verify checksum

1 files changed, 28 insertions, 0 deletions

diff --git a/scd4x.lua b/scd4x.lua
index 47495e2..89679c1 100644
--- a/scd4x.lua
+++ b/scd4x.lua
@@ -36,10 +36,38 @@ function scd4x.read()
 	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