yet another dht11/22 module added

lua_examples/yet-another-dht22.lua

@@ -0,0 +1,84 @@
+------------------------------------------------------------------------------
+-- DHT11/22 query module
+--
+-- LICENCE: http://opensource.org/licenses/MIT
+-- Vladimir Dronnikov <dronnikov@gmail.com>
+--
+-- Example:
+-- print("DHT11", require("dht22").read(4))
+-- print("DHT22", require("dht22").read(4, true))
+-- NB: the very first read sometimes fails
+------------------------------------------------------------------------------
+local M
+do
+  -- cache
+  local gpio = gpio
+  local val = gpio.read
+  local waitus = tmr.delay
+  --
+  local read = function(pin, dht22)
+    -- wait for pin value
+    local w = function(v)
+      local c = 255
+      while c > 0 and val(pin) ~= v do c = c - 1 end
+      return c
+    end
+    -- NB: we preallocate incoming data buffer
+    --     or precise timing in reader gets broken
+    local b = { 0, 0, 0, 0, 0 }
+
+    -- kick the device
+    gpio.mode(pin, gpio.INPUT, gpio.PULLUP)
+    gpio.write(pin, 1)
+    waitus(10)
+    gpio.mode(pin, gpio.OUTPUT)
+    gpio.write(pin, 0)
+    waitus(20000)
+    gpio.write(pin, 1)
+    gpio.mode(pin, gpio.INPUT, gpio.PULLUP)
+    -- wait for device presense
+    if w(0) == 0 or w(1) == 0 or w(0) == 0 then
+      return nil, 0
+    end
+    -- receive 5 octets of data, msb first
+    for i = 1, 5 do
+      local x = 0
+      for j = 1, 8 do
+        x = x + x
+        if w(1) == 0 then return nil, 1 end
+        -- 70us for 1, 27 us for 0
+        waitus(30)
+        if val(pin) == 1 then
+          x = x + 1
+          if w(0) == 0 then return nil, 2 end
+        end
+      end
+      b[i] = x
+    end
+    -- check crc. NB: calculating in receiver loop breaks timings
+    local crc = 0
+    for i = 1, 4 do
+      crc = (crc + b[i]) % 256
+    end
+    if crc ~= b[5] then return nil, 3 end
+    -- convert
+    local t, h
+    -- DHT22: values in tenths of unit, temperature can be negative
+    if dht22 then
+      h = b[1] * 256 + b[2]
+      t = b[3] * 256 + b[4]
+      if t > 0x8000 then t = -(t - 0x8000) end
+    -- DHT11: no negative temperatures, only integers
+    -- NB: return in 0.1 Celsius
+    else
+      h = 10 * b[1]
+      t = 10 * b[3]
+    end
+    return t, h
+  end
+  -- expose interface
+  M = {
+    read = read,
+  }
+end
+return M