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CMSIS-RTOS Tutorial

Introduction

This tutorial is an excerpt from “The Designers Guide to the Cortex-M Processor

Family” by Trevor Martin and is reproduced with permission of Elsevier. For

more details please see the Further Reading section at the end of this tutorial.

In this tutorial we are going to look at using a small footprint RTOS running on a

Cortex-M based microcontroller. Specifically we are going to use an RTOS that

meets the ‘Cortex Microcontroller Interface Standard’ (CMSIS) RTOS

Specification. This specification defines a standard RTOS API for use with

practice to get used to working with an RTOS but once you have made the leap

the advantages are such that you will not want to return to writing bare metal

code.

Getting Started- Installing the tools

To run the examples in this tutorial, it is first necessary to install the MDK-ARM

toolchain. First download the MDK-Core Version 5 using the embedded URL

below and run the installation file.

http://www.keil.com/mdk5/install

This installs the core toolchain which includes the IDE, compiler/linker and the

2 CMSIS-RTOS Tutorial

In the exercises we are going to use an STM32F103RB so we need to install

display the available device packs.

Select the Keil::STM32F1xx_DFP and press the install button. This will take a

few minutes to download and install the STM32F1xx support files.

If the pack installer has any problems accessing the remote pack you can

download it manually using the URL below

http://www.keil.com/dd2/Pack/

CMSIS-RTOS Tutorial

Again select the STM32F1xx pack and save it to your hard disk. The file may be

saved as a .zip file depending on the browser you are using. If it is saved as a .zip

change the .zip extension to .pack, you can then install it locally by double

clicking on the STM32F1xx.pack file.

The examples for this tutorial are provided as a CMSIS pack. You can install the

pack into the MDK-ARM by simply double clicking on the

4 CMSIS-RTOS Tutorial

What Hardware do I need?

Simple answer: none! The Keil toolchain contains simulators for each of the

Cortex-M processors. It also contains full simulation models (CPU + Peripherals)

for some of the earlier Cortex-M microcontrollers. This means we can run the

examples in the debugger using the simulation models and explore every aspect

of using the RTOS. In fact this method of working is a better way of learning

how to use the RTOS than going straight to a real microcontroller.

to understand as you work through this tutorial. In the first section we will focus

on creating and managing Threads. The key concept here is to consider them

running as parallel concurrent objects. In the second section we will look at how

to communicate between threads. In this section the key concept is

synchronization of the concurrent threads.

CMSIS-RTOS Tutorial

The RTOS itself consists of a scheduler which supports round-robin, pre-emptive

and co-operative multitasking of program threads, as well as time and memory

management services. Inter-thread communication is supported by additional

RTOS objects, including signal triggering, semaphores, mutex and a mailbox

#include <cmsis_os.h>

This header file is maintained by ARM as part of the CMSIS-RTOS standard.

For the CMSIS-RTOS Keil RTX this is the default API. Other RTOS will have

their own proprietary API but may provide a wrapper layer to implement the

CMSIS-RTOS API so they can be used where compatibility with the CMSIS

standard is required.

Threads

The building blocks of a typical ‘C’ program are functions which we call to

perform a specific procedure and which then return to the calling function. In

CMSIS-RTOS the basic unit of execution is a “Thread”. A Thread is very similar