dfrorp: Initial implementation

This module implements support for the DFRobot Analog ORP
(Oxidation/Reduction Potential) Meter.

It requires 5.0 volts, but the more accurate the voltage specified (to
the constructor), the more accurate the meter (paraphrased from the
wiki).

Signed-off-by: Jon Trulson <jtrulson@ics.com>

src/dfrorp/dfrorp.c

@@ -0,0 +1,165 @@
+/*
+ * Author: Jon Trulson <jtrulson@ics.com>
+ * Copyright (c) 2016 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.
+ */
+
+#include <string.h>
+#include <assert.h>
+
+#include "dfrorp.h"
+
+#include "upm_utilities.h"
+
+#define DFRORP_NUM_SAMPLES 10
+// Resistor R2 value in schematic, in KOhms
+#define DFRORP_R2 30.0
+// Resistor R3 value in schematic, in KOhms
+#define DFRORP_R3 75.0
+
+static float average(const dfrorp_context dev, int samples)
+{
+  int sum = 0;
+
+  if (samples < 1)
+    samples = 1;
+
+  int i;
+  for (i=0; i< samples; i++)
+    {
+      int j = mraa_aio_read(dev->aio);
+      if (j < 0)
+        {
+          printf("%s: mraa_aio_read() failed.\n", __FUNCTION__);
+          return -1.0;
+        }
+      sum += j;
+      upm_delay_ms(20);
+    }
+
+  return (float)(sum / samples);
+}
+
+dfrorp_context dfrorp_init(unsigned int apin, float a_ref)
+{
+  dfrorp_context dev =
+    (dfrorp_context)malloc(sizeof(struct _dfrorp_context));
+
+  if (!dev)
+    return NULL;
+
+  // zero out context
+  memset((void *)dev, 0, sizeof(struct _dfrorp_context));
+
+  dev->aio = NULL;
+
+  dev->a_ref = a_ref;
+
+  dev->offset = 0.0;
+  dev->scale = 1.0;
+
+  // initialize the MRAA context
+
+  if (!(dev->aio = mraa_aio_init(apin)))
+    {
+      printf("%s: mraa_aio_init() failed.\n", __FUNCTION__);
+      dfrorp_close(dev);
+      return NULL;
+    }
+
+  // set our analog resolution
+  dev->a_res = (float)(1 << mraa_aio_get_bit(dev->aio)) - 1;
+
+  return dev;
+}
+
+void dfrorp_close(dfrorp_context dev)
+{
+  assert(dev != NULL);
+
+  if (dev->aio)
+    mraa_aio_close(dev->aio);
+
+  free(dev);
+}
+
+void dfrorp_set_offset(const dfrorp_context dev, float offset)
+{
+  assert(dev != NULL);
+
+  dev->offset = offset;
+}
+
+void dfrorp_set_scale(const dfrorp_context dev, float scale)
+{
+  assert(dev != NULL);
+
+  dev->scale = scale;
+}
+
+upm_result_t dfrorp_update(const dfrorp_context dev)
+{
+  assert(dev != NULL);
+
+  float sample = average(dev, DFRORP_NUM_SAMPLES);
+  if (sample == -1.0)
+    return UPM_ERROR_OPERATION_FAILED;
+
+  dev->normalized = sample / dev->a_res;
+  dev->volts = dev->normalized * dev->a_ref;
+
+  float volts = dev->volts + dev->orp_cal_offset;
+
+  // From the DFRobot site
+  dev->orp = ( (DFRORP_R2 * dev->a_ref * 1000.0) -
+               (DFRORP_R3 * sample * dev->a_ref * 1000.0 / dev->a_res) ) /
+    DFRORP_R3 - dev->orp_cal_offset;
+
+  return UPM_SUCCESS;
+}
+
+float dfrorp_get_orp(const dfrorp_context dev)
+{
+  assert(dev != NULL);
+
+  return dev->orp * dev->scale + (dev->offset * dev->scale);
+}
+
+float dfrorp_get_volts(const dfrorp_context dev)
+{
+  assert(dev != NULL);
+
+  return dev->volts;
+}
+
+float dfrorp_get_normalized(const dfrorp_context dev)
+{
+  assert(dev != NULL);
+
+  return dev->normalized;
+}
+
+void dfrorp_set_calibration_offset(const dfrorp_context dev, float offset)
+{
+  assert(dev != NULL);
+
+  dev->orp_cal_offset = offset;
+}