samples: shields: lmp90100_evb: add rtd sample

Add sample for reading the temperature of a 3-wire PT100 sensor using
the Texas Instruments LMP90100 Sensor Analog Frontend (AFE) Evaluation
Board (EVB) shield.

Signed-off-by: Henrik Brix Andersen <hebad@vestas.com>

samples/shields/lmp90100_evb/lmp90100_evb.rst

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+.. _shields-lmp90100_evb-samples:
+
+LMP90100 Sensor AFE Evaluation Board Shield Samples
+###################################################
+
+.. toctree::
+   :maxdepth: 1
+   :glob:
+
+   **/*

samples/shields/lmp90100_evb/rtd/CMakeLists.txt

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+# SPDX-License-Identifier: Apache-2.0
+
+cmake_minimum_required(VERSION 3.13.1)
+
+# This sample is specific to lmp90100_evb shield. Enforce -DSHIELD option
+set(SHIELD lmp90100_evb)
+
+include($ENV{ZEPHYR_BASE}/cmake/app/boilerplate.cmake NO_POLICY_SCOPE)
+project(lmp90100_evb_rtd)
+
+FILE(GLOB app_sources src/*.c)
+target_sources(app PRIVATE ${app_sources})

samples/shields/lmp90100_evb/rtd/README.rst

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+.. _lmp90100_evb_rtd_sample:
+
+LMP90100 Sensor AFE Evaluation Board: RTD Sample
+################################################
+
+Overview
+********
+
+This sample is provided as an example of how to read the temperature
+of a Resistance Temperature Detector (RTD) sensor using the LMP90100
+Sensor AFE Evaluation Board shield. The sample is designed for use
+with a 3-wire PT100 sensor.
+
+Please refer to :ref:`lmp90100_evb_shield` for more information on
+this shield.
+
+Requirements
+************
+
+Prior to running the sample application, the LMP90100 EVB must be
+connected to the development board according to Example #3 ("3-wire
+RTD Application") in the `LMP90100 Sensor AFE Evaluation Board User's
+Guide`_.
+
+Building and Running
+********************
+
+This sample runs with the LMP90100 EVB connected to any development
+board with a matching Arduino connector. For this example, we use a
+:ref:`frdm_k64f` board.
+
+.. zephyr-app-commands::
+   :zephyr-app: samples/shields/lmp90100_evb/rtd
+   :board: frdm_k64f
+   :goals: build
+   :compact:
+
+Sample Output
+=============
+
+ .. code-block:: console
+
+    R: 111.15 ohm
+    T: 28.66 degC
+    R: 111.14 ohm
+    T: 28.64 degC
+    R: 111.14 ohm
+    T: 28.63 degC
+    R: 111.13 ohm
+    T: 28.61 degC
+
+.. _LMP90100 Sensor AFE Evaluation Board User's Guide:
+   http://www.ti.com/lit/pdf/snau028

samples/shields/lmp90100_evb/rtd/app.overlay

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+/*
+ * Copyright (c) 2019 Vestas Wind Systems A/S
+ *
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+&lmp90100 {
+	rtd-current = <1000>;
+};

samples/shields/lmp90100_evb/rtd/prj.conf

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+CONFIG_LOG=y
+CONFIG_ADC=y

samples/shields/lmp90100_evb/rtd/sample.yaml

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+sample:
+  description: Demonstration of the LMP90100 EVB with RTD sensor
+  name: LMP90100 EVB RTD
+tests:
+  sample.shields.lmp90100_evb.rtd:
+    platform_whitelist: frdm_k64f
+    harness: shield
+    tags: shield
+    depends_on: arduino_spi

samples/shields/lmp90100_evb/rtd/src/main.c

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+/*
+ * Copyright (c) 2019 Vestas Wind Systems A/S
+ *
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+#include <zephyr.h>
+#include <device.h>
+#include <drivers/adc.h>
+#include <stdio.h>
+
+#define LOG_LEVEL CONFIG_LOG_DEFAULT_LEVEL
+#include <logging/log.h>
+LOG_MODULE_REGISTER(main);
+
+#define RTD_NOMINAL_RESISTANCE 100
+
+static double sqrt(double value)
+{
+	double sqrt = value / 3;
+	int i;
+
+	if (value <= 0) {
+		return 0;
+	}
+
+	for (i = 0; i < 6; i++) {
+		sqrt = (sqrt + value / sqrt) / 2;
+	}
+
+	return sqrt;
+}
+
+static double rtd_temperature(int nom, double resistance)
+{
+	double a0 =  3.90802E-3;
+	double b0 = -0.58020E-6;
+	double temp;
+
+	temp = -nom * a0;
+	temp += sqrt((nom * nom) * (a0 * a0) - 4.0 * nom * b0 *
+		     (nom - resistance));
+	temp /= 2.0 * nom * b0;
+
+	return temp;
+}
+
+void main(void)
+{
+	struct device *lmp90100;
+	double resistance;
+	s32_t buffer;
+	int err;
+	const struct adc_channel_cfg ch_cfg = {
+		.channel_id = 0,
+		.differential = 1,
+		.input_positive = 0,
+		.input_negative = 1,
+		.reference = ADC_REF_EXTERNAL1,
+		.gain = ADC_GAIN_1,
+		.acquisition_time = ADC_ACQ_TIME(ADC_ACQ_TIME_TICKS, 0)
+	};
+	const struct adc_sequence seq = {
+		.options = NULL,
+		.channels = BIT(0),
+		.buffer = &buffer,
+		.buffer_size = sizeof(buffer),
+		.resolution = 24,
+		.oversampling = 0,
+		.calibrate = 0
+	};
+
+	lmp90100 = device_get_binding(DT_INST_0_TI_LMP90100_LABEL);
+	if (!lmp90100) {
+		LOG_ERR("LMP90100 device not found");
+		return;
+	}
+
+	err = adc_channel_setup(lmp90100, &ch_cfg);
+	if (err) {
+		LOG_ERR("failed to setup ADC channel (err %d)", err);
+		return;
+	}
+
+	while (true) {
+		err = adc_read(lmp90100, &seq);
+		if (err) {
+			LOG_ERR("failed to read ADC (err %d)", err);
+			return;
+		}
+
+		resistance = (buffer / 8388608.0) * 2000;
+		printf("R: %.02f ohm\n", resistance);
+		printf("T: %.02f degC\n",
+		       rtd_temperature(RTD_NOMINAL_RESISTANCE, resistance));
+
+		k_sleep(1000);
+	}
+}