FreeRTOS — task creation, priorities, xTaskCreate

5.5.10 · Coding › Embedded Systems & Real-Time Software

WHAT hai ek task?

Ek task hamesha exactly ek state mein hota hai:

State	Matlab
Running	Abhi CPU use kar raha hai (single core par ek waqt mein sirf ek)
Ready	Chalne ke liye taiyaar hai, CPU ka intezaar kar raha hai
Blocked	Kisi event ka intezaar kar raha hai (delay, queue, semaphore) — koi CPU nahi lagta
Suspended	Scheduling se hata diya gaya hai jab tak resume na ho

Figure — FreeRTOS — task creation, priorities, xTaskCreate

HOW karte hain task create: `xTaskCreate`

Formula Signature (har argument = WHY woh exist karta hai)

BaseType_t xTaskCreate(
    TaskFunction_t   pvTaskCode,    // the function = the task body
    const char *     pcName,        // human-readable name (debug only)
    configSTACK_DEPTH_TYPE usStackDepth, // stack size in WORDS, not bytes
    void *           pvParameters,  // arg passed into the task function
    UBaseType_t      uxPriority,    // 0..(configMAX_PRIORITIES-1)
    TaskHandle_t *   pxCreatedTask  // out: handle to control the task later
);

Return: pdPASS success par, ya errCOULD_NOT_ALLOCATE_REQUIRED_MEMORY agar heap mein stack + TCB fit nahi hua.

$StackBytes = usStackDepth \times sizeof(StackType_t)$

Priorities — scheduling ka dil

Worked Example 1 — do blinking LEDs

void vBlink(void *pvParameters) {
    int pin = (int)pvParameters;          // Why? pass which LED via param
    for (;;) {                            // Why? a task NEVER returns
        gpio_toggle(pin);
        vTaskDelay(pdMS_TO_TICKS(500));   // Why? Blocked → frees CPU for others
    }
}
 
void main(void) {
    xTaskCreate(vBlink, "L1", 128, (void*)13, 1, NULL);  // Why prio 1: above idle
    xTaskCreate(vBlink, "L2", 128, (void*)14, 1, NULL);  // Why same prio: fair share
    vTaskStartScheduler();   // Why? hands control to FreeRTOS; never returns
    for(;;);                 // Why? only reached if heap was too small
}

pdMS_TO_TICKS kyun? FreeRTOS time ko ticks mein count karta hai, ms mein nahi. Yeh macro configTICK_RATE_HZ use karke convert karta hai, code ko readable aur portable rakhta hai.

Worked Example 2 — priority preemption

void vHigh(void *p){ for(;;){ readSensor(); vTaskDelay(pdMS_TO_TICKS(10)); } }
void vLow (void *p){ for(;;){ heavyMath(); } }   // never blocks!
 
xTaskCreate(vLow,  "low",  256, NULL, 1, NULL);
xTaskCreate(vHigh, "high", 256, NULL, 3, NULL);   // Why prio 3 > 1: must preempt

Yeh kaam kyun karta hai? Har 10 ms mein vHigh Ready ho jaata hai. Kyunki 3 > 1 hai, scheduler vLow ko instantly preempt kar deta hai, sensor read chala deta hai, phir vHigh block ho jaata hai aur vLow apna math resume kar leta hai. Sensor ko deterministic timing milti hai chahe vLow CPU hoga kyu na kar raha ho.

Worked Example 3 — return value check karna

TaskHandle_t h;
if (xTaskCreate(vBlink, "L", 4096, NULL, 1, &h) != pdPASS) {
    // Why check? huge stack (4096 words = 16 KB) may exhaust the heap
    error_blink();   // fail loudly instead of silently not running
}
vTaskDelete(h);      // Why store handle? to delete/suspend this task later

Common mistake Steel-manned common errors

1. "Lower number = higher priority" (yeh UNIX nice values se sahi lagta hai). Kyun sahi lagta hai: kai OSes (Linux nice, classic RTOSes) isko invert karte hain. Fix: FreeRTOS mein bada number jeet ta hai. configMAX_PRIORITIES-1 sabse top hai.

2. "Stack depth bytes mein hai." Kyun sahi lagta hai: C ka malloc bytes leta hai. Fix: Yeh words mein hai. Bytes paane ke liye sizeof(StackType_t) se multiply karo (Cortex-M par 4). 1024 pass karna "1 KB" sochke actually 4 KB reserve karta hai.

3. Highest-priority task mein koi blocking call nahi hai. Kyun sahi lagta hai: "Yeh important hai, ise hamesha chalane do!" Fix: Yeh sabko starve kar deta hai, idle task ko bhi (toh power saving + watchdog feed break ho jaate hain). Hamesha vTaskDelay/block karo.

4. Task function ko return karne dena. Kyun sahi lagta hai: normal functions return karte hain. Fix: Returning task scheduler ko corrupt kar deta hai — body ko for(;;) mein wrap karo, ya cleanly self-destruct karne ke liye vTaskDelete(NULL) call karo.

5. ISR se xTaskCreate call karna. Kyun sahi lagta hai: "Mujhe is interrupt par ek task chahiye." Fix: Zyaadatar xTaskCreate paths ISR-safe nahi hain. Tasks pehle se create karo; ISRs se unhe signal karne ke liye ...FromISR APIs use karo.

Recall Feynman: ek 12-saal ke bachche ko samjhao

Socho ek cook (CPU) ek kitchen mein kai recipes (tasks) ke saath hai. Cook ek waqt mein sirf ek kaam kar sakta hai, lekin ek smart manager (scheduler) use itni tezi se switch karta hai ki lag ta hai sab dishes ek saath ban rahi hain. Har recipe ka ek urgency number hota hai — bada number = sab chhod do aur yeh pehle karo. xTaskCreate woh moment hai jab aap manager ko ek naya recipe card dete ho, use bata ke kitni counter space (stack) chahiye aur yeh kitna urgent hai. Ek achchi recipe kehti hai "ab 5 minute wait karo" (vTaskDelay) taaki cook doosron ki madad karne jaa sake bekar khade rehne ki jagah.

Flashcards

FreeRTOS mein, kya higher priority number zyada urgent hoti hai ya kam?

Zyada urgent — numerically sabse bada READY priority hamesha chalta hai.

xTaskCreate mein usStackDepth ka unit kya hai?

Words, bytes nahi (sizeof(StackType_t) se multiply karo, jaise Cortex-M par ×4).

xTaskCreate ke 6 parameters kya hain order mein?

Function, Name, StackDepth(words), Parameters(void*), Priority, &TaskHandle.

xTaskCreate success par kya return karta hai vs failure par?

Success par pdPASS; errCOULD_NOT_ALLOCATE_REQUIRED_MEMORY agar heap mein stack+TCB fit na ho.

High-priority task ko block kyun karna chahiye (jaise vTaskDelay)?

Warna woh CPU kabhi yield nahi karta aur sabhi lower-priority tasks starve ho jaate hain.

Do READY tasks ki equal priority ho toh kya hota hai?

Round-robin time-slicing (agar configUSE_TIME_SLICING=1 ho) — CPU fairly share hota hai.

Char task states kya hain?

Running, Ready, Blocked, Suspended.

TCB kya hai?

Task Control Block — kernel struct jo task ki priority, state, stack pointer store karta hai; har task ke saath uske stack ke saath allocate hota hai.

tskIDLE_PRIORITY (0) par kya chalta hai?

Idle task — sabse low priority, tab chalta hai jab kuch aur ready nahi hota.

Task function kabhi return kyun nahi karna chahiye?

Return karna scheduler ko corrupt kar deta hai; for(;;) mein wrap karo ya cleanly bahar nikalne ke liye vTaskDelete(NULL) call karo.

pdMS_TO_TICKS kya karta hai aur kyun zaroori hai?

Milliseconds ko configTICK_RATE_HZ use karke kernel ticks mein convert karta hai; FreeRTOS time ko ticks mein count karta hai, ms mein nahi.

Default FreeRTOS scheduler mein preemption kya hai?

Ek higher-priority task ka READY hona ek running lower-priority task ko immediately interrupt kar deta hai.

Connections

FreeRTOS — vTaskDelay vs vTaskDelayUntil
FreeRTOS — Queues and inter-task communication
FreeRTOS — Semaphores and Mutexes
Priority Inversion and Priority Inheritance
Context Switching and the TCB
ISR-safe APIs (FromISR) and deferred interrupt handling
Heap memory management (heap_1..heap_5) in FreeRTOS
Round-robin vs Preemptive Scheduling