samples: sensor: add MAX6675 sample application

Add a sample application to test MAX6675 cold-junction-compensated
K-thermocouple to digital converter.

Signed-off-by: Gerard Marull-Paretas <[email protected]>

Author: gmarull

samples/sensor/max6675/CMakeLists.txt

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+# Copyright (c) 2021 Teslabs Engineering S.L.
+# SPDX-License-Identifier: Apache-2.0
+
+cmake_minimum_required(VERSION 3.13.1)
+find_package(Zephyr REQUIRED HINTS $ENV{ZEPHYR_BASE})
+project(max6675)
+
+FILE(GLOB app_sources src/*.c)
+target_sources(app PRIVATE ${app_sources})

samples/sensor/max6675/README.rst

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+MAX6675 K-thermocouple to digital converter
+###########################################
+
+Overview
+********
+
+This is a sample application to read an external MAX6675
+cold-junction-compensated K-thermocouple to digital converter.
+
+Requirements
+************
+
+- MAX6675 wired to your board SPI bus
+- K-thermocouple connected to MAX6675 T+/T- inputs
+
+References
+**********
+
+ - MAX6675: https://datasheets.maximintegrated.com/en/ds/MAX6675.pdf
+
+Building and Running
+********************
+
+This sample can be built with any board that supports SPI. A sample overlay is
+provided for the NUCLEO-F030R8 board.
+
+.. zephyr-app-commands::
+   :zephyr-app: samples/sensor/max6675
+   :board: nucleo_f030r8
+   :goals: build
+   :compact:
+
+Sample Output
+=============
+
+The application will read and print sensor temperature every second. Note that
+temperature fetch will fail if the K-thermocouple is not connected. This is
+because MAX6675 is able to detect if the K-thermocouple is connected or not.
+
+.. code-block:: console
+
+   Temperature: 25.25 C
+   Temperature: 25.50 C
+
+   <repeats endlessly every second>

samples/sensor/max6675/boards/nucleo_f030r8.overlay

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+/*
+ * Copyright (c) 2021 Teslabs Engineering S.L.
+ *
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+&arduino_spi {
+	max6675@0 {
+		compatible = "maxim,max6675";
+		reg = <0>;
+		spi-max-frequency = <4300000>;
+		label = "MAX6675";
+	};
+};

samples/sensor/max6675/prj.conf

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+CONFIG_CBPRINTF_FP_SUPPORT=y
+CONFIG_SPI=y
+CONFIG_SENSOR=y
+CONFIG_MAX6675=y

samples/sensor/max6675/sample.yaml

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+sample:
+  name: MAX6675 K-thermocouple to digital converter
+tests:
+  sample.sensor.max6675:
+    tags: sensors
+    harness: console
+    depends_on: spi
+    harness_config:
+        type: multi_line
+        ordered: true
+        regex:
+            - "Temperature: ([0-9\\.]+) C"
+        fixture: fixture_spi_max6675
+    filter: dt_compat_enabled("maxim,max6675")

samples/sensor/max6675/src/main.c

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+/*
+ * Copyright (c) 2021 Teslabs Engineering S.L.
+ *
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+#include <stdio.h>
+
+#include <zephyr.h>
+#include <device.h>
+#include <drivers/sensor.h>
+
+/**
+ * @file Sample app using the MAX6675 cold-junction-compensated K-thermocouple
+ *	 to digital converter.
+ *
+ * This app will read and display the sensor temperature every second.
+ */
+
+void main(void)
+{
+	const struct device *dev;
+	struct sensor_value val;
+
+	dev = device_get_binding(DT_LABEL(DT_INST(0, maxim_max6675)));
+	if (dev == NULL) {
+		printf("Could not obtain MAX6675 device\n");
+		return;
+	}
+
+	while (1) {
+		int ret;
+
+		ret = sensor_sample_fetch_chan(dev, SENSOR_CHAN_AMBIENT_TEMP);
+		if (ret < 0) {
+			printf("Could not fetch temperature (%d)\n", ret);
+			return;
+		}
+
+		ret = sensor_channel_get(dev, SENSOR_CHAN_AMBIENT_TEMP, &val);
+		if (ret < 0) {
+			printf("Could not get temperature (%d)\n", ret);
+			return;
+		}
+
+		printf("Temperature: %.2f C\n", sensor_value_to_double(&val));
+
+		k_sleep(K_MSEC(1000));
+	}
+}