From 3aabcac3aa26b40b2712928b74e6bbfdf4f3efb4 Mon Sep 17 00:00:00 2001 From: Daniel Friesel Date: Thu, 26 May 2022 10:18:18 +0200 Subject: Verify CRC when reading data --- README.md | 1 + sen5x.lua | 31 +++++++++++++++++++++++++++++++ 2 files changed, 32 insertions(+) diff --git a/README.md b/README.md index 8b38677..ccfcfed 100644 --- a/README.md +++ b/README.md @@ -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. diff --git a/sen5x.lua b/sen5x.lua index afc9970..13bd972 100644 --- a/sen5x.lua +++ b/sen5x.lua @@ -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 -- cgit v1.2.3