC16). PIC18s have better performance and are faster (10MIPS againts 5MIPS of PIC16).