From d683d03fe802d65a419654451af8945b37949383 Mon Sep 17 00:00:00 2001
From: Leonardo Celente <leocelente@gmail.com>
Date: Thu, 17 Mar 2022 20:00:27 -0300
Subject: [PATCH] First release candidate of MAX6682 driver for #8

---
 MAX6682/README.md | 39 ++++++++++++++++++++++++++++
 MAX6682/max6682.c | 66 +++++++++++++++++++++++++++++++++++++++++++++++
 MAX6682/max6682.h | 30 +++++++++++++++++++++
 3 files changed, 135 insertions(+)
 create mode 100644 MAX6682/README.md
 create mode 100644 MAX6682/max6682.c
 create mode 100644 MAX6682/max6682.h

diff --git a/MAX6682/README.md b/MAX6682/README.md
new file mode 100644
index 0000000..29a5058
--- /dev/null
+++ b/MAX6682/README.md
@@ -0,0 +1,39 @@
+# MAX6682
+
+## Propósito
+Interface externa de termistores. O MAX6682 permite o controle da região de
+linearidade da relação entre resistência e temperatura de um termistor.
+
+
+## Exemplo
+```c
+spi_device_t spi = { .spi = &hspi1, 			// SPI Handle
+    .pin = { .port = MAX6682_CS_GPIO_Port, 	// CS GPIO Port
+        .pin = MAX6682_CS_Pin },		// CS GPIO Pin
+    };
+
+max6682_t max = { .dev = spi,		//
+    .Rext = 7680 // External Resistance (Rext)
+    };
+
+max6682_init(&max);
+result_float result = max6682_measure(&max);
+if(result.hasError){
+  printf("Invalid Resistance\r\n");
+}
+float R = result.value;
+printf("Thermistor Resistance: %.f\r\n", R);
+```
+
+## Documentação
+
+Para determinar a resistência externa $$R_ext$$ utilize a formula do datasheet
+
+![relação datasheet](https://i.imgur.com/9JAlXYt.png)
+
+Por último o parãmetro da resistência de calibração é a resistência 
+anunciada pelo termistor, isto é, a sua resistência esperada para temperatura
+de 25°C.
+
+## Notas
+Não esqueça de ativar printf com floats antes de rodar o exemplo.
diff --git a/MAX6682/max6682.c b/MAX6682/max6682.c
new file mode 100644
index 0000000..1806e51
--- /dev/null
+++ b/MAX6682/max6682.c
@@ -0,0 +1,66 @@
+/*
+ * thermistor.c
+ *
+ *  Created on: Mar 17, 2022
+ *      Author: leocelente
+ */
+
+#include "max6682.h"
+#include <math.h>
+
+// Vr+, voltage the sensor sets at pin R+, aka Rref
+#define V_Rp (1.220f)
+// Conversion constants: datasheet Page 4, Temperature Conversion
+#define C_1 (0.010404f)
+#define C_2 (0.174387f)
+
+/**
+ * Read from MAX6682 via SPI
+ */
+static result_uint16_t read(max6682_t const *max) {
+	uint8_t raw[2] = { 0 };
+	buffer_view_t view_raw = { .data = raw, .size = sizeof(raw) };
+	gpio_low(max->dev.pin);
+	error_t e = spi_receive(max->dev, view_raw);
+	gpio_high(max->dev.pin);
+	uint16_t value = (raw[0]) | (raw[1] << 8);
+	result_uint16_t out = { .hasError = e, .value = value };
+	return out;
+}
+
+error_t max6682_init(max6682_t *max) {
+	if (max->Rext <= 0.f) {
+		return ERROR;
+	}
+//	...
+	return SUCCESS;
+}
+
+// Convert raw data from SPI to resistance in Ohms
+static result_float convert(max6682_t const *max, uint16_t Dout) {
+	// voltage drop on Rext
+	float Vext = V_Rp * ((Dout * C_1)/8 + C_2);
+	// current passing through both resistors
+	float Iload = Vext / max->Rext;
+	// voltage drop in thermistor
+	float Vth = V_Rp - Vext;
+	// thermistor resistance
+	float R = Vth / Iload;
+
+	result_float result = { .value = R, .hasError = 0 };
+	if (R <= 0.f || isnan(R) || !isfinite(R)) {
+		result.hasError = ERROR;
+	}
+	return result;
+}
+
+result_float max6682_measure(max6682_t const *max) {
+	// output is 11 bits long
+	result_uint16_t result = read(max);
+	if (result.hasError) {
+		result_float failed = { .value = 0.f, .hasError = result.hasError };
+		return failed;
+	}
+	return convert(max, result.value);
+}
+
diff --git a/MAX6682/max6682.h b/MAX6682/max6682.h
new file mode 100644
index 0000000..c35d73d
--- /dev/null
+++ b/MAX6682/max6682.h
@@ -0,0 +1,30 @@
+/*
+ * thermistor.h
+ *
+ *  Created on: Mar 17, 2022
+ *      Author: leocelente
+ */
+
+#ifndef INC_MAX6682_H_
+#define INC_MAX6682_H_
+#include "platform/platform.h"
+
+
+typedef struct {
+	spi_device_t dev;
+	// Linearizing external resistance
+	float Rext;
+} max6682_t;
+
+/**
+ * Validates Rext and device connection
+ */
+error_t max6682_init(max6682_t * max);
+
+/**
+ * Reads SPI output and returns the thermistor resistance
+ */
+result_float max6682_measure(max6682_t const* max);
+
+
+#endif /* INC_MAX6682_H_ */