Hands-On: Multi-Task RTOS Application - Embedded C/C++ for Automotive

Lab Scope: Multi-Task FreeRTOS Application

Task	Period	Priority	Function
Task_Sensor	10 ms	3	Read ADC, compute filtered value
Task_Control	10 ms	4 (higher)	PI controller: read sensor value, compute actuator output
Task_Comms	100 ms	2	Send CAN frame with current status

Exercise 1: Three-Task Application

Cmultitask_app.c

#include "FreeRTOS.h"
#include "task.h"
#include "queue.h"
#include 
#include "Std_Types.h"

/* Shared data: sensor → control via queue (type-safe, thread-safe copy) */
static QueueHandle_t   g_sensor_queue;
static StaticQueue_t   g_sensor_queue_buf;
static uint16_t        g_sensor_queue_storage[4 * sizeof(uint16_t)];

typedef struct { uint16_t raw; uint32_t timestamp_ticks; } SensorData_t;

/* Task 1: Sensor acquisition at 10 ms */
static StackType_t  s_sensor_stack[256];
static StaticTask_t s_sensor_tcb;

void Task_Sensor(void *p)
{
    (void)p;
    TickType_t last_wake = xTaskGetTickCount();
    for (;;) {
        uint16_t adc_val = ADC_Read(ADC_CHANNEL_MAIN);
        SensorData_t data = { .raw = adc_val, .timestamp_ticks = xTaskGetTickCount() };
        xQueueOverwrite(g_sensor_queue, &data);  /* overwrite: keep latest only */
        vTaskDelayUntil(&last_wake, pdMS_TO_TICKS(10u));
    }
}

/* Task 2: Control at 10 ms — higher priority to ensure control runs after sensor */
void Task_Control(void *p)
{
    (void)p;
    SensorData_t data;
    TickType_t last_wake = xTaskGetTickCount();
    for (;;) {
        if (xQueuePeek(g_sensor_queue, &data, 0u) == pdTRUE) {
            uint16_t output = PID_Update(data.raw);
            DAC_Write(output);
        }
        vTaskDelayUntil(&last_wake, pdMS_TO_TICKS(10u));
    }
}

/* Task 3: CAN communication at 100 ms — lowest priority */
void Task_Comms(void *p)
{
    (void)p;
    SensorData_t data;
    TickType_t last_wake = xTaskGetTickCount();
    for (;;) {
        xQueuePeek(g_sensor_queue, &data, 0u);
        uint8_t can_payload[8] = { (uint8_t)(data.raw >> 8u),
                                    (uint8_t)(data.raw & 0xFFu), 0,0,0,0,0,0 };
        CAN_Send(0x100u, can_payload, 2u);
        vTaskDelayUntil(&last_wake, pdMS_TO_TICKS(100u));
    }
}

Exercise 3: Measure Task Timing with STM

Ctask_timing.c

/* Measure actual task execution time using STM hardware timer */
#include "FreeRTOS.h"
#include "task.h"
#include 

#define STM_TICKS_PER_US  300u

typedef struct {
    uint32_t start_tick;
    uint32_t wcet_ticks;   /* worst-case execution time */
    uint32_t count;
} TaskProfile_t;

static TaskProfile_t g_sensor_profile;

void Task_Sensor_Profiled(void *p)
{
    (void)p;
    TickType_t last_wake = xTaskGetTickCount();
    for (;;) {
        uint32_t t_start = STM0_TIM0;

        uint16_t adc_val = ADC_Read(ADC_CHANNEL_MAIN);
        SensorData_t data = { .raw = adc_val, .timestamp_ticks = (uint32_t)last_wake };
        xQueueOverwrite(g_sensor_queue, &data);

        uint32_t elapsed = STM0_TIM0 - t_start;
        if (elapsed > g_sensor_profile.wcet_ticks) {
            g_sensor_profile.wcet_ticks = elapsed;
        }
        g_sensor_profile.count++;

        /* Log WCET via DID if > 80% of 10ms budget */
        if (g_sensor_profile.wcet_ticks > (8000u * STM_TICKS_PER_US)) {
            Dem_ReportErrorStatus(DEM_SENSOR_TASK_OVERRUN, DEM_EVENT_STATUS_FAILED);
        }

        vTaskDelayUntil(&last_wake, pdMS_TO_TICKS(10u));
    }
}

Summary

The three-task application demonstrates the producer-consumer pattern: the sensor task (lower priority, runs first at period start) writes to a queue; the control task (higher priority, triggered by the deadline) reads from it. Using vTaskDelayUntil instead of vTaskDelay is critical for periodic accuracy: vTaskDelay adds jitter equal to task execution time; vTaskDelayUntil maintains a constant period regardless of how long the task took to execute. The profiling exercise reveals the real WCET — always instrument before optimising.

🔬 Deep Dive — Core Concepts Expanded

This section builds on the foundational concepts covered above with additional technical depth, edge cases, and configuration nuances that separate competent engineers from experts. When working on production ECU projects, the details covered here are the ones most commonly responsible for integration delays and late-phase defects.

Key principles to reinforce:

Configuration over coding: In AUTOSAR and automotive middleware environments, correctness is largely determined by ARXML configuration, not application code. A correctly implemented algorithm can produce wrong results due to a single misconfigured parameter.
Traceability as a first-class concern: Every configuration decision should be traceable to a requirement, safety goal, or architecture decision. Undocumented configuration choices are a common source of regression defects when ECUs are updated.
Cross-module dependencies: In tightly integrated automotive software stacks, changing one module's configuration often requires corresponding updates in dependent modules. Always perform a dependency impact analysis before submitting configuration changes.

🏭 How This Topic Appears in Production Projects

Project integration phase: The concepts covered in this lesson are most commonly encountered during ECU integration testing — when multiple software components from different teams are combined for the first time. Issues that were invisible in unit tests frequently surface at this stage.
Supplier/OEM interface: This is a topic that frequently appears in technical discussions between Tier-1 ECU suppliers and OEM system integrators. Engineers who can speak fluently about these details earn credibility and are often brought into critical design review meetings.
Automotive tool ecosystem: Vector CANoe/CANalyzer, dSPACE tools, and ETAS INCA are the standard tools used to validate and measure the correct behaviour of the systems described in this lesson. Familiarity with these tools alongside the conceptual knowledge dramatically accelerates debugging in real projects.

⚠️ Common Mistakes and How to Avoid Them

Assuming default configuration is correct: Automotive software tools ship with default configurations that are designed to compile and link, not to meet project-specific requirements. Every configuration parameter needs to be consciously set. 'It compiled' is not the same as 'it is correctly configured'.
Skipping documentation of configuration rationale: In a 3-year ECU project with team turnover, undocumented configuration choices become tribal knowledge that disappears when engineers leave. Document why a parameter is set to a specific value, not just what it is set to.
Testing only the happy path: Automotive ECUs must behave correctly under fault conditions, voltage variations, and communication errors. Always test the error handling paths as rigorously as the nominal operation. Many production escapes originate in untested error branches.
Version mismatches between teams: In a multi-team project, the BSW team, SWC team, and system integration team may use different versions of the same ARXML file. Version management of all ARXML files in a shared repository is mandatory, not optional.

📊 Industry Note

Engineers who master both the theoretical concepts and the practical toolchain skills covered in this course are among the most sought-after professionals in the automotive software industry. The combination of AUTOSAR standards knowledge, safety engineering understanding, and hands-on configuration experience commands premium salaries at OEMs and Tier-1 suppliers globally.