samples/boards: stm32: Add low power blinky sample

This sample demonstrates a low power blinky application.

Signed-off-by: Erwan Gouriou <erwan.gouriou@linaro.org>

samples/boards/stm32/power_mgmt/blinky/CMakeLists.txt

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+# SPDX-License-Identifier: Apache-2.0
+
+cmake_minimum_required(VERSION 3.13.1)
+find_package(Zephyr REQUIRED HINTS $ENV{ZEPHYR_BASE})
+project(stm32_pm_blinky)
+
+target_sources(app PRIVATE src/main.c)

samples/boards/stm32/power_mgmt/blinky/README.rst

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+.. _stm32-pm-blinky-sample:
+
+Blinky
+######
+
+Overview
+********
+
+Blinky is a simple application which blinks an LED forever using the :ref:`GPIO
+API <gpio_api>` in low power context.
+
+.. _stm32-pm-blinky-sample-requirements:
+
+Requirements
+************
+
+You will see this error if you try to build Blinky for an unsupported board:
+
+.. code-block:: none
+
+   Unsupported board: led0 devicetree alias is not defined
+
+The board must have an LED connected via a GPIO pin. These are called "User
+LEDs" on many of Zephyr's :ref:`boards`. The LED must be configured using the
+``led0`` :ref:`devicetree <dt-guide>` alias. This is usually done in the
+:ref:`BOARD.dts file <devicetree-in-out-files>` or a :ref:`devicetree overlay
+<set-devicetree-overlays>`.
+
+The board should support enabling PM.
+
+Building and Running
+********************
+
+Build and flash Blinky as follows, changing ``nucleo_l476rg`` for your board:
+
+.. zephyr-app-commands::
+   :zephyr-app: samples/basic/blinky
+   :board: nucleo_l476rg
+   :goals: build flash
+   :compact:
+
+After flashing, the LED starts to blink. Blinky does not print to the console.

samples/boards/stm32/power_mgmt/blinky/prj.conf

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+CONFIG_GPIO=y
+
+CONFIG_PM=y
+CONFIG_PM_DEVICE=y
+CONFIG_PM_DEVICE_RUNTIME=y
+
+# Disable debug to enter core low power states
+# Warning: requires to erase the device for next flashing actions
+CONFIG_DEBUG=y
+
+
+# FIXME: Early console not supported due to uart init postponed
+CONFIG_BOOT_BANNER=n
+# Disable serial to ease debug
+CONFIG_SERIAL=n

samples/boards/stm32/power_mgmt/blinky/sample.yaml

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+sample:
+  name: STM32 GPIO Power Management
+tests:
+  sample.boards.stm32.power_mgmt.blinky:
+    tags: LED power
+    filter: dt_compat_enabled("zephyr,power-state") and
+            dt_enabled_alias_with_parent_compat("led0", "gpio-leds")
+    depends_on: gpio

samples/boards/stm32/power_mgmt/blinky/src/main.c

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+/*
+ * Copyright (c) 2021 Linaro Limited
+ *
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+#include <zephyr.h>
+#include <device.h>
+#include <devicetree.h>
+#include <drivers/gpio.h>
+
+
+#define SLEEP_TIME_MS   2000
+
+static const struct gpio_dt_spec led =
+	GPIO_DT_SPEC_GET(DT_ALIAS(led0), gpios);
+
+void main(void)
+{
+	bool led_is_on = true;
+
+	__ASSERT_NO_MSG(device_is_ready(led.port));
+
+	while (true) {
+		gpio_pin_configure_dt(&led, GPIO_OUTPUT_ACTIVE);
+		gpio_pin_set(led.port, led.pin, (int)led_is_on);
+		if (led_is_on == false) {
+			/* Release resource to release device clock */
+			gpio_pin_configure(led.port, led.pin, GPIO_DISCONNECTED);
+		}
+		k_msleep(SLEEP_TIME_MS);
+		if (led_is_on == true) {
+			/* Release resource to release device clock */
+			gpio_pin_configure(led.port, led.pin, GPIO_DISCONNECTED);
+		}
+		led_is_on = !led_is_on;
+	}
+}