samples: uart: Add sample for single line uart

The sample uses st single-line uart mode on stm32 boards.

Signed-off-by: Jonathan Hahn <Jonathan.Hahn@t-online.de>

samples/drivers/uart/stm32/single_wire/CMakeLists.txt

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+# SPDX-License-Identifier: Apache-2.0
+
+cmake_minimum_required(VERSION 3.20.0)
+find_package(Zephyr REQUIRED HINTS $ENV{ZEPHYR_BASE})
+project(uart_stm32_single_wire)
+
+target_sources(app PRIVATE
+    src/main.c
+    )

samples/drivers/uart/stm32/single_wire/README.rst

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+.. _sample_uart_stm32_single_wire:
+
+STM32 Single Wire UART
+######################
+
+Overview
+********
+
+A simple application demonstrating how to use the single wire / half-duplex UART
+functionality of STM32. Without adaptions this example runs on STM32F3 discovery
+board. You need to establish a physical connection between pins PA2 (USART2_TX) and
+PC10 (UART4_TX).
+
+Add a `single_wire_uart_loopback` fixture to your board in the hardware map to allow
+twister to verify this sample's output automatically.
+
+Building and Running
+********************
+
+Build and flash as follows, replacig ``stm32f3_disco`` with your board:
+
+ .. zephyr-app-commands::
+    :zephyr-app: samples/drivers/uart/stm32/single_wire
+    :board: stm32f3_disco
+    :goals: build flash
+    :compact:
+
+After flashing the console output should not show any failure reports,
+but the following message repeated every 2s:
+
+.. code-block:: none
+
+    Received c

samples/drivers/uart/stm32/single_wire/boards/stm32f3_disco.overlay

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+/*
+ * Copyright (c) 2021 Jonathan Hahn
+ *
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+/ {
+	 aliases {
+		single-line-uart1 = &usart2;
+		single-line-uart2 = &uart4;
+	};
+};
+
+&usart2 {
+	pinctrl-0 = <&usart2_tx_pa2>;
+	single-wire;
+};
+
+&uart4 {
+	pinctrl-0 = <&uart4_tx_pc10>;
+	single-wire;
+};

samples/drivers/uart/stm32/single_wire/prj.conf

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+# Nothing needed here

samples/drivers/uart/stm32/single_wire/sample.yaml

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+sample:
+  name: STM32 Single Wire UART sample
+tests:
+  sample.drivers.uart.stm32.single_wire:
+    platform_allow: stm32f3_disco
+    tags: drivers uart
+    harness: console
+    harness_config:
+      fixture: single_line_uart_loopback
+      type: one_line
+      regex:
+        - "Received c"

samples/drivers/uart/stm32/single_wire/src/main.c

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+/*
+ * Copyright (c) 2021 Jonathan Hahn
+ *
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+#include <zephyr.h>
+#include "kernel.h"
+#include <device.h>
+#include <devicetree.h>
+#include <drivers/uart.h>
+
+#define UART_NODE1 DT_ALIAS(single_line_uart1)
+#define UART_NODE2 DT_ALIAS(single_line_uart2)
+
+const struct device *sl_uart1 = DEVICE_DT_GET(UART_NODE1);
+const struct device *sl_uart2 = DEVICE_DT_GET(UART_NODE2);
+
+void main(void)
+{
+	unsigned char recv;
+
+	if (!device_is_ready(sl_uart1) || !device_is_ready(sl_uart2)) {
+		printk("uart devices not ready\n");
+		return;
+	}
+
+	while (true) {
+
+		uart_poll_out(sl_uart1, 'c');
+
+		/* give the uarts some time to get the data through */
+		k_sleep(K_MSEC(50));
+
+		int ret = uart_poll_in(sl_uart2, &recv);
+
+		if (ret < 0) {
+			printk("Receiving failed. Error: %d", ret);
+		} else {
+			printk("Received %c\n", recv);
+		}
+
+		k_sleep(K_MSEC(2000));
+	}
+}