ds18b20: negatives handled

lua_examples/yet-another-ds18b20.lua

@@ -5,10 +5,11 @@
 -- Vladimir Dronnikov <dronnikov@gmail.com>
 --
 -- Example:
--- for k, v in pairs(require("ds18b20").read(4)) do print(k, v) end
+-- require("ds18b20").read(4, function(r) for k, v in pairs(r) do print(k, v) end end)
 ------------------------------------------------------------------------------
 local M
 do
+  local bit = bit
   local format_addr = function(a)
     return ("%02x-%02x%02x%02x%02x%02x%02x"):format(
         a:byte(1),
@@ -16,7 +17,7 @@ do
         a:byte(4), a:byte(3), a:byte(2)
       )
   end
-  local read = function(pin, delay)
+  local read = function(pin, cb, delay)
     local ow = require("ow")
     -- get list of relevant devices
     local d = { }
@@ -37,7 +38,7 @@ do
     ow.skip(pin)
     ow.write(pin, 0x44, 1)
     -- wait a bit
-    tmr.delay(delay or 100000)
+    tmr.alarm(0, delay or 100, 0, function()
     -- iterate over devices
       local r = { }
       for i = 1, #d do
@@ -49,13 +50,20 @@ do
         -- read data
         local x = ow.read_bytes(pin, 9)
         if ow.crc8(x) == 0 then
-        local t = (x:byte(1) + x:byte(2) * 256) * 625
+          local t = (x:byte(1) + x:byte(2) * 256)
-        -- NB: temperature in Celcius * 10^4
+          -- negatives?
+          if bit.isset(t, 15) then t = 1 - bit.bxor(t, 0xffff) end
+          -- NB: temperature in Celsius * 10^4
+          t = t * 625
+          -- NB: due 850000 means bad pullup. ignore
+          if t ~= 850000 then
             r[format_addr(d[i])] = t
+          end
           d[i] = nil
         end
       end
-    return r
+      cb(r)
+    end)
   end
   -- expose
   M = {