The SDK is based on FreeRTOS (components/os/freertos); tasks, queues, semaphores, event groups and software timers are standard interfaces. See FreeRTOS Scheduling Fundamentals for the theory and FreeRTOS OS for the example.

Common FreeRTOS APIs (Standard Interfaces)

xTaskCreate(task, name, stack, arg, prio, handle)

Create a task (a task is an independent piece of functionality).

Parameters:

• task: task function
• name: task name (for debugging)
• stack: stack size in words
• arg: argument passed to the task
• prio: priority — larger number means higher priority
• handle: task handle output

Return value: pdPASS on success

vTaskDelete(task)

Delete a task (pass NULL to delete the current task).

Parameters:

• task: task handle

Return value: none

vTaskDelay(ticks)

Delay the task: move it to Blocked and yield the CPU.

Parameters:

• ticks: delay in ticks

Return value: none

vTaskDelayUntil(&prev, ticks)

Precise periodic delay: wakes at a fixed period regardless of task run time.

Parameters:

• prev: previous wake-time record
• ticks: period in ticks

Return value: none

vTaskStartScheduler()

Start the scheduler; tasks begin running. It normally never returns.

Parameters: none

Return value: none (does not return)

xTaskGetTickCount()

Get the current system tick (often used for timeout checks).

Parameters: none

Return value: current tick

xQueueCreate(len, size)

Create a queue (a data pipe between tasks).

Parameters:

• len: queue length
• size: bytes per item

Return value: queue handle; NULL on failure

xQueueSend(queue, item, timeout)

Send a message to the queue.

Parameters:

• queue: queue handle
• item: data to send
• timeout: max wait when the queue is full

Return value: pdPASS on success

xQueueReceive(queue, buf, timeout)

Receive a message from the queue (blocks when empty).

Parameters:

• queue: queue handle
• buf: receive buffer
• timeout: max wait

Return value: pdPASS on success

xSemaphoreCreateBinary()

Create a binary semaphore (for event notification).

Parameters: none

Return value: semaphore handle

xSemaphoreCreateMutex()

Create a mutex (ownership + priority inheritance; protects shared resources).

Parameters: none

Return value: mutex handle

xSemaphoreTake / xSemaphoreGive(sem, timeout) / (sem)

Take (wait for) a semaphore / give a semaphore.

Parameters:

• sem: semaphore handle
• timeout: wait timeout (Take only)

Return value: pdPASS on success

xEventGroupCreate / xEventGroupSetBits / xEventGroupWaitBits()

Event groups: bit flags for "multi-condition triggers".

Parameters: see the FreeRTOS documentation (create has none; set/wait take the group handle and bit mask)

Return value: event group handle / current event bits

xTimerCreate / xTimerStart / xTimerStop(...)

Software timers: run a callback when the period expires, without occupying a hardware timer.

Parameters:

• xTimerCreate: name, period, auto-reload, callback

Return value: timer handle / pdPASS

xStreamBufferCreate / xStreamBufferSend / xStreamBufferReceive(...)

Stream buffers: byte-stream send/receive (e.g., UART data → processing task).

Parameters: see the FreeRTOS documentation

Return value: handle / number of bytes sent or received

Note: the SDK uses the standard FreeRTOS API; full declarations are in components/os/freertos/include/ (task.h, queue.h, semphr.h, timers.h, event_groups.h, stream_buffer.h).

Common Exam & Interview Questions

Difference between task delay and busy-wait delay?

vTaskDelay moves the task to Blocked and yields the CPU; a busy while-loop keeps spinning, wasting CPU and preventing sleep.

Semaphore or mutex?

Semaphores are for event notification/counting (no ownership); mutexes protect shared resources (ownership + priority inheritance against priority inversion).

Have questions?

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