From 64bb06882f96883ad60a26b2c14f86513e4b9500 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Mario=20H=C3=BCttel?= <mario.huettel@gmx.net>
Date: Tue, 2 Feb 2021 19:32:12 +0100
Subject: [PATCH] Issue #26: Add reverse lookup function to temp converter that
 converts a temperature to a PT1000 resistance value

---
 .../reflow-controller/temp-converter.h        |  9 ++++
 stm-firmware/temp-converter.c                 | 47 +++++++++++++++++++
 2 files changed, 56 insertions(+)

diff --git a/stm-firmware/include/reflow-controller/temp-converter.h b/stm-firmware/include/reflow-controller/temp-converter.h
index febd392..a126b13 100644
--- a/stm-firmware/include/reflow-controller/temp-converter.h
+++ b/stm-firmware/include/reflow-controller/temp-converter.h
@@ -29,4 +29,13 @@
  */
 int temp_converter_convert_resistance_to_temp(float resistance, float *temp_out);
 
+/**
+ * @brief Convert temperature to PT1000 resistance
+ * @param temp Temperature in degrees celsius
+ * @param[out] resistance_out Resistance value
+ * @return 0 if ok, -1 if tmeperature is below the lookup table, 1 if value is above the lookup table, -1000 in case of a pointer error,
+ *	   -100 if an internal error occured. This should never happen, if the lookup table is correct
+ */
+int temp_convertet_convert_temp_to_resistance(float temp, float *resistance_out);
+
 #endif /* __TEMP_CONVERTER_H__ */
diff --git a/stm-firmware/temp-converter.c b/stm-firmware/temp-converter.c
index b0649ac..0c87c41 100644
--- a/stm-firmware/temp-converter.c
+++ b/stm-firmware/temp-converter.c
@@ -21,6 +21,7 @@
 #include <reflow-controller/temp-converter.h>
 #include <reflow-controller/temp-converter-data.h>
 #include <helper-macros/helper-macros.h>
+#include <stdbool.h>
 
 static const float temp_lookup[(TEMP_CONVERSION_MAX_RES-TEMP_CONVERSION_MIN_RES) / TEMP_CONVERSION_RES_STEP+1] = {TEMP_CONVERSION_ARRAY_DATA};
 
@@ -73,3 +74,49 @@ int temp_converter_convert_resistance_to_temp(float resistance, float *temp_out)
 return_ret_val:
 	return ret_val;
 }
+
+int temp_convertet_convert_temp_to_resistance(float temp, float *resistance_out)
+{
+	int retcode = 0;
+	unsigned int i;
+	float lower_temp;
+	float upper_temp;
+	float lower_resistance;
+	float upper_resistance;
+	float temp_ratio;
+	bool found = false;
+
+	if (!resistance_out)
+		return -1000;
+
+	if (temp < temp_lookup[0]) {
+		/* Requested temperature is smaller than minimum value in lookup table */
+		*resistance_out = temp_lookup[0];
+		retcode = -1;
+		goto exit;
+	} else if (temp > temp_lookup[COUNT_OF(temp_lookup) - 1]) {
+		/* Requested temperature is higher than maximum value in lookup table */
+		*resistance_out = temp_lookup[COUNT_OF(temp_lookup) -1];
+		retcode = 1;
+		goto exit;
+	}
+
+	for (i = 0U; i < (COUNT_OF(temp_lookup) - 1); i++) {
+		if (temp >= temp_lookup[i] && temp <= temp_lookup[i+1]) {
+			upper_temp = temp_lookup[i+1];
+			lower_temp = temp_lookup[i];
+			lower_resistance = TEMP_CONVERSION_MIN_RES + TEMP_CONVERSION_RES_STEP * (i);
+			upper_resistance = lower_resistance + TEMP_CONVERSION_RES_STEP;
+			found = true;
+		}
+	}
+
+	if (!found)
+		return -100;
+
+	temp_ratio = (temp - lower_temp) / (upper_temp - lower_temp);
+	*resistance_out = lower_resistance + (upper_resistance - lower_resistance) * temp_ratio;
+
+exit:
+	return retcode;
+}