STM32 Nucleo-F429ZI MIP FreeRTOS

Overview

This tutorial shows how to use Mongoose Library over FreeRTOS, using MIP (a bare-metal embedded TCP/IP stack designed specifically for Mongoose), and running on an STM32 Nucleo-F429ZI development board.

We implement a very basic web server that responds "hi" to any URL and provides a minimal example of a REST API to change the debug level. The full source code for this project is at https://github.com/cesanta/mongoose/tree/master/examples/stm32/nucleo-f429zi-freertos-mip

This example is a plain gcc make-based project. The relevant files are:

  • cmsis/: provides all necessary MCU support, this project is CMSIS-based
  • FreeRTOS-Kernel/: provides the RTOS functionality and memory management functions; see FreeRTOS integration below
  • mcu.h: provides a minimal HAL to perform the most common activities in a uniform way
  • startup.c: provides the MCU low-level initialization, vector table and default exception handlers
  • syscalls.c: provides an abstraction layer for those system functions Mongoose expects
  • main.c: here we do our main job, initializing the MCU, the FreeRTOS scheduler, Mongoose, the network, and calling the event manager
  • Makefile: a standard make file that performs the compilation and linking process

Below is a general process outline:

  • The board IP addressing will be provided by a DHCP server. If you want to set a static configuration, set IP address, network mask and gateway in main.c; see below
  • Build the example (see below) and run it on a development board
  • The firmware initializes the network
  • After initialization, the application starts Mongoose's event loop and blinks a blue LED
  • Once the blue LED starts blinking, the example is ready
  • Open your web browser and navigate to the board IP address, you should see a greeting

Build and run

  • It is assumed you're using Linux or Mac as a workstation, you have Docker installed, and your user is able to run it. If in doubt, check

    $ docker ps
    
  • We will also use the stlink flash utility

  • Start a terminal in the project directory; clone the Mongoose Library repo, and run the make build command:

    $ git clone https://github.com/cesanta/mongoose
    $ cd mongoose/examples/stm32/nucleo-f429zi-freertos-mip
    $ make build
    
  • In order to flash this recently built firmware to your board, plug it in a USB port and execute:

    $ make flash
    

    As long as there is only one board plugged in, stlink will find it; though we need to know the serial port device to be able to get the log information. It is probably /dev/ttyACM0

  • When the firmware is flashed, the board should signal its state by blinking the blue LED. We now need to know the IP address of the board to connect to it. If we used DHCP, as it is the default, we can check our DHCP server logs or see the device logs. Let's do this.

  • To connect to the board, in this example we'll be using picocom; we configure it for 115200bps and to add a carriage return. Use the proper serial device.

    $ picocom /dev/ttyACM0 -i -b 115200 --imap=lfcrlf
    picocom v2.2
    ...
    Terminal ready
    0 2 main.c:48:server                    Initializing Ethernet driver
    6 2 main.c:59:server                    Starting Mongoose v7.8
    c 3 mongoose.c:3496:mg_listen           1 0x0 http://0.0.0.0
    2ef 2 main.c:72:blinker                 blink :),  RAM: 71080
    ...
    11e0 2 mongoose.c:6815:onstatechange    READY, IP: 192.168.69.232
    
  • Now start a browser on http://IP_ADDRESS, where IP_ADDRESS is the board's IP address printed on the serial console. You should see a greeting.

How it works

This example can be divided in the following blocks:

  1. FreeRTOS integration for this microcontroller
  2. Initialize the microcontroller for this particular board
  3. Start the FreeRTOS scheduler to run the desired tasks
  4. Initialize the Ethernet controller
  5. Initialize Mongoose
  6. Initialize the networking stack
  7. Run Mongoose

FreeRTOS integration

Mongoose supports a number of well-known architectures, among them FreeRTOS. To tell Mongoose in which architecture it is running, we need to define the macro MG_ARCH; and when this symbol is not defined and there is no other clue, Mongoose will default to try to include mongoose_custom.h. In that file, we add the proper definition:

Network operations need a time base to calculate timeouts; this will be provided by FreeRTOS and Mongoose now knows how to link to it. We'll configure it at a 1000 Hz rate, to provide a 1ms time base.

In the Makefile, we clone the FreeRTOS-Kernel repository at a stable branch; then we set the proper paths for include files and required code:

  • FreeRTOS-Kernel/include/: headers with FreeRTOS-Kernel API definitions
  • FreeRTOS-Kernel/portable/GCC/ARM_CM4F: headers specific for the GCC compiler and the ARM Cortex-M4F core
  • FreeRTOS-Kernel/*.c: the generic kernel code
  • FreeRTOS-Kernel/portable/GCC/ARM_CM4F/port.c: the specific kernel code for the GCC compiler and the ARM Cortex-M4F core
  • FreeRTOS-Kernel/portable/MemMang/heap_4.c: the memory management strategy we chose
  • FreeRTOSConfig.h: here we do our main integration job, defining the required exception handlers, preferences, and stack and heap sizes. The number of priority bits in the NVIC for this particular MCU is provided by the CMSIS layer

MCU and board initialization

This example uses CMSIS (though the MIP stack itself does not). To keep things easier we provided a thin HAL layer in the mcu.h file; while the startup.c file handles the reset vector and calls our main function. There, we call these HAL functions to initialize the MCU and those peripherals we are going to use:

Create tasks and start scheduler

We create the necessary tasks and call the function that starts the scheduler. Then, FreeRTOS takes over and we have a server task, where we initialize and run Mongoose with MIP, and a blinker task to blink an LED

Note we provide ample stack space for the Mongoose task. It doesn't actually need to be that big for such a simple example, but more complex interfaces will need plenty of room.

Ethernet controller initialization

The Ethernet controller initialization follows. We need to enable the MAC GPIO pins to connect to the dev board PHY using RMII, configure the clocks, and reset the controller:

Mongoose initialization

Then we initialize Mongoose, this is no different from what we always do in any example; we are a task in FreeRTOS and we can run an infinite loop.

MIP initialization

MIP has to be enabled to be compiled in, and so Mongoose will work in association with it. This is done in the Makefile by defining MG_ENABLE_MIP=1.

Then this networking stack has to be configured and initialized. This is done by calling mip_init() and passing it a pointer to a struct mip_if. Inside this structure:

  • have pointers to a struct mip_driver and any extra data that it could need
  • For DHCP: set ip as zero
  • For a static configuration, specify ip, mask, and gw in network byte order

In this example, we use DHCP:

Note that, we also need to specify a unique MAC address. For this example we chose a fancy unicast locally administered address; for production runs you'll have to consider among several options, from adding a MAC address chip in your hardware design to registering with the IEEE Registration Authority.

Some drivers, as you have probably noticed, require extra data. In this case the STM32 driver can accept the setting for the divider that generates the MDIO clock. You can pass a null pointer in the driver data or a negative value for this parameter and the driver will calculate it for you, based on the clock configuration.

MIP will run as part of Mongoose, it is a part of it. The Ethernet controller will interrupt when it has received a frame and the IRQ handler will push it to a queue, from where MIP will pick it up later; no multi-threading issues to worry about, just follow Mongoose documentation as usual and call all mg_* API functions from the same FreeRTOS task where Mongoose is running.

Run Mongoose

Then we run Mongoose. This is no different from what we always do in any example, though note that it should be run after network initialization. The logic is standard: initialize the event manager (as we already did), start a listener on port 80, and fall into an infinite event loop:

The listener will call this function when it receives an HTTP request:

There we check the requested URL and either parse a JSON message or just reply "hi"

Blinker task

There is a simple FreeRTOS task that initializes the GPIO and loops to blink the blue LED

Custom application

In order to create your own Mongoose-enabled application you have several ways:

  1. The obvious way, is to add the required functionality to this example. The example includes an important set of drivers. As we've seen in previous sections, there are functions to read and write to GPIOs.

  2. If, for some reason, you can't use this example as a base (e.g.: you have your own big project to which you need to add Mongoose, or you'd rather use your preferred IDE), you can do the following:

    • Add relevant project files to your project
    • Add mongoose.c and .h files to your project
    • Add Mongoose-specific configuration flags, see the Makefile
    • Add the required preprocessor symbols:
      • MG_ARCH=MG_ARCH_FREERTOS, MG_ENABLE_MIP=1
    • Since your environment will surely be initializing the MCU, make sure the AHB clock frequency is at least 25 MHz; then follow the example server() task and the Ethernet controller initialization
    • Now write code similar to that in main.c; for that you can read Mongoose documentation and follow our examples and tutorials