From cc9b3e65ca93b8a761b056123b0a21b6faf82a61 Mon Sep 17 00:00:00 2001
From: Jeonghoon Park <jh1979.park@samsung.com>
Date: Thu, 9 Aug 2018 19:02:16 +0900
Subject: [PATCH 1/1] implement voltage to ppm for example

---
 inc/co2-sensor.h |  6 +++--
 src/co2-sensor.c | 70 +++++++++++++++++++++++++++++++++++++++++++++++++-------
 2 files changed, 66 insertions(+), 10 deletions(-)

diff --git a/inc/co2-sensor.h b/inc/co2-sensor.h
index d9ee219..d3b2b39 100644
--- a/inc/co2-sensor.h
+++ b/inc/co2-sensor.h
@@ -34,9 +34,11 @@ void co2_sensor_close(void);
 /* utility functions */
 double co2_sensor_value_to_voltage(unsigned int value);
 
-/* Not implemented, please see comment in c source code */
+
+/* Fallowing functions art Not implemented, please see comments in c source code */
+int co2_sensor_set_calibration_values(unsigned int zero_point_v,
+		unsigned int second_point_ppm, unsigned int second_point_v);
 unsigned int co2_sensor_voltage_to_ppm(double voltage);
 unsigned int co2_sensor_value_to_ppm(unsigned int value);
 
 #endif /* __CO2_SENSOR_H__ */
-
diff --git a/src/co2-sensor.c b/src/co2-sensor.c
index 363f2c6..ce85c21 100644
--- a/src/co2-sensor.c
+++ b/src/co2-sensor.c
@@ -17,13 +17,26 @@
  ******************************************************************/
 
 #include <peripheral_io.h>
+#include <math.h>
 #include "adc-mcp3008.h"
 #include "log.h"
 
+#define CO2_SENSOR_VOLTAGE_GAIN (8.5)
+
 #define CO2_SENSOR_REF_VOLTAGE (5)
 #define CO2_SENSOR_VALUE_MAX (1024)
 
+/* CAUTION !!  These data are EXAMPLE Data, NOT REAL data */
+#define CO2_SENSOR_DEFAULT_VALUE_ZP (690) // 400ppm
+#define CO2_SENSOR_DEFAULT_VALUE_1000 (644)
+
+#define USE_EXAMPLE_CODE
+
+static const double log400 = 2.602;
+
 static bool initialized = false;
+static double co2_zp_volt = -1;
+static double slope_value = -1;
 
 void co2_sensor_close(void)
 {
@@ -41,6 +54,7 @@ int co2_sensor_read(int ch_num, unsigned int *out_value)
 		retv_if(ret != 0, -1);
 		initialized = true;
 	}
+
 	ret = adc_mcp3008_read(ch_num, &read_value);
 	retv_if(ret != 0, -1);
 
@@ -49,24 +63,64 @@ int co2_sensor_read(int ch_num, unsigned int *out_value)
 	return 0;
 }
 
-double co2_sensor_value_to_voltage(unsigned int value)
+static inline double __value_to_volt(unsigned int value)
+{
+	return (double)value
+		* (double)(CO2_SENSOR_REF_VOLTAGE / CO2_SENSOR_VALUE_MAX)
+		/ CO2_SENSOR_VOLTAGE_GAIN;
+}
+
+static double __calc_slope(double zp_volt, double sec_volt, double x_axis_diff)
+{
+	return (zp_volt - sec_volt) / (x_axis_diff);
+}
+
+/* To use to calibrate ppm value*/
+int co2_sensor_set_calibration_values(unsigned int zero_point_v,
+		unsigned int second_point_ppm, unsigned int second_point_v)
 {
-	double v = 0;
+	double sec_volt = -1;
+	double x_axis_diff = -1;
 
-	v = (double)value * CO2_SENSOR_REF_VOLTAGE / CO2_SENSOR_VALUE_MAX;
+	retv_if(zero_point_v <= second_point_v, -1);
+	retv_if(second_point_ppm <= 400, -1);
 
-	return v;
+	co2_zp_volt = __value_to_volt(zero_point_v);
+	sec_volt = __value_to_volt(second_point_v);
+	x_axis_diff = log400 - log(second_point_ppm);
+
+	slope_value = __calc_slope(co2_zp_volt, sec_volt, x_axis_diff);
+
+	return 0;
 }
 
-/* Not implemented, please see c source code */
-unsigned int co2_sensor_voltage_to_ppm(double voltage)
+double co2_sensor_value_to_voltage(unsigned int value)
 {
-	int ppm = 0;
+	retv_if(value > 1023, -100000000.0); // out of value range
+
+	return __value_to_volt(value);
+}
 
+/* Not implemented !!! */
+unsigned int co2_sensor_voltage_to_ppm(double voltage)
+{
+	double ppm = 0;
 	/* Make this function yourself */
 	/* Please ref. 'Theory' section in https://sandboxelectronics.com/?p=147#Theory */
 
-	return ppm;
+	/* Example */
+#ifdef USE_EXAMPLE_CODE
+	if (slope_value < 0)
+		co2_sensor_set_calibration_values(CO2_SENSOR_DEFAULT_VALUE_ZP,
+			CO2_SENSOR_DEFAULT_VALUE_1000, 1000);
+
+	if (voltage >= co2_zp_volt)
+		return 400;
+
+	ppm = pow(10, (voltage - co2_zp_volt)/slope_value + log400);
+#endif
+
+	return (unsigned int)ppm;
 }
 
 unsigned int co2_sensor_value_to_ppm(unsigned int value)
-- 
2.7.4