Unverified Commit 3aabcac3 authored by Birte Kristina Friesel's avatar Birte Kristina Friesel
Browse files

Verify CRC when reading data

parent 3f09903d

README.md

+1 −0
Original line number Diff line number Diff line
@@ -10,6 +10,7 @@ sen5x.lua has been tested with Lua 5.1 on NodeMCU firmware 3.0.1 (Release 
202112300746, integer build). It requires the following modules.



* i2c

* bit



The MQTT HomeAssistant integration in init.lua additionally needs the following

modules.

sen5x.lua

+31 −0
Original line number Diff line number Diff line
@@ -48,6 +48,9 @@ function sen5x.read() 
	end

	local data = i2c.read(sen5x.bus_id, 24)

	i2c.stop(sen5x.bus_id)

	if not sen5x.crc_valid(data, 24) then

		return false

	end

	sen5x.pm1 = sen5x.read_value(data, 1)

	sen5x.pm2_5 = sen5x.read_value(data, 4)

	sen5x.pm4 = sen5x.read_value(data, 7)
@@ -76,6 +79,9 @@ function sen5x.get_product() 
	end

	local data = i2c.read(sen5x.bus_id, 48)

	i2c.stop(sen5x.bus_id)

	if not sen5x.crc_valid(data, 48) then

		return false

	end

	local ret = ""

	ret = ret .. string.sub(data, 1, 2)

	ret = ret .. string.sub(data, 4, 5)
@@ -83,4 +89,29 @@ function sen5x.get_product() 
	return ret

end



function sen5x.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 sen5x.crc_valid(data, length)

	for i = 1, length, 3 do

		if sen5x.crc_word(data, i) ~= string.byte(data, i+2) then

			return false

		end

	end

	return true

end



return sen5x