verify checksum

	end

	local data = i2c.read(scd4x.bus_id, 9)

	i2c.stop(scd4x.bus_id)

	if not scd4x.crc_valid(data, 9) then

		return nil

	end

	local co2 = string.byte(data, 1) * 256 + string.byte(data, 2)

	local temp_raw = 175 * (string.byte(data, 4) * 256 + string.byte(data, 5))

	local humi_raw = 100 * (string.byte(data, 7) * 256 + string.byte(data, 8))

	return co2, temp_raw, humi_raw

end

function scd4x.crc_word(data, index)

	local crc = 0xff

	for i = index, index+1 do

		crc = bit.bxor(crc, string.byte(data, i))

		for j = 8, 1, -1 do

			if bit.isset(crc, 7) then

				crc = bit.bxor(bit.lshift(crc, 1), 0x31)

			else

				crc = bit.lshift(crc, 1)

			end

			crc = bit.band(crc, 0xff)

		end

	end

	return bit.band(crc, 0xff)

end

function scd4x.crc_valid(data, length)

	for i = 1, length, 3 do

		if scd4x.crc_word(data, i) ~= string.byte(data, i+2) then

			return false

		end

	end

	return true

end

return scd4x