This section has additional useful information related to FreeRTOS and its integration within MCU+ SDK. It also compares some key points and config in FreeRTOS vs TI SysBIOS RTOS which was used in previous TI SOCs and SDKs.
FreeRTOS configuration
Using and adding FreeRTOS to your project
- You can start using FreeRTOS using one of the many example projects in the SDK. The Empty Project at
${SDK_INSTALL_PATH}/examples/empty is a good starting point
- Given below are some details to add FreeRTOS to a project if you decide to start from scratch
- Below paths must be added to your project include path to use FreeRTOS Kernel APIs- (below shows example for R5F)
${MCU_PLUS_SDK_PATH}/source/kernel/freertos/FreeRTOS-Kernel/include
${MCU_PLUS_SDK_PATH}/source/kernel/freertos/portable/TI_ARM_CLANG/ARM_CR5F
${MCU_PLUS_SDK_PATH}/source/kernel/freertos/config/am263x/r5f
- To use FreeRTOS+POSIX, you need to add the below additional paths
${MCU_PLUS_SDK_PATH}/source/kernel/freertos/FreeRTOS-POSIX/include
${MCU_PLUS_SDK_PATH}/source/kernel/freertos/FreeRTOS-POSIX/include/private
${MCU_PLUS_SDK_PATH}/source/kernel/freertos/FreeRTOS-POSIX/FreeRTOS-Plus-POSIX/include
${MCU_PLUS_SDK_PATH}/source/kernel/freertos/FreeRTOS-POSIX/FreeRTOS-Plus-POSIX/include/portable
- To link to FreeRTOS library, add below path to you library path
${MCU_PLUS_SDK_PATH}/source/kernel/freertos/lib
- And link to below library for your SOC and CPU of choice
freertos.{soc}.{cpu}.{compiler}.{profile}.lib
- Before using FreeRTOS APIs in your application, you need to do below
- Initialize cache, MPU, HW interrupt as needed.
- Setup one timer to tick at a frequency defined in
FreeRTOSConfig.h, typically 1 ms
- Create at least one task
- Finally call
vTaskStartScheduler to start FreeRTOS.
- This starts the FreeRTOS schedular and it jumps to the highest priority task previously created.
- More tasks and OS resources like semaphores can be created from within this initial task.
- Once again refer to the many FreeRTOS examples to see the project settings and startup code.
Summary of comparison with SysBIOS
This section has a quick summary of comparision between SysBIOS and FreeRTOS features. Refer rest of the sections on this page for more details
Key similarities between modules
| SysBIOS Module | FreeRTOS Module | Additional Remarks |
| XDC based config | FreeRTOSConfig.h | #define based config, most code in C files for FreeRTOS |
| Static alloc | Static alloc | “construct” APIs in SysBIOS, “CreateStatic” APIs in FreeRTOS |
| Task | Task | Same scheduling policy as SysBIOS |
| Semaphore | Semaphore, Mutex | Same as SysBIOS. Binary, counting semaphore’s, recursive, priority inheritance mutex’s |
| Clock | Timer | Similar features, see details below |
| Event | Event | Similar features, see details below |
| Idle | Idle | Only one idle hook in FreeRTOS |
| Queue | List | Similar features, see details below |
| Mailbox | Queue | Similar features, see details below |
| Hwi | HwiP in DPL | Similar features, see details below |
| Cache | CacheP in DPL | Similar features, see details below |
| HeapMem | HeapP in DPL | Similar features, see details below |
| MPU/MMU | MPU/MMU in DPL | Similar features, see details below |
| Swi | “Pend” function call | Similar features, see details below |
| Load | Run-time stats APIs | Similar features, see details below |
| POSIX | FreeRTOS+POSIX | Similar features, see details below |
Key differences between modules
| FreeRTOS difference vs SysBIOS | Alternative in FreeRTOS |
When configUSE_TIME_SLICING is 1, FreeRTOS will time slice between tasks at the same priority | Keep this at 0 (SDK default config), if this is not desired |
| Timer callbacks are called in task context vs ISR context in SysBIOS | Keep the timer task to highest priority so that timer callback gets called immediately after all ISRs are done (SDK default config) |
| No SWI support in FreeRTOS | Use “pend" function call, where functions are executed in timer task context. Again keep timer task as highest priority (SDK default config), see xTimerPendFunctionCall() and xTimerPendFunctionCallFromISR() |
| No ability to create arbitrary heaps | SDK DPL provides APIs to create arbitrary heaps to match SysBIOS (see Heap) |
Only FreeRTOS APIs ending with FromISR() can be called from within ISRs | SDK DPL provides a function HwiP_inISR() to detect if code is in ISR. This can be used to keep the application functions generic. SDK DPL takes care of this by default |
| Number of task priorities is configurable (upto max 32) vs SysBIOS default of 16 | SDK default config keeps number of task priorities to 16 in FreeRTOS |
FreeRTOS Tasks
Overview
- FreeRTOS has ability to create tasks with below parameters
- Entry function
- One
void * entry function argument
- Stack memory (when NULL, FreeRTOS uses the default heap to allocate the stack memory)
- Stack size in units of “stack words”, i.e 32b or 4 bytes in case of R5F, M4F and 8 bytes in case of A53
- Priority (0 is lowest,
configMAX_PRIORITY-1 is the highest)
- SDK default config is 16 priorities to match SysBIOS priorities
- Static task object memory allocation is supported
- SDK examples and DPL by default use static alloc APIs
- Scheduler policy is configurable
configUSE_PREEMPTION, SDK default is 1 to match SysBIOS scheduler policy
- This enables pre-emptive priority based scheduling
configUSE_TIME_SLICING, SDK default is 0, i.e disabled, to match SysBIOS scheduler policy
- When enabled, when two tasks are of the same priority, FreeRTOS will time-slice between the tasks at units of timer tick
- NOTE, this is unlike SysBIOS where unless
Task_yeild is called scheduler will not switch to another task at same priority
Initial Tasks
- Two tasks are created inside FreeRTOS on startup, idle task and timer task
- Idle task is similar to any other task, only it runs at lowest priority.
- User can configure a “hook” function to call inside of IDLE, e.g, WFI can be called here.
- SDK default config is shown below,
vApplicationIdleHook calls wfi instruction for R5F, M4F, A53.
\code
/* in FreeRTOSConfig.h file */
/* when 1, make sure to implement void vApplicationIdleHook(void) as the hook function */
#define configUSE_IDLE_HOOK (1)
/* in port.c file for R5F, M4F, A53 */
void vApplicationIdleHook( void )
{
__asm__ volatile ("wfi");
}
\endcode
- Timer task (SDK default config enables timer task to match SysBIOS),
- Timer callbacks and “deferred” interrupt handler functions are called in context of the timer task
- NOTE, unlike SysBIOS, SW timer callbacks are not called in ISR context
- “deferred” handler functions, can be used as equivalent of SW interrupt of SysBIOS
- NOTE, unlike SysBIOS, there is no SWI module in FreeRTOS
- SDK config keeps the priority of this task to highest so that timer function will run immediately after a ISR
- All of above can be controlled via FreeRTOS Config, SDK defaults are listed below
#define configUSE_TIMERS (1) /* enable timer task and SW timers */
#define configTIMER_TASK_PRIORITY (configMAX_PRIORITIES - 1) /* highest priority */
#define configTIMER_TASK_STACK_DEPTH (256) /* 1KB */
Task load
- To measure task and CPU load, in FreeRTOS config, we need to set
configGENERATE_RUN_TIME_STATS to 1 (SDK default is 1 when configOPTIMIZE_FOR_LATENCY is 0)
- When load measurement is enabled,
portGET_RUN_TIME_COUNTER_VALUE() is called to get a high resolution timer counter value.
- In MCU+ SDK porting layer, we implement
portGET_RUN_TIME_COUNTER_VALUE() using ClockP_getTimeUsec to return time stamp in units of usecs
- In FreeRTOS Kernel, the load measurement counter will overflow after 32b, i.e around 1 hr with usec resolution timer.
- To avoid this, in FreeRTOS DPL, a periodic load update API is called in IDLE task to accumulate the measured task load and take care of overflow condition.
- To get task load with overflow condition taken care of, it is recommended to create the task using Task DPL APIs
- Total CPU load is calculated by measuring the time spent in IDLE task and then subtracting it from 100%, i.e CPU load = 100 - CPU idle load
Important tips for application writers
- Task function should not return in FreeRTOS, instead it should call vTaskDelete(NULL) to destroy itself, e.g
void myTaskMain(void *args)
{
/* ... do something ... */
vTaskDelete(NULL);
/* do not call “return” */
}
- FreeRTOS schedular, by default task switch does not save/restore FPU (floating point unit) registers, tasks which need FPU need to call
portTASK_USES_FLOATING_POINT once before using FPU operations. If in doubt always call this function
- ISR handler may or may not save FPU state depending on the CPU, see Features Supported to check if your CPU of interest supports FPU save/restore
- On task delete, FreeRTOS will free any memory allocated internally, if dynamically memory allocation mode was used. This memory free is done in “IDLE” task, so “IDLE” needs to get opportunity to run at some point.
vTaskStartScheduler() starts FreeRTOS and the highest priority task is executed first.
- Tasks, semaphores and OS objects can be created before
vTaskStartScheduler() is called.
- Recommend to create one task and then call
vTaskStartScheduler() and then do everything from within that task including creating other tasks.
- **
vTaskStartScheduler() is similar to BIOS_start(), i.e it never returns and switches to the created tasks or idle task**
- .stack is the stack used by code before FreeRTOS scheduler is started, i.e from
_c_int00 to vTaskStartSchedular()
FreeRTOS Interrupts
Overview
- Interrupt handler is not directly invoked by FreeRTOS Kernel, porting layer handles this
- In theory, FreeRTOS can work without interrupts. To switch tasks, interrupts are not needed. However that’s not very useful.
- Porting layer does below
- Setup (any) one timer to be configured at
configTICK_RATE_HZ, typically 1ms
- For R5F, A53, we use one of the SOC level general purpose DM timers.
- For M4F, we use the M4F internal SysTick timer.
- Timer ISR is outside FreeRTOS kernel
- When the timer ISR happens the porting layer calls
xTaskIncrementTick() FreeRTOS API to maintain FreeRTOS timer tick state
- Porting layer also implements the common interrupt entry and exit logic
- Esp before interrupt exit, porting layer needs to invoke a task switch if during ISR handling a semaphore was posted that needed a task switch on ISR exit
- Interrupt nesting is also taken care by this common interrupt handler
- Porting layer also implements functions to protect critical sections of FreeRTOS via below APIs
portENTER_CRITICAL, disable interrupt, track nesting of callsportEXIT_CRITICAL, reenable interrupt, if nesting call depth is 0portSET_INTERRUPT_MASK_FROM_ISR, interrupt disable and return old interrupt stateportCLEAR_INTERRUPT_MASK_FROM_ISR, restore interrupt state
Nested interrupts and ISR stack
- On R5F,
- When a interrupt is triggered, the CPU switches to IRQ mode and uses IRQ stack.
- IRQ interrupt are disabled by HW at this point.
- In the ISR handler, some CPU state is saved to IRQ stack and mode is switched to SVC mode and therefore SVC stack
- IRQs are then enabled, i.e nested interrupts are enabled
- The user ISR code is executed
- At this point more IRQs can occur to interrupt the user ISR code
- After all IRQs are handled, IRQ is disabled
- Now mode is switched back to IRQ mode
- If a task switch was requested by any of the ISRs, tasks are switched and control returns from IRQ to the highest priority pending task
- If a task switch was NOT requested, control returns from IRQ to the interrupted task
- On R5F,
- Thus the user ISR code get called in context of SVC stack, so size of SVC stack MUST be kept large enough to handle worst case ISR requirement.
- The IRQ stack size itself can be kept small since it only saves few bytes (8 bytes) of state for every nested IRQ invocation.
- SDK default examples keep IRQ stack as 256 bytes and SVC stack size as 4096 bytes.
- This stack size is specified in the linker command file, a sample snippet is shown below,
__IRQ_STACK_SIZE = 256;
/* This is the size of stack when R5 is in IRQ mode
* - In both NORTOS and FreeRTOS nesting is disabled for FIQ
*/
__FIQ_STACK_SIZE = 256;
__SVC_STACK_SIZE = 4096; /* This is the size of stack when R5 is in SVC mode */
__ABORT_STACK_SIZE = 256; /* This is the size of stack when R5 is in ABORT mode */
__UNDEFINED_STACK_SIZE = 256; /* This is the size of stack when R5 is in UNDEF mode */
- On M4F, nested interrupt work by default without any special handling.
- M4F provides a
pendSV exception which when triggered is invoked after all nested ISRs are handled.
- The porting layer calls FreeRTOS task switch logic in the
pendSV exception handler
- On A53,
- The user ISR gets called in context of EL1 stack and is represented by the
.stack section in the linker command file.
Interrupts outside of FreeRTOS
- On R5F,
- When FreeRTOS enter its critical section, it only disables IRQ but not FIQ
- Hence, FreeRTOS API calls MUST NOT be done inside FIQ.
- FIQs can be used for extreme low latency interrupt bypassing the OS completely.
- NOTE: this is same as the case with SysBIOS
- On M4F,
- When FreeRTOS enter its critical section, all interrupt levels numerically below
configMAX_SYSCALL_INTERRUPT_PRIORITY defined in FreeRTOSConfig.h are disabled.
- So interrupts at numerical priority level above
configMAX_SYSCALL_INTERRUPT_PRIORITY MUST not use FreeRTOS API calls.
- SDK default keeps
configMAX_SYSCALL_INTERRUPT_PRIORITY at 0xE0, i.e all interrupt priority levels are disabled when FreeRTOS critical section is entered.
- On A53,
- Only IRQ is supported, FIQ is not supported
Additional important tips for application writers
- Only FreeRTOS APIs that end with
FromISR can be called from ISR context BaseType_t xHigherPriorityTaskWoken = 0;
xSemaphoreGiveFromISR(pSemaphore->semHndl, &xHigherPriorityTaskWoken);
portYIELD_FROM_ISR(xHigherPriorityTaskWoken);
- The
FromISR API returns a flag xHigherPriorityTaskWoken to indicate task switch should be requested, portYIELD_FROM_ISR requests the task switch. Actual task switch happens after all nested ISRs have executed.
- This is different vs SysBIOS, where the same APIs can be used inside ISR and outside ISR.
- The SDK DPL APIs takes care of this internally as below, e.g,
SemaphoreP_post(...) {
if( HwiP_inISR() ) {
BaseType_t xHigherPriorityTaskWoken = 0;
xSemaphoreGiveFromISR(pSemaphore->semHndl, &xHigherPriorityTaskWoken);
portYIELD_FROM_ISR(xHigherPriorityTaskWoken);
} else {
xSemaphoreGive(pSemaphore->semHndl);
}
}
FreeRTOS Semaphores and Mutex
Overview
- FreeRTOS has text book implementation of binary semaphore, counting semaphore, mutex with priority inheritance and recursive mutex
- Timeouts can be specified when waiting on a semaphore/mutex, with timeout error code if semaphore/mutex is not available
- Timeout is specified in units of OS ticks. Use
pdMS_TO_TICKS() to convert from msec to OS ticks
Important tips for application writers
- Binary and counting semaphores should be used to signal from ISR to task and task to task
- SDK default config enables binary and counting semaphores
- Mutex should be used for mutual exclusion of critical sections in applications.
- SDK default config enables mutex and recursive mutex's
- Once again, only APIs that end with
FromISR can be used from within ISR. These are non-blocking APIs.
FreeRTOS Task Notification
Overview
- Task notification is low overhead API to signal a task from ISR or a task from another task
- We see about 25% reduction in cycles needed for task switch using task notification vs using semaphores in ideal standalone conditions.
- This is a very freertos specific API and SDK drivers and DPL will NOT use this.
Important tips for application writers
- In practical use-cases, typically cache effects due to cache miss on the larger application code/data, will have higher effect than overheads of task switch in the smaller OS code.
- So we recommend to NOT use this feature unless really the savings is critical to end application.
- Here one needs to know the task handle to signal, so its difficult to use this API from within a driver library since driver ISR for example does not know the task in which the driver API is called
- Thus use these APIs in applications after thorough analysis and profiling on benefit for the use-case.
FreeRTOS Event Groups
Overview
- An event group is a set of event bits. Event bits are used to indicate if an event has occurred or not. Event bits are often referred to as event flags.
- The number of bits (or flags) stored within an event group is 24 bits
Important tips for application writers
- Unlike "Event" in SysBIOS where one could give a "AND" mask and "OR" mask to wait on, in FreeRTOS one can only give one "event mask" and then tell if one should use "AND" wait or "OR" wait on the single event mask
- When "set bits" is called from ISR, actual action of setting bits is done within the "Timer task" as a "deferred" function call.
FreeRTOS Timers
Overview
- Allows users to create one shot or periodic callbacks at time resolution of one OS tick, typically 1ms.
- Similar to “Clock” module in SysBIOS
- A timer task executes the callbacks, this timer task is like any other task, nothing special about it.
- SDK default is to set highest priority for this task so that after all ISRs this task will execute
- A SW queue is used to post callbacks or functions to execute on timer expiry, the queue depth is config option. SDK default is 16
Important tips for application writers
- Since a single task executes all the multiple callbacks, the callbacks are recommended to not block and spend too much time, even though in theory blocking calls are allowed.
- Use
xTimerPendFunctionCall() to do “deferred ISR” handling , i.e do the most critical work in ISR and do the rest of work in a another function call which gets called right after all ISRs are done. This feature can be used as alternative to SWI in SysBIOS
- Note, do not use
FromISR APIs in callbacks, use regular FreeRTOS APIs
FreeRTOS Heaps
Default heap
- FreeRTOS kernel has multiple heap implementations as defined in
portable/MemMang
- A port should compile one of below
heap_{n}.c files to pick the required heap implementation
heap_1.c
- linear heap allocation,
free not allowed. Typically used when true dynamic alloc is prohibited.
- Obsolete now and not used anymore, since FreeRTOS natively support static alloc mode
heap_2.c
- Linked list based heap but adjacent free blocks not merged
- Obsolete now and not used anymore,
heap_4.c is a better implementation
heap_3.c SDK default
- Uses compiler provided,
malloc() and free(), FreeRTOS only makes the calls task safe
- Heap size specified via
--heap compiler option and placed in .heap section in linker command file.
heap_4.c
- Linked list based heap with adjacent free block merging
- More or less similar to
malloc() from compiler and SysBIOS heap implementation
- Memory needs to be provided as a global static array, and placed appropriately in linker command file.
heap_5.c
- Same as
heap_4.c, + it allows memory to be specified as multiple memory blocks When memory in one block is full, it will alloc from the next block and so on.
User defined heaps
- Unlike SysBIOS, FreeRTOS does not support user defined heaps at arbitrary user defined memory locations
- On TI SOCs with multiple levels of memory like L2, L3, DDR, it is very convenient to have ability to create multiple application specified heaps.
heap_4.c almost does this, but it uses a global variable for heap memory base and size- Therefore, SDK DPL implements heap APIs to match SysBIOS as below
- SDK has adapted
heap_4.c to take memory base and size as input parameters during heap create. This allows creation of arbitrary number of application heaps to match SysBIOS features
HeapP.h is the user API (Heap)HeapP_freertos.c, HeapP_internal.c is the implementation (Note, FreeRTOS kernel code is not modified)
Important tips for application writers
- Use
--heap in linker command file to specify heap size and place .heap section in appropriate memory in linker command file. This is used by FreeRTOS (heap_3.c) when it needs dynamic memory allocation.
- In general recommend to use FreeRTOS static alloc APIs for tasks, semaphores to keep the application deterministic.
- SDK DPL uses FreeRTOS static alloc APIs
- Use SDK provided
HeapP.h to create arbitrary application heaps as needed.
FreeRTOS Additional Modules
Queues
- Highly efficient SW queue implementation
- Lowest level primitive in FreeRTOS used internally by almost all modules like tasks, semaphores, mutex
- Task/ISR safe
- Blocking/non-blocking with timeouts
- Fixed queue depth and fixed queue element size at queue create
- Can be used by application users
Queue Sets, Stream Buffers, Co-routines
- Queue Sets, more higher level than queues, built using queues, semaphores. FreeRTOS recommends to use these very carefully. https://www.freertos.org/Pend-on-multiple-rtos-objects.html
- Stream buffers, built using queues, semaphores
- Croutines, used when tasks are deemed too expensive to use, almost unlikely we will ever use these
- SDK default config keeps these disabled
FreeRTOS Hook Functions
Debug Hook Functions
- FreeRTOS provides "hook" function callbacks specified in FreeRTOS config.
- These callbacks are invoked by FreeRTOS at specific points or when certain conditions are met
- Stack overflow check,
configCHECK_FOR_STACK_OVERFLOW ( SDK default enabled )
- Enables stack overflow checks, two options to detect overflows,
- Option 1: Check if stack pointer has gone beyond limit ( SDK default )
- Option 2: Put a pattern at top of stack and if pattern is overwritten then call out stack overflow
- NOTE: Depending on nature of stack overflow, this logic may or may not catch all stack overflows
- Malloc failed hook,
configUSE_MALLOC_FAILED_HOOK ( SDK default disabled )
- Called when
pvPortMalloc fails inside FreeRTOS, not very useful, since all APIs return error on memory alloc failure.
- Assert,
configASSERT ( SDK default enabled )
- Called when unrecoverable assert condition happens inside FreeRTOS or porting layer
- SDK defines a new config flag
configOPTIMIZE_FOR_LATENCY, this can be used to disable all debug hooks and some other configs in FreeRTOS config to minimize FreeRTOS overheads due to debug hooks,
- Recommend to enable all hooks in
debug mode and disable all hooks in release mode after applications are reasonably sure assert and overflows will not happen.
- By default
configOPTIMIZE_FOR_LATENCY is set to 1, i.e enabled
Statistics Hook Functions
- FreeRTOS provides APIs to query and return time spent in each task and output the results in a nice formatted table
- The hook functions for this feature are enabled in SDK default config when
configOPTIMIZE_FOR_LATENCY is 0.
- It uses same timer as the tick timer to measure task load.
- See also https://www.freertos.org/rtos-run-time-stats.html
Trace Hooks Functions
- FreeRTOS provides more than 70+ hook marcos at key points in the kernel to log trace info
- Example,
traceTASK_SWITCHED_IN, called when a task is being switched intotraceTASK_SWITCHED_OUT, called when a task is begin switched to another task
- This is used by tools like
Tracealyzer to log and then visualize task execution and lot more in a GUI tool, see https://www.freertos.org/FreeRTOS-Plus/FreeRTOS_Plus_Trace/FreeRTOS_Plus_Trace.html
- Not enabled in SDK default config
FreeRTOS POSIX
Overview
Important tips for application writers
- Stack used by threads created with pthread is allocated using dynamic memory alloc, so make sure heap size is sufficiently large
- mqueue, timer also uses dynamic alloc APIs
- Use this layer if application portability to other POSIX OS is important, else it is recommended to use direct FreeRTOS APIs or SDK DPL APIs
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