board: arm: add support for the nucleo_f031k6 board

The configuration is based on
the f303k6 and the ST reference manual.

I had successfully tested the following sample code:
blinky
blink_led (uses TIM2_CH2 on PB3)
hello_world

Signed-off-by: Sebastian Schwabe <sebastian-schwabe@gmx.de>

boards/arm/nucleo_f031k6/Kconfig.board

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+# STM32 Nucleo-32 development board with STM32F031K6 MCU configuration
+
+# Copyright (c) 2021 Sebastian Schwabe <sebastian.schwabe@mailbox.tu-dresden.de>
+# SPDX -License-Identifier: Apache-2.0
+
+config BOARD_NUCLEO_F031K6
+	bool "NUCLEO-32 F031K6 Development Board"
+	depends on SOC_STM32F031X6

boards/arm/nucleo_f031k6/Kconfig.defconfig

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+# STM32 Nucleo-32 development board with STM32F031K6 MCU
+
+# Copyright (c) 2021 Sebastian Schwabe <sebastian.schwabe@mailbox.tu-dresden.de>
+# SPDX-License-Identifier: Apache-2.0
+
+if BOARD_NUCLEO_F031K6
+
+config BOARD
+	default "nucleo_f031k6"
+
+endif # BOARD_NUCLEO_F031K6

boards/arm/nucleo_f031k6/board.cmake

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+# SPDX-License-Identifier: Apache-2.0
+
+board_runner_args(jlink "--device=STM32F031K6" "--speed=4000")
+
+include(${ZEPHYR_BASE}/boards/common/openocd.board.cmake)
+include(${ZEPHYR_BASE}/boards/common/jlink.board.cmake)

boards/arm/nucleo_f031k6/doc/index.rst

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+.. _nucleo_f031k6_board:
+
+ST Nucleo F031K6
+################
+
+Overview
+********
+The STM32 Nucleo-32 development board with STM32F031K6 MCU, supports Arduino nano connectivity.
+
+The STM32 Nucleo board provides an affordable, and flexible way for users to try out new concepts,
+and build prototypes with the STM32 microcontroller, choosing from the various
+combinations of performance, power consumption and features.
+
+The STM32 Nucleo board integrates the ST-LINK/V2-1 debugger and programmer.
+
+The STM32 Nucleo board comes with the STM32 comprehensive software HAL library together
+with various packaged software examples.
+
+.. image:: img/nucleo_f031k6.jpg
+   :width: 250px
+   :height: 250px
+   :align: center
+   :alt: Nucleo F031k6
+
+More information about the board can be found at the `Nucleo F031K6 website`_.
+
+Hardware
+********
+Nucleo F031K6 provides the following hardware components:
+
+- STM32 microcontroller in LQFP32 package
+
+- On-board ST-LINK/V2-1 debugger/programmer with SWD connector:
+
+- Flexible board power supply:
+
+  - USB VBUS or external source (3.3V, 5V, 7 - 12V)
+
+- Three LEDs:
+
+  - USB communication (LD1), user LED (LD2), power LED (LD3)
+
+- reset push button
+
+More information about STM32F031K6 can be found here:
+
+- `STM32F031 reference manual`_
+- `STM32F031 data sheet`_
+
+Supported Features
+==================
+
+The Zephyr nucleo_f031k6 board configuration supports the following hardware features:
+
++-----------+------------+-------------------------------------+
+| Interface | Controller | Driver/Component                    |
++===========+============+=====================================+
+| NVIC      | on-chip    | nested vector interrupt controller  |
++-----------+------------+-------------------------------------+
+| UART      | on-chip    | serial port-polling;                |
+|           |            | serial port-interrupt               |
++-----------+------------+-------------------------------------+
+| PINMUX    | on-chip    | pinmux                              |
++-----------+------------+-------------------------------------+
+| GPIO      | on-chip    | gpio                                |
++-----------+------------+-------------------------------------+
+| CLOCK     | on-chip    | reset and clock control             |
++-----------+------------+-------------------------------------+
+| FLASH     | on-chip    | flash memory                        |
++-----------+------------+-------------------------------------+
+| I2C       | on-chip    | i2c controller                      |
++-----------+------------+-------------------------------------+
+| SPI       | on-chip    | spi controller                      |
++-----------+------------+-------------------------------------+
+
+Other hardware features are not yet supported in this Zephyr port.
+
+The default configuration can be found in the defconfig file:
+``boards/arm/nucleo_f031k6/nucleo_f031k6_defconfig``
+
+Connections and IOs
+===================
+
+Each of the GPIO pins can be configured by software as output (push-pull or open-drain), as
+input (with or without pull-up or pull-down), or as peripheral alternate function. Most of the
+GPIO pins are shared with digital or analog alternate functions.
+
+Board connectors:
+-----------------
+.. image:: img/nucleo_f031k6_connectors.png
+   :width: 960px
+   :align: center
+   :height: 720px
+   :alt: Nucleo F031K6 connectors
+
+Default Zephyr Peripheral Mapping:
+----------------------------------
+
+- UART_1 TX/RX : PA2/PA15 (ST-Link Virtual COM Port)
+- I2C1 SCL/SDA : PB6/PB7 (Arduino I2C)
+- SPI1 NSS/SCK/MISO/MOSI : PA4/PA5/PA6/PA7 (Arduino SPI)
+- LD2 : PB3
+
+
+For mode details please refer to `STM32 Nucleo-32 board User Manual`_.
+
+Programming and Debugging
+*************************
+
+Applications for the ``nucleo_f031k6`` board configuration can be built and
+flashed in the usual way (see :ref:`build_an_application` and
+:ref:`application_run` for more details).
+
+Flashing
+========
+
+Nucleo F031K6 board includes an ST-LINK/V2-1 embedded debug tool interface.
+This interface is supported by the openocd version included in the Zephyr SDK.
+
+Flashing an application to Nucleo F030R8
+----------------------------------------
+
+Here is an example for the :ref:`blinky-sample` application.
+
+.. zephyr-app-commands::
+   :zephyr-app: samples/basic/blinky
+   :board: nucleo_f031k6
+   :goals: build flash
+
+You will see the LED blinking every second.
+
+Debugging
+=========
+
+You can debug an application in the usual way.  Here is an example for the
+:ref:`blinky-sample` application.
+
+.. zephyr-app-commands::
+   :zephyr-app: samples/basic/blinky
+   :board: nucleo_f031k6
+   :maybe-skip-config:
+   :goals: debug
+
+References
+**********
+
+.. target-notes::
+
+.. _Nucleo F031K6 website:
+   https://www.st.com/en/evaluation-tools/nucleo-f031k6.html
+
+.. _STM32F031 reference manual:
+   https://www.st.com/resource/en/reference_manual/dm00031936-stm32f0x1stm32f0x2stm32f0x8-advanced-armbased-32bit-mcus-stmicroelectronics.pdf
+
+.. _STM32F031 data sheet:
+   https://www.st.com/resource/en/datasheet/stm32f031k6.pdf
+
+.. _STM32 Nucleo-32 board User Manual:
+   https://www.st.com/resource/en/user_manual/dm00231744-stm32-nucleo32-boards-mb1180-stmicroelectronics.pdf

boards/arm/nucleo_f031k6/nucleo_f031k6.dts

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+/*
+ * Copyright (c) 2021 Sebastian Schwabe <sebastian.schwabe@mailbox.tu-dresden.de>
+ *
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+/dts-v1/;
+#include <st/f0/stm32f031X6.dtsi>
+#include <st/f0/stm32f031k6tx-pinctrl.dtsi>
+
+/ {
+	model = "STMicroelectronics STM32F031K6-NUCLEO board";
+	compatible = "st,stm32f031k6-nucleo";
+
+	chosen {
+		zephyr,console = &usart1;
+		zephyr,shell-uart = &usart1;
+		zephyr,sram = &sram0;
+		zephyr,flash = &flash0;
+	};
+
+	leds {
+		compatible = "gpio-leds";
+		green_led_3: led_3 {
+			gpios = <&gpiob 3 GPIO_ACTIVE_HIGH>;
+			label = "User LD3";
+		};
+	};
+
+	pwmleds {
+		compatible = "pwm-leds";
+		green_pwm_led: green_pwm_led {
+			pwms = <&pwm2 2 0 PWM_POLARITY_NORMAL>;
+		};
+	};
+
+	aliases {
+		led0 = &green_led_3;
+		pwm-led0 = &green_pwm_led;
+	};
+};
+
+/* Due to limited available memory, don't enable gpiod and gpiof */
+/* (Test cases fail due to 'SRAM' region overflow) */
+&gpiod {status = "disabled";};
+&gpiof {status = "disabled";};
+
+&timers2 {
+	status = "okay";
+
+	pwm2: pwm {
+		status = "okay";
+		pinctrl-0 = <&tim2_ch2_pb3>;
+	};
+};
+
+&usart1 {
+	pinctrl-0 = <&usart1_tx_pa2 &usart1_rx_pa15>;
+	current-speed = <115200>;
+	status = "okay";
+};
+
+&i2c1 {
+	pinctrl-0 = <&i2c1_scl_pb6 &i2c1_sda_pb7>;
+	status = "okay";
+	clock-frequency = <I2C_BITRATE_FAST>;
+};
+
+&spi1 {
+	pinctrl-0 = <&spi1_nss_pa4 &spi1_sck_pa5
+		     &spi1_miso_pa6 &spi1_mosi_pa7>;
+	status = "okay";
+};
+
+&adc1 {
+	pinctrl-0 = <&adc_in0_pa0>;
+	status = "okay";
+};
+
+&rtc {
+	status = "okay";
+};

boards/arm/nucleo_f031k6/nucleo_f031k6.yaml

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+identifier: nucleo_f031k6
+name: ST Nucleo F031K6
+type: mcu
+arch: arm
+toolchain:
+  - zephyr
+  - gnuarmemb
+  - xtools
+ram: 4
+flash: 32
+supported:
+  - i2c
+  - spi
+  - gpio
+testing:
+  ignore_tags:
+    - net
+    - bluetooth

boards/arm/nucleo_f031k6/nucleo_f031k6_defconfig

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+# SPDX-License-Identifier: Apache-2.0
+
+# Platform Configuration
+CONFIG_SOC_SERIES_STM32F0X=y
+CONFIG_SOC_STM32F031X6=y
+
+# General Kernel Options
+CONFIG_SYS_CLOCK_HW_CYCLES_PER_SEC=48000000
+
+# Kernel Options due to Low Memory (4k)
+CONFIG_MAIN_STACK_SIZE=512
+CONFIG_IDLE_STACK_SIZE=150
+CONFIG_ISR_STACK_SIZE=256
+CONFIG_LOG_BUFFER_SIZE=256
+# Prevent Interrupt Vector Table in RAM
+CONFIG_IS_BOOTLOADER=y
+CONFIG_BOOTLOADER_SRAM_SIZE=4
+
+# Serial Drivers
+CONFIG_SERIAL=y
+CONFIG_UART_INTERRUPT_DRIVEN=y
+# enable console
+CONFIG_CONSOLE=y
+CONFIG_UART_CONSOLE=y
+
+# Pinmux Driver
+CONFIG_PINMUX=y
+
+# GPIO Controller
+CONFIG_GPIO=y
+
+# Clock configuration
+CONFIG_CLOCK_CONTROL=y
+# SYSCLK selection
+CONFIG_CLOCK_STM32_SYSCLK_SRC_PLL=y
+# use HSI as PLL input
+CONFIG_CLOCK_STM32_PLL_SRC_HSI=y
+# produce 48MHz clock at PLL output
+CONFIG_CLOCK_STM32_PLL_PREDIV=4
+CONFIG_CLOCK_STM32_PLL_MULTIPLIER=12
+CONFIG_CLOCK_STM32_AHB_PRESCALER=1
+CONFIG_CLOCK_STM32_APB1_PRESCALER=1

boards/arm/nucleo_f031k6/support/openocd.cfg

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+source [find board/st_nucleo_f0.cfg]
+
+$_TARGETNAME configure -event gdb-attach {
+        echo "Debugger attaching: halting execution"
+        reset halt
+        gdb_breakpoint_override hard
+}
+
+$_TARGETNAME configure -event gdb-detach {
+        echo "Debugger detaching: resuming execution"
+        resume
+}