Adding a new system task

This howto describes how to create a new system task. A FreeRTOS task is similar to a thread on a standard operating system. It has its own function call stack, and can be preempted by the RTOS scheduler. Typical hazards and techniques of shared-memory multithreaded programming apply.

A new task is most often needed when adding a fundamentally new subsystem to the Crazyflie firmware.

If the new subsystem needs to do significant computation in response to inputs such as CRTP radio messages, it should receive those inputs on a queue and perform the computation within its own task instead of blocking the radio task. In this example, we will set up the skeleton for such a new subsystem.

Development environment

You should have the crazyflie-firmware and crazyflie-clients-python cloned in the same folder. If you are using the Bitcraze VM this is already the case.

The Crazyflie firmware should be on Master branch.

For the rest of the howto you will work in the crazyflie-firmware project.

Setting the task constants

Each task has a few constants that are stored globally in config/config.h.

First, set the task priority. The FreeRTOS scheduler always prefers to run a higher-priority task. High priorities should be reserved for very time-sensitive tasks like the main stabilizer loop.

In config.h, after the existing task priorities, add the line

#define EXAMPLE_TASK_PRI        1

Next, each task has a name. In config.h, after the existing task names, add the line


Finally, we must select a fixed size for the task’s function call stack. configMINIMAL_STACK_SIZE gives a fairly small stack. Tasks that do lots of computation will almost certainly need larger stacks.

In config.h, after the existing task stack sizes, add the line


Implementing the task

High level tasks (those that do not directly talk to hardware) usually go in the modules directory. We will walk through the sections needed in a new file modules/src/example.c.

First, include the necessary system header files:

#include "config.h"
#include "debug.h"
#include "FreeRTOS.h"
#include "queue.h"
#include "static_mem.h"
#include "task.h"

File-static variables

Next, we statically allocate the queue to hold inputs from other parts of the system such as the radio. Here we just hold integers in the queue. Often it will be a struct instead. A queue of length 1 is appropriate if new inputs supersede old ones. For example, if the input controls the current LED color, we do not care which other colors were requested in the past.

Note that the STATIC_MEM_*_ALLOC family of macros expand to static variable declarations, and can be used outside of any function scope.

static xQueueHandle inputQueue;
STATIC_MEM_QUEUE_ALLOC(inputQueue, 1, sizeof(int));

Next, we allocate the task call stack and other necessary memory for OS bookkeeping. The FreeRTOS task API requires that task functions take a void * argument.

static void exampleTask(void*);


Next, we implement the Init() function. This should contain all of the FreeRTOS allocations and initializations that need to last as long as the firmware is running. Note the use of the constants we defined in config.h. Later in this howto, we will need to edit another file to call the Init().

static bool isInit = false;

void exampleTaskInit() {
  inputQueue = STATIC_MEM_QUEUE_CREATE(inputQueue);
  // TODO
  isInit = true;

Next, we implement the Test() function. Usually this does not do much, besides verify that Init() has been called. Later in this howto, we will need to edit another file to call the Test().

bool exampleTaskTest() {
  return isInit;

Now we come to the main task function. It is usually an infinite loop that waits on inputs from something. In our case, we wait on inputs from the queue. Our xQueueReceive uses the special timeout value portMAX_DELAY, which means that the xQueueReceive call will (potentially) block forever and only return when the queue is not empty. Other tasks will use integer or zero timeout values if they have some work to do regardless of whether or not inputs are received.

static void exampleTask(void* parameters) {
  DEBUG_PRINT("Example task main function is running!");
  while (true) {
    int input;
    if (pdTRUE == xQueueReceive(inputQueue, &input, portMAX_DELAY)) {
      // Respond to input here!

Finally, we implement the public function that is called by other parts of the system to pass inputs to our new subsystem.

xQueueOverwrite specifies that, if the queue is full, we overwrite the existing contents instead of blocking on the queue’s consumer to remove an item from the other end of the queue. In subsystems where we do not want to throw away inputs from the queue, use the xQueueSend* family instead.

Subsystems should not expose their queues directly. It should always be wrapped in a function, like this.

void exampleTaskEnqueueInput(int value) {
  xQueueOverwrite(inputQueue, &value);

See the FreeRTOS docs for more details on the many xQueue API functions available.

Design Note: For complex subsystems, consider implementing the algorithmic core as a library that does not depend on FreeRTOS- and ARM-related headers, and calling this library from your task. This makes it easier to write unit tests for your algorithmic core that can be compiled, run, and debugged on a PC.

Writing the public interface

Subsystems should expose a public interface to other parts of the firmware that hide implementation details as much as is practical. We create the new file in modules/interface/example.h:

#ifndef __EXAMPLE_TASK_H__
#define __EXAMPLE_TASK_H__

#include <stdbool.h>

void exampleTaskInit();
bool exampleTaskTest();

void exampleTaskEnqueueInput(int value);

#endif // __EXAMPLE_TASK_H__

In a real task, make sure to comment the public API thoroughly.

Initializing the task

In modules/src/system.c, make the following changes.

Include the header for our new task:

#include "example.h"

In systemTask(), after the lines where the other Init() functions are called, add the line


In systemTask(), after the lines where the other pass &= ...Test() lines, add the line

pass &= exampleTaskTest();

Adding the task to the build

Add this to the Makefile, after the end of the Modules block:

PROJ_OBJ += example.o

Compile, flash and run!

Now the last step is to compile and flash your new firmware. Launch the following commands in a shell:

crazyflie-firmware$ make
crazyflie-firmware$ make cload

The output will be similar to the following:

crazyflie-firmware$ make
  CC    hello.o
Crazyflie 2.0 build!
Build 44:61f3d878233d (2015.08.1-44) MODIFIED
Version extracted from git
Crazyloader build!
   text    data     bss     dec     hex filename
 130660    1636   29828  162124   2794c cf2.elf
rm version.c
crazyflie-firmware$ make cload
../crazyflie-clients-python/bin/cfloader flash cf2.bin stm32-fw
Restart the Crazyflie you want to bootload in the next
 10 seconds ...
Connected to bootloader on Crazyflie 2.0 (version=0x10)
Target info: nrf51 (0xFE)
Flash pages: 232 | Page size: 1024 | Buffer pages: 1 | Start page: 88
144 KBytes of flash available for firmware image.
Target info: stm32 (0xFF)
Flash pages: 1024 | Page size: 1024 | Buffer pages: 10 | Start page: 16
1008 KBytes of flash available for firmware image.

Flashing 1 of 1 to stm32 (fw): 161867 bytes (159 pages) ..........10..........10..........10..........10..........10..........10..........10..........10..........10..........10..........10..........10..........10..........10..........10.........9
Reset in firmware mode ...

Now you can connect your Crazyflie with the client and see the “Example task main function is running!” in the debug console.