Calculate a USBISS SPI divisor value from the input SPI clock speed

        :param sck: SPI clock frequency
        :type sck: int
        :returns: ISS SCK divisor
        :rtype: int

def iss_spi_divisor(self, sck):
        """
        Calculate a USBISS SPI divisor value from the input SPI clock speed

        :param sck: SPI clock frequency
        :type sck: int
        :returns: ISS SCK divisor
        :rtype: int
        """
        _divisor = (6000000 / sck) - 1
        divisor = int(_divisor)

        if divisor != _divisor:
            raise ValueError('Non-integer SCK divisor.')

        if not 1 <= divisor < 256:
            error = (
                "The value of sck_divisor, {}, "
                "is not between 0 and 255".format(divisor)
            )
            raise ValueError(error)
        return divisor