init_SPI(): set Slave Select pin high prior to enabling the SPI module, so that no slave peripheral is accidentally selected by an unintentional low signal on the SS pin.
Created separate header to store pin definitions, to promote better data encapsulation and code reusability.
Abstracted register/port/pin definitions from spi_master.h to pin_definitions.h.
Renamed register/port/pin definitions/mnemonics header from 'pin_defintions.h' to 'spi_mnemonics.h'.
Removed pin_definitions.h.
Updated #include statement for SPI mnemonics from 'pin_definitions.h' to 'spi_mnemonics.h'.
Defined macros for initiating & ending SPI communication cycles by setting Slave Select pin LOW (initiate) or HIGH (end).