code: def set_mode(self, mode):
          
          if mode < 0 or mode > 3:
              raise ValueError('Mode must be a value 0, 1, 2, or 3.')
          if mode & 0x02:
              # Clock is normally high in mode 2 and 3.
              self._clock_base = GPIO.HIGH
          else:
              # Clock is normally low in mode 0 and 1.
              self._clock_base = GPIO.LOW
          if mode & 0x01:
              # Read on trailing edge in mode 1 and 3.
              self._read_leading = False
          else:
              # Read on leading edge in mode 0 and 2.
              self._read_leading = True
          # Put clock into its base state.
          self._gpio.output(self._sclk, self._clock_base)
docs: Set SPI mode which controls clock polarity and phase.  Should be a
          numeric value 0, 1, 2, or 3.  See wikipedia page for details on meaning:
          http://en.wikipedia.org/wiki/Serial_Peripheral_Interface_Bus