Trace hooks and FreeRTOS+Trace

Introduction

Trace hooks help you watch what your FreeRTOS program is doing step-by-step. FreeRTOS+Trace shows this information in a clear way so you can find and fix problems easily.

You want to see when tasks start and stop in your FreeRTOS program.

You need to find why your program is slow or stuck.

You want to check how tasks share the CPU over time.

You want to learn how FreeRTOS schedules tasks by watching real events.

Syntax

FreeRTOS

/* Enable trace hooks in FreeRTOSConfig.h */
#define configUSE_TRACE_FACILITY 1

/* Include FreeRTOS+Trace header in your main file */
#include "trcRecorder.h"

/* Start trace recording */
vTraceEnable(TRC_START);

/* Stop trace recording */
vTraceEnable(TRC_STOP);

Trace hooks are enabled by setting configUSE_TRACE_FACILITY to 1 in FreeRTOSConfig.h.

FreeRTOS+Trace uses special functions to start and stop trace recording. Use the FreeRTOS+Trace tool to retrieve data from the RAM buffer.

Examples

This line in FreeRTOSConfig.h turns on trace hooks so FreeRTOS records task events.

FreeRTOS

/* Enable trace hooks */
#define configUSE_TRACE_FACILITY 1

This example shows how to start trace recording.

FreeRTOS

#include "trcRecorder.h"

int main(void) {
    vTraceEnable(TRC_START);  // Start tracing
    // Your FreeRTOS code here
    return 0;
}

Sample Program

This program enables trace hooks, creates a simple task, starts trace recording, and starts the scheduler. Use the FreeRTOS+Trace tool to read the trace buffer from RAM.

FreeRTOS

/* FreeRTOSConfig.h snippet */
#define configUSE_TRACE_FACILITY 1

/* main.c */
#include "FreeRTOS.h"
#include "task.h"
#include "trcRecorder.h"

void Task1(void *pvParameters) {
    for (;;) {
        // Simulate work
        vTaskDelay(pdMS_TO_TICKS(100));
    }
}

int main(void) {
    xTaskCreate(Task1, "Task1", 1000, NULL, 1, NULL);

    vTraceEnable(TRC_START);  // Start trace

    vTaskStartScheduler();    // Start FreeRTOS scheduler

    // Normally never reached
    return 0;
}

Important Notes

Trace hooks add a little overhead but help you see what tasks do in real time.

FreeRTOS+Trace shows a timeline of tasks switching, delays, and interrupts.

Always disable trace hooks in production to keep your program fast.

Summary

Trace hooks let you watch FreeRTOS tasks and events as they happen.

FreeRTOS+Trace helps you see this information clearly to debug and learn.

Enable trace hooks in config and use vTraceEnable to record data. Retrieve with the tool.

Practice

1. What is the main purpose of trace hooks in FreeRTOS?

easy

A. To reduce memory usage of the FreeRTOS kernel

B. To increase the speed of task switching

C. To monitor tasks and events as they happen in the system

D. To automatically fix bugs in the code

Solution

1. Step 1: Understand what trace hooks do

   Trace hooks are special points in FreeRTOS code that let you watch tasks and events when they occur.

2. Step 2: Identify the main purpose

   The main use is to monitor and record system behavior for debugging or learning.

3. Final Answer:

   To monitor tasks and events as they happen in the system -> Option C

4. Quick Check:

   Trace hooks = monitor tasks/events [OK]

Hint: Trace hooks watch events live in FreeRTOS [OK]

Common Mistakes:

• Thinking trace hooks speed up tasks
• Confusing trace hooks with memory optimization
• Believing trace hooks fix bugs automatically

2. Which configuration option must be enabled to use trace hooks in FreeRTOS?

easy

A. configTRACE_TASK_EVENTS

B. configENABLE_TASK_SWITCH

C. configUSE_FREERTOS_TRACE

D. configUSE_TRACE_HOOKS

Solution

1. Step 1: Identify the config option for trace hooks

   The FreeRTOS configuration option to enable trace hooks is named configUSE_TRACE_HOOKS.

2. Step 2: Confirm other options are incorrect

   Other options like configENABLE_TASK_SWITCH or configUSE_FREERTOS_TRACE do not exist or are unrelated.

3. Final Answer:

   configUSE_TRACE_HOOKS -> Option D

4. Quick Check:

   Enable trace hooks = configUSE_TRACE_HOOKS [OK]

Hint: Look for configUSE_TRACE_HOOKS to enable hooks [OK]

Common Mistakes:

• Choosing options that sound similar but don't exist
• Confusing trace hooks with task switching configs
• Assuming FreeRTOS+Trace needs a different config

3. Given this snippet enabling trace hooks and starting FreeRTOS+Trace recording:

vTraceEnable(TRC_START);
vTaskStartScheduler();

What is the expected result?

medium

A. Trace recording starts and scheduler runs tasks

B. Scheduler starts but trace recording is disabled

C. Trace recording starts but scheduler does not run

D. Compilation error due to wrong function call

Solution

1. Step 1: Understand vTraceEnable(TRC_START)

   This function call starts the FreeRTOS+Trace recording of events.

2. Step 2: Understand vTaskStartScheduler()

   This function starts the FreeRTOS scheduler to run tasks.

3. Final Answer:

   Trace recording starts and scheduler runs tasks -> Option A

4. Quick Check:

   vTraceEnable + vTaskStartScheduler = recording + running [OK]

Hint: vTraceEnable(TRC_START) starts recording before scheduler [OK]

Common Mistakes:

• Thinking scheduler won't run after vTraceEnable
• Assuming trace recording is off by default
• Confusing function names causing compile errors

4. You added trace hooks but see no data in FreeRTOS+Trace viewer. What is a likely cause?

medium

A. configUSE_TRACE_HOOKS is not set to 1 in FreeRTOSConfig.h

B. vTraceEnable(TRC_START) was called twice

C. FreeRTOS+Trace viewer is not installed

D. Scheduler was started before enabling trace hooks

Solution

1. Step 1: Check configUSE_TRACE_HOOKS setting

   If this is not set to 1, trace hooks do not run, so no data is recorded.

2. Step 2: Verify other options

   Calling vTraceEnable twice or starting scheduler early usually does not prevent data. Viewer missing is unrelated to data generation.

3. Final Answer:

   configUSE_TRACE_HOOKS is not set to 1 in FreeRTOSConfig.h -> Option A

4. Quick Check:

   Trace hooks disabled = no data [OK]

Hint: Always enable configUSE_TRACE_HOOKS = 1 [OK]

Common Mistakes:

• Forgetting to enable trace hooks in config
• Blaming viewer instead of config settings
• Assuming multiple vTraceEnable calls cause issues

5. You want to record only task switch events using FreeRTOS+Trace. Which approach is best?

hard

A. Use configUSE_TRACE_HOOKS = 0 and enable task switch trace only

B. Enable trace hooks and filter events in FreeRTOS+Trace viewer

C. Disable trace hooks and use printf in task switch hook

D. Modify FreeRTOS kernel to record only task switches

Solution

1. Step 1: Understand trace hooks and filtering

   Trace hooks record many events; filtering in the viewer lets you see only task switches easily.

2. Step 2: Evaluate other options

   Modifying kernel is complex and error-prone. Using printf is inefficient. Disabling trace hooks stops all tracing.

3. Final Answer:

   Enable trace hooks and filter events in FreeRTOS+Trace viewer -> Option B

4. Quick Check:

   Filter in viewer = easiest selective trace [OK]

Hint: Filter events in viewer instead of changing kernel [OK]

Common Mistakes:

• Trying to modify kernel for selective tracing
• Disabling trace hooks disables all tracing
• Using printf slows system and is not trace hook