FreeRTOS Course - Semaphore/Mutex Management

Introduction to FreeRTOS V6.0.5

About SwiftACT
A Technology services startup company
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Areas of specialties:
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Embedded systems development
Types of services:
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Training
Introduction to FreeRTOS V6.0.5 Amr Ali Abdel-Naby@2010

About Me
Graduated 2004
ECE, ASU: 5 yrs distinction
6+ years in embedded systems development
SDLC, Apps, MW, DD, Porting, ...
4+ years in SW engineering
PSP, CMMI, Systematic reuse, ...
4+ years in SW testing
IBM certified, ISTQB certified, ...

Copyright
Materials in this course is the property of Amr Ali Abdel-Naby.
Reproduction or transmission of the materials outside [Customer Name] in any manner without the copyright owner permission is a law violation.
[Customer Name] is free to use this material as is within its head quarters for noncommercial uses.
Snapshots or trademarks used are retained to their original owners.

Course Objectives
After completing this course, you will:
Understand what is FreeRTOS
Experience different FreeRTOS services and APIs
Understand the porting process of FreeRTOS

Course Notes
I assume you are familiar with RTOS concepts.
I assume you know C.
I assume you know 80x86 architecture and assembly.
Ask any time.

Course References
http://www.freertos.org

Outline
Introduction to FreeRTOS
Lab 0: Getting Started
Kernel Structure
Lab 1: Kernel Structure
Task Management
Lab 2: Task Management
Queue Management
Lab 3: Queue Management
Semaphore/Mutex Management
Lab 4: Semaphore/Mutex Management
Co-routine Management
Lab 5: Co-routine Management
Advanced Features
Lab 6: Advanced Features
Porting FreeRTOS
Lab 7: Porting FreeRTOS
Amr Ali Abdel-Naby@2010 Introduction to FreeRTOS V6.0.5

Binary Semaphore
Used for mutual exclusion and synchronization
Critical Section
Initial count = 1
wait_sem to enter critical section
signal_sem to leave critical section
Synchronization
Initial count = 0
wait_sem to wait
signal_sem to signal
Amr Ali Abdel-Naby@2010 Introduction to FreeRTOS V6.0.5 HW Device 1 ISR 1 SW Task

Counting Semaphore
Used for resource limit and credit tracking
Resource limit
Initial count = # of equivalent resources
wait_sem to gain access to a resource
signal_sem to release a resource
Credit tracking
Initial count = 0
signal_sem to increment (Credit)
wait_sem to consume (Debit)

Mutex
A special type of binary semaphore
Preferred over binary semaphore for mutual exclusion
Mutex vs. Semaphore
Mutex ownership
Recursive locking
Minimize priority inversion using inheritance protocol
Can’t be used from ISRs

More About Semaphore/Mutex
Same queues’ implementation APIs are used to implement semaphores/mutexes.
Their APIs are macros that point to queues’ implementation APIs.
Queues and semaphores/mutexes APIs are inter-operable.
You should be careful if your are going to do this.
Binary semaphore/Mutex = A queue that can hold 1 item of data size 0
Either full or empty  Binary
Counting semaphore = A queue that can hold n items each of data size 0
You can extend their APIs by defining new macros to the queue’s implementation APIs.

Semaphore/Mutex Management APIs
They fall under 4 categories:
Amr Ali Abdel-Naby@2010 Introduction to FreeRTOS V6.0.5 Creation vSemaphoreCreateBinary xSemaphoreCreateCounting xSemaphoreCreateMutex xSemaphoreCreateRecursiveMutex Fully Featured xSemaphoreTake xSemaphoreTakeRecursive xSemaphoreGive xSemaphoreGiveRecursive Light Weight xSemaphoreGiveFromISR Alternative xSemaphoreAltTake xSemaphoreAltGive

Creating a Binary Semaphore, vSemaphoreCreateBinary
vSemaphoreCreateBinary(xSemaphoreHandle xSemaphore)
xSemaphore: Handle to the created semaphore
!NULL if created
NULL if failed
Note the initial binary semaphore value is 1

Creating a Counting Semaphore, xSemaphoreCreateCounting
xSemaphoreHandle xSemaphoreCreateCounting(unsigned portBASE_TYPE uxMaxCount, unsigned portBASE_TYPE uxInitialCount )
uxMaxCount: The maximum count value that can be reached
uxInitialCount: The count value assigned to the semaphore when it is created
Return value:
!NULL if created
NULL if failed

Creating a Mutex, xSemaphoreCreateMutex
xSemaphoreHandle xSemaphoreCreateMutex(void)
Return value:
!NULL if created
NULL if failed

Creating a Recursive Mutex, xSemaphoreCreateRecursiveMutex
xSemaphoreHandle xSemaphoreCreateRecursiveMutex(void)
Return value:
!NULL if created
NULL if failed

Obtaining a Semaphore/Mutex, xSemaphoreTake
xSemaphoreTake(xSemaphoreHandle xSemaphore, portTickType xBlockTime)
xSemaphore: A handle to the semaphore/mutex to be obtained
Created with vSemaphoreCreateBinary, xSemaphoreCreateMutex or xSemaphoreCreateCounting
xBlockTime: The time in ticks to wait for the semaphore/mutex to become available
Return value:
pdTRUE if the semaphore/mutex was obtained
Otherwise pdFALSE

Obtaining a Recursive Mutex, xSemaphoreTakeRecursive
xSemaphoreTake(RecursivexSemaphoreHandle xSemaphore, portTickType xBlockTime)
xSemaphore: A handle to the mutex to be obtained
Created with xSemaphoreCreaterecursiveMutex
xBlockTime: The time in ticks to wait for the mutex to become available
Return value:
pdTRUE if the mutex was obtained
Otherwise pdFALSE

Releasing a Semaphore/Mutex, xSemaphoreGive
xSemaphoreGive(xSemaphoreHandle xSemaphore)
xSemaphore: A handle to the semaphore/mutex to be released
Created with vSemaphoreCreateBinary, xSemaphoreCreateMutex or xSemaphoreCreateCounting
Return value:
pdTRUE if the semaphore/mutex was released
Otherwise pdFALSE

Releasing a Recursive Mutex, xSemaphoreGiveRecursive
xSemaphoreGiveRecursive(xSemaphoreHandle xMutex)
xSemaphore: A handle to the mutex to be released
Created with xSemaphoreCreaterecursiveMutex
Return value:
pdTRUE if the mutex was released
Otherwise pdFALSE

Releasing a Semaphore From ISR, xSemaphoreGiveFromISR
xSemaphoreGiveFromISR(xSemaphoreHandle xSemaphore, signed portBASE_TYPE * pxHigherPriorityTaskWoken)
= xSemaphoreGive but used within ISRs
pxTaskWoken : Is set to pdTRUE if releasing caused a higher priority task to unblock
A context switch is needed after ISR exits.

Alternative Obtaining a Semaphore/Mutex, xSemaphoreAltTake
xSemaphoreAltTake(xSemaphoreHandle xSemaphore, portTickType xBlockTime)
= xSemaphoreTake but with alternative implementation

Alternative Releasing a Semaphore/Mutex, xSemaphoreAltGive
xSemaphoreAltGive(xSemaphoreHandle xSemaphore)
= xSemaphoreGive but with alternative implementation

More About Semaphore/Mutex Revisited
Always remember that you can extend semaphores/mutexes with queues’ implementation APIs.
There is no deletion API!!!
Possible candidates that can be used are vQueueDelete and vPortFree.
Before using any, you have to check:
Configuration for the queue registry functions
Port memory management functions

Outline
Introduction to FreeRTOS
Lab 0: Getting Started
Kernel Structure
Lab 1: Kernel Structure
Task Management
Lab 2: Task Management
Queue Management
Lab 3: Queue Management
Semaphore/Mutex Management
Lab 4: Semaphore/Mutex Management
Co-routine Management
Lab 5: Co-routine Management
Advanced Features
Lab 6: Advanced Features
Porting FreeRTOS
Lab 7: Porting FreeRTOS

Lab4: Semaphore/Mutex Management
Please follow the instructions in the lab handout

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FreeRTOS Course - Semaphore/Mutex Management

by amraldo on Jul 25, 2010

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