mqx: C gas sensors mq2-mq9

Implement a single source file for the following:

    * mq2: H2/LPG/CH4/CO/alcohol/smoke/propane/etc
    * mq3: Alcohol/benzine/CH4/hexane/LPG/CO
    * mq4: CNG (mostly methane)
    * mq5: H2/LPG/CH4/CO/alcohol
    * mq6: LPG (mostly propane/butane)
    * mq7: CO
    * mq8: H
    * mq9: CO/CH4/LPG

The documentation for these sensors notes that they do not read an exact
gas concentration.  Some of the sensors have examples for how to
calibrate with a 'clean air environment' for outputing ppm, but this is
not the case for all mqx sensors.  Because of this, the output can be
read as a normalized output (0.0->1.0), raw volts (normalized * aRef),
or a scaled/offset voltage.

Signed-off-by: Noel Eck <noel.eck@intel.com>

src/gas/mqx.h

@@ -0,0 +1,153 @@
+/*
+ * Author:
+ * Copyright (c) 2015 Intel Corporation.
+ *
+ * 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.
+ */
+
+#pragma once
+
+#include "upm.h"
+#include "mraa/aio.h"
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+/*
+ * The mqx gas sensor driver can be used to read a wide range of sensors.  The
+ * list below shows the currently supported sensors:
+ *
+ *      mq2: H2/LPG/CH4/CO/alcohol/smoke/propane/etc
+ *          LPG/propane 200-5000 ppm
+ *          butane 300-5000 ppm
+ *          CH4 5000-2000 ppm
+ *          H2 300-5000 ppm
+ *          alcohol 100-2000 ppm
+ *      mq3: Alcohol/benzine/CH4/hexane/LPG/CO .05-10 mg/L
+ *      mq4: CNG (mostly methane) 200-10000 ppm
+ *      mq5: H2/LPG/CH4/CO/alcohol 200-10000 ppm
+ *      mq6: LPG (mostly propane/butane) 200-10000 ppm
+ *      mq7: CO 20-2000 ppm
+ *      mq8: H 100-10000 ppm
+ *      mq9: CO/CH4/LPG 200-1000/10000/10000 ppm
+ *
+ */
+
+/**
+ * driver context
+ */
+typedef struct _mqx_context {
+    /* mraa aio pin context */
+    mraa_aio_context aio;
+    /* Analog voltage reference */
+    float m_aRef;
+    /* Scale */
+    float m_scale;
+    /* Offset in sensor units */
+    float m_offset;
+} *mqx_context;
+
+/**
+ * Initialize analog sensor
+ * @param pin Analog pin
+ * @return sensor context
+ */
+mqx_context mqx_init(int16_t pin);
+
+/**
+ * Analog sensor destructor
+ * @param sensor context pointer
+ */
+void mqx_close(mqx_context dev);
+
+/**
+ * Set ADC reference voltage
+ * @param dev sensor context pointer
+ * @param aref ADC reference voltage
+ * @return Function result code
+ */
+upm_result_t mqx_set_aref(const mqx_context dev, float aref);
+
+/**
+ * Set sensor scale.  This scale is applied to the return value:
+ *     counts = counts * scale
+ * @param dev sensor context pointer
+ * @param scale count scale value used
+ * @return Function result code
+ */
+upm_result_t mqx_set_scale(const mqx_context dev, float scale);
+
+/**
+ * Set sensor offset.  This offset is applied to the return value:
+ *     counts = counts + offset
+ * @param dev sensor context pointer
+ * @param offset count offset value used
+ * @return Function result code
+ */
+upm_result_t mqx_set_offset(const mqx_context dev, float offset);
+
+/**
+ * Get sensor aref
+ * @param dev sensor context pointer
+ * @return Sensor ADC reference voltage
+ */
+float mqx_get_aref(const mqx_context dev);
+
+/**
+ * Get sensor scale
+ * @param dev sensor context pointer
+ * @return Sensor scale
+ */
+float mqx_get_scale(const mqx_context dev);
+
+/**
+ * Get sensor offset
+ * @param dev sensor context pointer
+ * @return Sensor offset
+ */
+float mqx_get_offset(const mqx_context dev);
+
+/**
+ * Read normalized value for sensor
+ * @param dev sensor context pointer
+ * @param *value Normalized value (0.0 -> 1.0)
+ * @return Function result code
+ */
+upm_result_t mqx_get_normalized(const mqx_context dev, float *value);
+
+/**
+ * Read raw voltage from the sensor
+ * @param dev sensor context pointer
+ * @param *value Raw sensor voltage
+ * @return Function result code
+ */
+upm_result_t mqx_get_raw_volts(const mqx_context dev, float *value);
+
+/**
+ * Read scaled/offset voltage from the sensor
+ * @param dev sensor context pointer
+ * @param *value Adjusted sensor voltage
+ * @return Function result code
+ */
+upm_result_t mqx_get_volts(const mqx_context dev, float *value);
+#ifdef __cplusplus
+}
+#endif