85 lines
2.1 KiB
Lua
85 lines
2.1 KiB
Lua
------------------------------------------------------------------------------
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-- DHT11/22 query module
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--
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-- LICENCE: http://opensource.org/licenses/MIT
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-- Vladimir Dronnikov <dronnikov@gmail.com>
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--
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-- Example:
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-- print("DHT11", dofile("dht22.lua").read(4))
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-- print("DHT22", dofile("dht22.lua").read(4, true))
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-- NB: the very first read sometimes fails
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------------------------------------------------------------------------------
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local M
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do
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-- cache
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local gpio = gpio
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local val = gpio.read
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local waitus = tmr.delay
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--
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local read = function(pin, dht22)
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-- wait for pin value
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local w = function(v)
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local c = 255
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while c > 0 and val(pin) ~= v do c = c - 1 end
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return c
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end
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-- NB: we preallocate incoming data buffer
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-- or precise timing in reader gets broken
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local b = { 0, 0, 0, 0, 0 }
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-- kick the device
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gpio.mode(pin, gpio.INPUT, gpio.PULLUP)
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gpio.write(pin, 1)
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waitus(10)
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gpio.mode(pin, gpio.OUTPUT)
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gpio.write(pin, 0)
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waitus(20000)
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gpio.write(pin, 1)
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gpio.mode(pin, gpio.INPUT, gpio.PULLUP)
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-- wait for device presense
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if w(0) == 0 or w(1) == 0 or w(0) == 0 then
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return nil, 0
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end
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-- receive 5 octets of data, msb first
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for i = 1, 5 do
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local x = 0
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for j = 1, 8 do
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x = x + x
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if w(1) == 0 then return nil, 1 end
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-- 70us for 1, 27 us for 0
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waitus(30)
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if val(pin) == 1 then
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x = x + 1
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if w(0) == 0 then return nil, 2 end
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end
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end
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b[i] = x
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end
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-- check crc. NB: calculating in receiver loop breaks timings
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local crc = 0
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for i = 1, 4 do
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crc = (crc + b[i]) % 256
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end
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if crc ~= b[5] then return nil, 3 end
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-- convert
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local t, h
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-- DHT22: values in tenths of unit, temperature can be negative
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if dht22 then
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h = b[1] * 256 + b[2]
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t = b[3] * 256 + b[4]
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if t > 0x8000 then t = -(t - 0x8000) end
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-- DHT11: no negative temperatures, only integers
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-- NB: return in 0.1 Celsius
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else
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h = 10 * b[1]
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t = 10 * b[3]
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end
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return t, h
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end
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-- expose interface
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M = {
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read = read,
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}
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end
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return M
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