Add RTOS page

Author: imuguruza

rtos/index.md

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+# RTOS
+
+## Introduction
+
+The use of Real-Time Operating Systems (RTOS) is a general practice in nowadays embedded systems. These embedded devices typically consist of a resource-constrained microcontroller that executes an application where the interaction with external components is performed. In many cases, this application contains a time-critical task where a time-deadline or deterministic response is required.
+
+Bare-metal applications are also used nowadays, but it requires a very low-level programming skills and lacks of hardware abstraction layers that RTOSes offers. RTOSes typically uses hardware abstraction layers (HAL) that eases the use of hardware resources such a timers and communication buses, making easier the development and allowing the code reuse. In addition, they offer thread and tasks entities that together with the use of schedulers, provides the necessary tools to implement application determinism. The scheduling normally consists on different algorithms where the user can choose from. Another feature that RTOSes normally offers is the stack management, helping the memory usage of the MCU, a valuable resource in embedded-systems.
+
+## RTOS in micro-ROS
+
+Due to the benefits explained in the introduction, micro-ROS integrates RTOS in its software stack. The use of such a tool enhances the micro-ROS features and allows reusing all the tools and implementations they provide. As the micro-ROS software stack is modular, the exchange of software entities is expected and desired. Same happens with the RTOS. Even that NuttX is the *default* RTOS for the project, it is expected that several of them could replace it.
+
+(NuttX)[http://www.nuttx.org/] is a RTOS that emphasizes its compliance with standards (such us POSIX) and small footprint, where could be git in microcontrollers from 8 to 32 bits.  This approach makes it kind for developers that are used to Linux. This RTOS is licensed under BSD license and makes use of GNU toolchain. In order to obtain more information, please visit [NuttX overview page](http://nuttx.org/Documentation/NuttX.html#overview).
+
+![NuttX_logo](https://upload.wikimedia.org/wikipedia/en/b/b0/NuttX_logo.png)
+
+For development purposes, project consortium has stablished two development boards as development blueprints: Olimex-STM32_e407 and the STM32LDiscovery.
+
+The first one consists on a ARM Cortex-M4F MCU with 196 KB of RAM and 1 MB of flash. It also offers Arduino-like expansion pins and Ethernet communication means.
+
+The STM32LDiscovery board contains a STM32L ultra-low power packaging and consists of a ARM Cortex-M3 MCU that integrates 32KB of RAM, 256KB of flash memory. This microcontroller aims to target low-power applications.
+
+## Getting started with NuttX and micro-ROS
+
+In order to obtain more information about how to get started using this RTOS, please check our [documentation repository](https://github.com/microROS/micro-ROS-doc), where tutorials and getting started material is offered.
+
+We have created several Docker containers for development purposes and precompiled examples, in order to rapidly start testing. These Docker files have been uploaded under the [micro-ROS Docker repository](https://github.com/microROS/docker).