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Customisation
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FreeRTOS is customised using a configuration file called FreeRTOSConfig.h. Every
FreeRTOS application must have a FreeRTOSConfig.h header file in its pre-processor
include path. FreeRTOSConfig.h tailors the RTOS kernel to the application being
built. It is therefore specific to the application, not the RTOS, and should be
located in an application directory, not in one of the RTOS kernel source code
directories.
Each demo application included in the RTOS source code download has its own FreeRTOSConfig.h file. Some of the demos are quite old and do not contain all the available configuration options. Configuration options that are omitted are set to a default value within an RTOS source file.
Here is a typical FreeRTOSConfig.h definition, followed by an explanation of each parameter:
#ifndef FREERTOS_CONFIG_H
#define FREERTOS_CONFIG_H
/* Here is a good place to include header files that are required across
your application. */
#include "something.h"
#define configUSE_PREEMPTION 1
#define configUSE_PORT_OPTIMISED_TASK_SELECTION 0
#define configUSE_TICKLESS_IDLE 0
#define configCPU_CLOCK_HZ 60000000
#define configTICK_RATE_HZ 250
#define configMAX_PRIORITIES 5
#define configMINIMAL_STACK_SIZE 128
#define configTOTAL_HEAP_SIZE 10240
#define configMAX_TASK_NAME_LEN 16
#define configUSE_TRACE_FACILITY 0
#define configUSE_16_BIT_TICKS 0
#define configIDLE_SHOULD_YIELD 1
#define configUSE_MUTEXES 0
#define configUSE_RECURSIVE_MUTEXES 0
#define configUSE_COUNTING_SEMAPHORES 0
#define configUSE_ALTERNATIVE_API 0 /* Deprecated! */
#define configQUEUE_REGISTRY_SIZE 10
#define configUSE_QUEUE_SETS 0
/* Hook function related definitions. */
#define configUSE_IDLE_HOOK 0
#define configUSE_TICK_HOOK 0
#define configCHECK_FOR_STACK_OVERFLOW 0
#define configUSE_MALLOC_FAILED_HOOK 0
/* Run time stats gathering related definitions. */
#define configGENERATE_RUN_TIME_STATS 0
/* Co-routine related definitions. */
#define configUSE_CO_ROUTINES 0
#define configMAX_CO_ROUTINE_PRIORITIES 1
/* Software timer related definitions. */
#define configUSE_TIMERS 1
#define configTIMER_TASK_PRIORITY 3
#define configTIMER_QUEUE_LENGTH 10
#define configTIMER_TASK_STACK_DEPTH configMINIMAL_STACK_SIZE
/* Interrupt nesting behaviour configuration. */
#define configKERNEL_INTERRUPT_PRIORITY [dependent of processor]
#define configMAX_SYSCALL_INTERRUPT_PRIORITY [dependent on processor and application]
/* Define to trap errors during development. */
#define configASSERT( ( x ) ) if( ( x ) == 0 ) vCallAssert( __FILE__, __LINE__ )
/* Optional functions - most linkers will remove unused functions anyway. */
#define INCLUDE_vTaskPrioritySet 1
#define INCLUDE_uxTaskPriorityGet 1
#define INCLUDE_vTaskDelete 1
#define INCLUDE_vTaskSuspend 1
#define INCLUDE_xResumeFromISR 1
#define INCLUDE_vTaskDelayUntil 1
#define INCLUDE_vTaskDelay 1
#define INCLUDE_xTaskGetSchedulerState 1
#define INCLUDE_xTaskGetCurrentTaskHandle 1
#define INCLUDE_uxTaskGetStackHighWaterMark 0
#define INCLUDE_xTaskGetIdleTaskHandle 0
#define INCLUDE_xTimerGetTimerDaemonTaskHandle 0
#define INCLUDE_pcTaskGetTaskName 0
#define INCLUDE_eTaskGetState 0
/* A header file that defines trace macro can be included here. */
#endif /* FREERTOS_CONFIG_H */
The Generic method:
A port specific method:
The malloc() failed hook function is a hook (or callback) function that, if defined and configured, will be called if pvPortMalloc() ever returns NULL. NULL will be returned only if there is insufficient FreeRTOS heap memory remaining for the requested allocation to succeed.
If configUSE_MALLOC_FAILED_HOOK is set to 1 then the application must define a malloc() failed hook function. If configUSE_MALLOC_FAILED_HOOK is set to 0 then the malloc() failed hook function will not be called, even if one is defined. Malloc() failed hook functions must have the name and prototype shown below.
void vApplicationMallocFailedHook( void );
The tick interrupt is used to measure time. Therefore a higher tick frequency means time can be measured to a higher resolution. However, a high tick frequency also means that the RTOS kernel will use more CPU time so be less efficient. The RTOS demo applications all use a tick rate of 1000Hz. This is used to test the RTOS kernel and is higher than would normally be required.
More than one task can share the same priority. The RTOS scheduler will share processor time between tasks of the same priority by switching between the tasks during each RTOS tick. A high tick rate frequency will therefore also have the effect of reducing the 'time slice' given to each task.
Each available priority consumes RAM within the RTOS kernel so this value should not be set any higher than actually required by your application.
This value will only be used if your application makes use of one of the sample memory allocation schemes provided in the FreeRTOS source code download. See the memory configuration section for further details.
Defining configUSE_16_BIT_TICKS as 1 causes portTickType to be defined (typedef'ed) as an unsigned 16bit type. Defining configUSE_16_BIT_TICKS as 0 causes portTickType to be defined (typedef'ed) as an unsigned 32bit type.
Using a 16 bit type will greatly improve performance on 8 and 16 bit architectures, but limits the maximum specifiable time period to 65535 'ticks'. Therefore, assuming a tick frequency of 250Hz, the maximum time a task can delay or block when a 16bit counter is used is 262 seconds, compared to 17179869 seconds when using a 32bit counter.
When tasks share the idle priority the behaviour can be slightly different. When configIDLE_SHOULD_YIELD is set to 1 the idle task will yield immediately should any other task at the idle priority be ready to run. This ensures the minimum amount of time is spent in the idle task when application tasks are available for scheduling. This behaviour can however have undesirable effects (depending on the needs of your application) as depicted below:

This diagram shows the execution pattern of four tasks at the idle priority. Tasks A, B and C are application tasks. Task I is the idle task. A context switch occurs with regular period at times T0, T1, ..., T6. When the idle task yields task A starts to execute - but the idle task has already taken up some of the current time slice. This results in task I and task A effectively sharing a time slice. The application tasks B and C therefore get more processing time than the application task A.
This situation can be avoided by:
configQUEUE_REGISTRY_SIZE defines the maximum number of queues and semaphores that can be registered. Only those queues and semaphores that you want to view using a RTOS kernel aware debugger need be registered. See the API reference documentation for vQueueAddToRegistry() and vQueueUnregisterQueue() for more information.
configKERNEL_INTERRUPT_PRIORITY and configMAX_SYSCALL_INTERRUPT_PRIORITY
Ports that contain a configKERNEL_INTERRUPT_PRIORITY setting include ARM Cortex-M3, PIC24, dsPIC, PIC32, SuperH and RX600.
Ports that contain a configMAX_SYSCALL_INTERRUPT_PRIORITY setting include PIC32, RX600 and ARM Cortex-M ports.
ARM Cortex-M3 and ARM Cortex-M4 users please take heed of the special note at the end of this section!
configKERNEL_INTERRUPT_PRIORITY should be set to the lowest priority.
Note in the following discussion that only API functions that end in "FromISR" can be called from within an interrupt service routine.
For ports that only implement configKERNEL_INTERRUPT_PRIORITY
configKERNEL_INTERRUPT_PRIORITY sets the interrupt priority used by the RTOS kernel itself. Interrupts that call API functions must also execute at this
priority. Interrupts that do not call API functions can execute at higher priorities and therefore never have their execution delayed by the RTOS kernel
activity (within the limits of the hardware itself).
For ports that implement both configKERNEL_INTERRUPT_PRIORITY and configMAX_SYSCALL_INTERRUPT_PRIORITY:
configKERNEL_INTERRUPT_PRIORITY sets the interrupt priority used by the RTOS kernel itself. configMAX_SYSCALL_INTERRUPT_PRIORITY sets the highest
interrupt priority from which interrupt safe FreeRTOS API functions can be called.
A full interrupt nesting model is achieved by setting configMAX_SYSCALL_INTERRUPT_PRIORITY above (that is, at a higher priority level) than configKERNEL_INTERRUPT_PRIORITY. This means the FreeRTOS kernel does not completely disable interrupts, even inside critical sections. Further, this is achieved without the disadvantages of a segmented kernel architecture. Note however, certain microcontroller architectures will (in hardware) disable interrupts when a new interrupt is accepted - meaning interrupts are unavoidably disabled for the short period between the hardware accepting the interrupt, and the FreeRTOS code re-enabling interrupts.
Interrupts that do not call API functions can execute at priorities above configMAX_SYSCALL_INTERRUPT_PRIORITY and therefore never be delayed by the RTOS kernel execution.
For example, imagine a hypothetical microcontroller that has 8 interrupt priority levels - 0 being the lowest and 7 being the highest (see the special note for ARM Cortex-M3 users at the end of this section). The picture below describes what can and cannot be done at each priority level should the two configuration constants be set to 4 and 0 as shown:

These configuration parameters allow very flexible interrupt handling:
A special note for ARM Cortex-M3 and ARM Cortex-M4 users: Please read the page dedicated to interrupt priority settings on ARM Cortex-M devices. As a minimum, remember that ARM Cortex-M3 cores use numerically low priority numbers to represent HIGH priority interrupts, which can seem counter-intuitive and is easy to forget! If you wish to assign an interrupt a low priority do NOT assign it a priority of 0 (or other low numeric value) as this can result in the interrupt actually having the highest priority in the system - and therefore potentially make your system crash if this priority is above configMAX_SYSCALL_INTERRUPT_PRIORITY.
The lowest priority on a ARM Cortex-M3 core is in fact 255 - however different ARM Cortex-M3 vendors implement a different number of priority bits and supply library functions that expect priorities to be specified in different ways. For example, on the STM32 the lowest priority you can specify in an ST driver library call is in fact 15 - and the highest priority you can specify is 0.
configASSERT() is equivalent to the standard C assert() macro. It is used in place of the standard C assert() macro because not all the compilers that can be used to build FreeRTOS provide an assert.h header file.
Each macro takes the form ...
To exclude vTaskDelete() from your build use:
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