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local scd4x = {}
local addr = 0x62
function scd4x.start()
i2c.start(0)
if not i2c.address(0, addr, i2c.TRANSMITTER) then
return false
end
i2c.write(0, {0x21, 0xb1})
i2c.stop(0)
return true
end
function scd4x.stop()
i2c.start(0)
if not i2c.address(0, addr, i2c.TRANSMITTER) then
return false
end
i2c.write(0, {0x3f, 0x86})
i2c.stop(0)
return true
end
function scd4x.read()
i2c.start(0)
if not i2c.address(0, addr, i2c.TRANSMITTER) then
return nil
end
i2c.write(0, {0xec, 0x05})
i2c.stop(0)
i2c.start(0)
if not i2c.address(0, addr, i2c.RECEIVER) then
return nil
end
local data = i2c.read(0, 9)
i2c.stop(0)
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
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