Lab Assignment: Watchdogs


  • Learn File I/O API to read and write data to the SD card
    • This requires micro SD card that is formatted with FAT32
  • Design a simple application that communicates over the RTOS queue
  • Learn about a Watchdog

Prerequisite Knowledge

You will be using a "file system" API to read (or write) a file. This is a third party library and is not part of standard C library, and it is connected to the SD card using the SPI bus.

Please read this page for API details.

Part 0: Setup Producer and Consumer Task

  1. Create a producer task that reads a sensor value every 1ms.
    • The sensor can be any input type, such as a light sensor, or an acceleration sensor
    • After collecting 100 samples (after 100ms), compute the average
    • Write average value every 100ms (avg. of 100 samples) to the sensor queue
    • Use medium priority for this task
  2. Create a consumer task that pulls the data off the sensor queue
    • Use infinite timeout value during queue receive API
    • Open a file (i.e.: sensor.txt), and append the data to an output file on the SD card
    • Save the data in this format: printf("%i, %i\n", time, light)"
      • Note that you can get the time using xTaskGetTickCount()
      • The sensor type is your choice (such as light or acceleration)
    • Note that if you write and close a file every 100ms, it may be very inefficient, so try to come up with a better method such that the file is only written once a second or so...
      • Also note that periodically you may have to "flush" the file (or close it) otherwise data on the SD card may be cached and the file may not get written
    • Use medium priority for this task
#include "ff.h"
#include <string.h>

// Sample code to write a file to the SD Card
void write_file_using_fatfs_pi(void) {
  const char *filename = "file.txt";
  FIL file; // File handle
  UINT bytes_written = 0;
  FRESULT result = f_open(&file, filename, (FA_WRITE | FA_CREATE_ALWAYS));

  if (FR_OK == result) {
    char string[64];
    sprintf(string, "Value,%i\n", 123);
    if (FR_OK == f_write(&file, string, strlen(string), &bytes_written)) {
    } else {
      printf("ERROR: Failed to write data to file\n");
  } else {
    printf("ERROR: Failed to open: %s\n", filename);

Part 1: Use FreeRTOS EventGroup API

  1. At the end of the loop of each task, set a bit using FreeRTOS event group API.
    • At the end of each loop of the tasks, set a bit using the xEventGroupSetBits()
    • producer task should set bit1, consumer task should set bit2 etc.
    • You are expected to read about the FreeRTOS Event Group API yourself
  2. Create a watchdog task that monitors the operation of the two tasks.
    • Use high priority for this task.
    • Use a task delay of 1 second, and wait for all the bits to set. If there are two tasks, wait for bit1, and bit2 etc.
    • If you fail to detect the bits are set, that means that the other tasks did not reach the end of the loop.
      • Print a message when the Watchdog task is able to verify the check-in of other tasks
      • Print an error message clearly indicating which task failed to check-in with the RTOS Event Groups API

void producer_task(void *params) {
  while(1) {
    // ...
    xEventGroupSetBits(...); // TODO

void consumer_task(void *params) {
  while(1) {
    // ...
    xEventGroupSetBits(...); // TODO

void watchdog_task(void *params) {
  while(1) {
    // ...
    if (xEventGroupWaitBits(...)) { // TODO
      // TODO

Part 2: Thoroughly test the Application

  1. Create a CLI to "suspend" and "resume" a task by name.
    • "task suspend task1" should suspend a task named "task1"
    • "task resume task2" should suspend a task named "task2"
  2. Run the system, and under normal operation, you will see a file being saved with sensor data values.
    • Collect the data over several seconds, and then verify by inserting the micro-SD card to your computer
    • Plot the file data in Excel to demonstrate.
  3. Suspend the producer task
    • The watchdog task should display a message and save relevant info to the SD card.
  4. Observe the CPU utilization while your file is being saved
    • You should observe that the SD card task should utilize more CPU

What you created is a "software watchdog". This means that in an event when a task is stuck, or a task is frozen, you can save relevant information such that you can debug at a later time.

You may use any built in libraries for this lab assignment such as a sensor API

What to turn in

  • Positive test case scenario with serial terminal indicating tasks are running normally
  • Suspension of a task, and then negative test case scenario with serial terminal indicating which task failed to check-in
  • All relevant source code (compiled and tested)
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