mg811: Initial implementation

This driver was developed on a DFRobot CO2 sensor based on the MG811:
http://www.dfrobot.com/index.php?route=product/product&product_id=1023

It seems to *require* very precise calibration at 400ppm and 1000ppm
to be accurate.  It also gets pretty hot (due to the heater) and
consumes significant current.

Signed-off-by: Jon Trulson <jtrulson@ics.com>
Signed-off-by: sisinty sasmita patra <sisinty.s.patra@intel.com>

src/mg811/mg811.cxx

@@ -0,0 +1,94 @@
+/*
+ * Author: Jon Trulson <jtrulson@ics.com>
+ * Copyright (c) 2015 Intel Corporation.
+ *
+ * Thanks to public domain code by Martin Liddament for some useful clues!
+ * http://www.veetech.org.uk/CO2_Monitor_Sketch_2_Operation.txt
+ * and sandbox electronics (http://sandboxelectronics.com/?p=147).
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining
+ * a copy of this software and associated documentation files (the
+ * "Software"), to deal in the Software without restriction, including
+ * without limitation the rights to use, copy, modify, merge, publish,
+ * distribute, sublicense, and/or sell copies of the Software, and to
+ * permit persons to whom the Software is furnished to do so, subject to
+ * the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be
+ * included in all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
+ * EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
+ * MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
+ * NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE
+ * LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION
+ * OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION
+ * WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
+ */
+
+#include <iostream>
+#include <math.h>
+#include "mg811.h"
+
+using namespace std;
+using namespace upm;
+
+// voltage gain of the DC amplifier
+static const float dcGain = 8.5;
+
+MG811::MG811(int pin, int dpin, float aref) :
+  m_aio(pin), m_gpio(dpin)
+{
+  m_aRes = m_aio.getBit();
+  m_aref = aref;
+
+  m_gpio.dir(mraa::DIR_IN);
+
+  // these are just placeholder values (coarsely measured during
+  // development), you should determine the appropriate values (in
+  // volts) for your environment at the specified concentrations.  Use
+  // the getReferenceVoltage() method to get these values at 400ppm
+  // and 1000ppm respectively.  Good luck.
+  setCalibration(0.5514, 0.370);
+}
+
+MG811::~MG811()
+{
+}
+
+float MG811::volts()
+{
+  int val = m_aio.read();
+
+  return(float(val) * (m_aref / float(1 << m_aRes)));
+}
+
+void MG811::setCalibration(float ppm400, float ppm1000)
+{
+  m_zeroPointValue = ppm400;
+  m_reactionValue = ppm1000;
+}
+
+float MG811::getReferenceVoltage()
+{
+  return (volts() / dcGain);
+}
+
+float MG811::ppm()
+{
+  static const float log400 = log10f(400);
+  static const float log1000 = log10f(1000);
+
+  float val = volts();
+
+  if ((val / dcGain) >= m_zeroPointValue)
+    return 0.0;
+  else
+    return powf(10.0, ((val/dcGain)-m_zeroPointValue) / 
+                      (m_reactionValue / (log400-log1000))+log400);
+}
+
+bool MG811::thresholdReached()
+{
+  return (m_gpio.read() ? true : false);
+}