freertos

FreeRTOS

Purpose

Guide agents through FreeRTOS application development: task creation and priorities, inter-task communication with queues and semaphores, stack overflow detection, configASSERT, and FreeRTOS-aware debugging with GDB and OpenOCD.

Triggers

• "How do I create a FreeRTOS task?"
• "How do I pass data between FreeRTOS tasks?"
• "My FreeRTOS task is crashing — how do I detect stack overflow?"
• "How do I use FreeRTOS mutexes?"
• "How do I debug FreeRTOS tasks with GDB?"
• "How do I configure FreeRTOSConfig.h?"

Workflow

1. Task creation and priorities

#include "FreeRTOS.h"
#include "task.h"

// Task function signature
void vMyTask(void *pvParameters) {
    const char *name = (const char *)pvParameters;
    for (;;) {
        // Task body — must never return
        printf("Task %s running\n", name);
        vTaskDelay(pdMS_TO_TICKS(500));  // yield for 500ms
    }
}

int main(void) {
    // xTaskCreate(function, name, stack_depth_words, param, priority, handle)
    TaskHandle_t xHandle = NULL;
    xTaskCreate(vMyTask, "MyTask",
                configMINIMAL_STACK_SIZE + 128,  // words, not bytes!
                (void *)"sensor",
                tskIDLE_PRIORITY + 2,            // higher = more urgent
                &xHandle);

    vTaskStartScheduler();  // never returns if heap is sufficient
    for (;;);               // should never reach here
}

Priority guidelines:

• tskIDLE_PRIORITY (0) — idle task, never block here
• ISR-deferred tasks — highest priority to service interrupts quickly
• Avoid priorities above configMAX_PRIORITIES - 1

2. Queues — inter-task data passing

#include "queue.h"

typedef struct { uint32_t sensor_id; float value; } SensorReading_t;

QueueHandle_t xSensorQueue;

void vProducerTask(void *pvParam) {
    SensorReading_t reading;
    for (;;) {
        reading.sensor_id = 1;
        reading.value = read_adc();
        // Send; block max 10ms if queue full
        xQueueSend(xSensorQueue, &reading, pdMS_TO_TICKS(10));
        vTaskDelay(pdMS_TO_TICKS(100));
    }
}

void vConsumerTask(void *pvParam) {
    SensorReading_t reading;
    for (;;) {
        // Block forever until item available
        if (xQueueReceive(xSensorQueue, &reading, portMAX_DELAY) == pdTRUE) {
            process(reading.value);
        }
    }
}

// Create before starting scheduler
xSensorQueue = xQueueCreate(10, sizeof(SensorReading_t));