boards: shields: support for x_nucleo_s2868a1

Added a new shield for the x_nucleo_s2868a1 expansion module

Signed-off-by: Nick Oikonomou <nikoikonomou92@gmail.com>

Author: nikooiko

boards/shields/x_nucleo_s2868a1/doc/img/x-nucleo-s2868a1.jpg

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+.. _x-nucleo-s2868a1:
+
+X-NUCLEO-S2868A1: S2LP radio shield
+###################################
+
+Overview
+********
+The X-NUCLEO-S2868A1 expansion board is based on the S2-LP radio and operates
+in the 868 MHz ISM frequency band. The expansion board is compatible with ST
+morpho and Arduino UNO R3 connectors. The X-NUCLEO-S2868A1 interfaces with
+the STM32 Nucleo microcontroller via SPI connections and GPIO pins. You can
+change some of the GPIOs by mounting or removing the resistors.
+
+.. image:: img/x-nucleo-s2868a1.jpg
+     :width: 350px
+     :height: 489px
+     :align: center
+     :alt: X-NUCLEO-S2868A1
+
+More information about the board can be found at the
+`X-NUCLEO-S2868A1 website`_.
+
+Hardware
+********
+
+X-NUCLEO-S2868A1 provides the following key features:
+
+ - Based on S2-LP radio
+ - S2-LP narrow band ultra-low power sub-1 GHz transceiver tuned for 860 - 940 MHz frequency band
+ - Programmable RF output power up to +16 dBm
+ - Modulation schemes: 2-FSK, 2-GFSK, 4-FSK, 4-GFSK, OOK and ASK
+ - Air data rate from 0.1 to 500 kbps
+ - Ultra-low power consumption: 7 mA RX and 10 mA TX at +10 dBm
+ - IEEE 802.15.4g hardware packet support with whitening, FEC, CRC and dual SYNC word detection
+ - RX and TX 128 byte FIFO buffers
+ - Support to wireless M-Bus
+ - Excellent performance of receiver sensitivity (up to -130 dBm)
+ - Automatic acknowledgment, retransmission and timeout protocol engine
+ - Compatible with STM32 Nucleo boards
+ - Compatible with Arduino UNO R3 connectors
+ - Support to SMD and SMA antennas
+ - BALF-SPI2-01D3 IPD balun for matching network and harmonics filter
+ - Sigfox compatible
+ - 6LoWPAN compatible
+ - RoHS compliant
+
+
+More information about X-NUCLEO-S2868A1 can be found in the
+`X-NUCLEO-S2868A1 data sheet`_.
+
+References
+**********
+
+.. target-notes::
+
+.. _X-NUCLEO-S2868A1 website:
+  https://www.st.com/en/ecosystems/x-nucleo-s2868a1.html
+
+.. _X-NUCLEO-S2868A1 data sheet:
+  https://www.st.com/resource/en/data_brief/x-nucleo-s2868a1.pdf

boards/shields/x_nucleo_s2868a1/doc/index.rst

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+/*
+ * Copyright (c) 2019 Nikos Oikonomou <nikoikonomou92@gmail.com>
+ *
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+&spi1 {
+	status = "ok";
+	cs-gpios = <&gpioa 1 (GPIO_DIR_OUT|GPIO_INT_ACTIVE_LOW)>;
+
+	s2lp@0 {
+		compatible = "st,s2lp";
+		reg = <0x0>;
+		label = "s2lp";
+		spi-max-frequency = <4000000>;
+		sdn-gpios = <&gpioa 8 (GPIO_DIR_OUT|GPIO_INT_ACTIVE_LOW)>;
+		rx-rdy-irq-gpios = <&gpioc 0 (GPIO_DIR_IN|GPIO_INT|
+			GPIO_INT_EDGE|GPIO_PUD_PULL_UP|GPIO_INT_ACTIVE_LOW)>;
+	};
+};