🟢 FreeRTOS: First Task & SWV ITM Tracing

Your first step into the RTOS world. One task. One LED. One console message. But an entirely new paradigm.

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🎯 Aim

Create a single FreeRTOS task on STM32F446RE using CMSIS-RTOS v2 that toggles the onboard LED every 500 ms, and observe real-time execution via the SWV ITM Data Console in STM32CubeIDE.

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🔧 Hardware Required

Component	Qty
STM32 Nucleo-F446RE	1
USB Type-A to Mini-B cable	1

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🧩 How FreeRTOS Changes Everything

Old Way (Super Loop)

main() → while(1) { toggle; HAL_Delay(500); toggle; ... }
         CPU is STUCK during HAL_Delay — can't do anything else

New Way (FreeRTOS)

Scheduler starts
    │
    └─► Task_1 (RUNNING)
            │
            ├── HAL_GPIO_TogglePin()
            ├── printf("Task_1 Executing...")
            └── osDelay(500)  ◄── Task enters BLOCKED state
                                   CPU is FREE for other tasks
                    │
                   500ms later...
                    │
            Task_1 → READY → RUNNING again

Task State Machine

              osThreadNew()
                   │
                   ▼
              ┌─────────┐
              │  READY  │◄──────────────────────┐
              └────┬────┘                       │
                   │  Scheduler selects it      │
                   ▼                            │
              ┌─────────┐                       │
              │ RUNNING │──── osDelay(500) ───► │
              └─────────┘                  ┌────┴────┐
                                           │ BLOCKED │
                                           └─────────┘

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⚙️ CubeMX Configuration

Step 1 — GPIO

• PA5 → GPIO Output (LED LD2)

Step 2 — System Settings

Setting	Value
RCC	BYPASS Clock Source
HCLK	84 MHz
SYS → Debug	Trace Asynchronous Sw
SYS → Timebase Source	TIM6 (not SysTick — FreeRTOS uses it)

Step 3 — FreeRTOS

• Middleware & Software Packs → FreeRTOS → Interface: CMSIS_V2
• Tasks & Queues tab → configure the default task:

Field	Value
Task Name	Task_1
Priority	osPriorityNormal
Entry Function	Task1_function
Stack Size	128 words (default)

Step 4 — Printf Float Support

• Project Properties → C/C++ Build → Settings → check printf and scanf float support

Step 5 — Generate Code

• Project name → STM32CubeIDE → Generate Code → Open Project

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🛠️ Debugger / SWV Setup

Debug icon → Edit Debug Configurations
    │
    ├── Debugger tab:
    │     └── ST-LINK → click SCAN → board S/N auto-detected
    │
    └── Serial Wire Viewer (SWV):
          ├── ☑ Enable
          ├── Core Clock: 84.0 MHz   ← must match your clock config!
          └── Apply → Close

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💻 Code — main.c

Include

/* USER CODE BEGIN Includes */
#include <stdio.h>
/* USER CODE END Includes */

Retarget printf → ITM Port 0

/* USER CODE BEGIN 0 */
int _write(int file, char *ptr, int len) {
    for (int i = 0; i < len; i++) {
        ITM_SendChar(*ptr++);
    }
    return len;
}
/* USER CODE END 0 */

ITM_SendChar() sends one character at a time through the Instrumentation Trace Macrocell — a built-in Cortex-M4 debug channel that requires zero UART pins.

Task Function

/* USER CODE END Header_Task1_function */
void Task1_function(void *argument) {
    /* USER CODE BEGIN 5 */
    for (;;) {
        HAL_GPIO_TogglePin(LED_GPIO_Port, LED_Pin);
        printf("Task_1 Executing for LED Toggle \n");
        osDelay(500);   // Blocks task for 500ms; CPU is free
    }
    /* USER CODE END 5 */
}

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🖥️ Launching the SWV ITM Console

Step 1:  Window → Show View → Other → SWV → SWV ITM Data Console
Step 2:  Click Debug button → Switch to Debug perspective
Step 3:  In SWV ITM Data Console:
           ☑ Enable PC Sampling
           ☑ Port 0  ← this is where ITM_SendChar(0) outputs
Step 4:  Click ● Start Trace (red dot)
Step 5:  Click ▶ Resume

Expected output on Port 0:
   Task_1 Executing for LED Toggle
   Task_1 Executing for LED Toggle
   Task_1 Executing for LED Toggle
   ...  (every 500 ms)

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🆚 FreeRTOS osDelay vs. Super Loop HAL_Delay

Feature	HAL_Delay(500)	osDelay(500)
CPU during delay	Spinning (busy)	Released to scheduler
Other tasks can run	❌ No	✅ Yes
Timing accuracy	Depends on SysTick	Managed by RTOS kernel
Use in ISR	Allowed	❌ Not allowed

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✅ Result

A single FreeRTOS task (Task_1) was created using the CMSIS-RTOS v2 interface. The LED toggled at a precise 500 ms interval driven by osDelay(), and real-time execution was verified non-intrusively via the SWV ITM Data Console on Port 0.