Running FreeRTOS on the Keil MCBSTM32 Board with the RVMDK Evaluation Tools

Application Note 210

Release information

Change history




June 2008


First release

Proprietary notice

Words and logos marked with Ó and Ô are registered trademarks owned by ARM Limited, except as otherwise stated below in this proprietary notice. Other brands and names mentioned herein may be the trademarks of their respective owners.

Neither the whole nor any part of the information contained in, or the product described in, this document may be adapted or reproduced in any material form except with the prior written permission of the copyright holder.

The product described in this document is subject to continuous developments and improvements. All particulars of the product and its use contained in this document are given by ARM in good faith. However, all warranties implied or expressed, including but not limited to implied warranties of merchantability, or fitness for purpose, are excluded.

This document is intended only to assist the reader in the use of the product. ARM Limited shall not be liable for any loss or damage arising from the use of any information in this document, or any error or omission in such information, or any incorrect use of the product.

Confidentiality status

This document is Open Access. This document has no restriction on distribution.

Feedback on this Application Note

If you have any comments on this Application Note, please send email to giving:

·                      the document title

·                      the document number

·                      the page number(s) to which your comments refer

·                      an explanation of your comments.

General suggestions for additions and improvements are also welcome.

ARM web address

1        Introduction

This application note describes how to modify an existing port demo of the FreeRTOS operating system that targets the Luminary Micro LM3S102 evaluation board. It uses Keil’s (an ARM company) RealView Microcontroller Development Kit (RVMDK) evaluation tools.  The modifications will be made so that the operating system can run on the Keil MCBSTM32 board using the RVMDK evaluation software.  The MCBSTM32 is Keil’s first board based on the Cortex-M3 processor core.

FreeRTOS was the first real-time kernel to be available for production Cortex-M3 based microcontrollers.

Note: Although this document does not describe the complete details and functionality of the FreeRTOS kernel, they are not required to complete the modifications.  Information on the operating system should be obtained from

In addition to being targeted for the evaluation version of RVMDK, the original port is also targeted at ARM’s RealView Development Suite (RVDS) because both environments use the same compilation tools, (the compiler, linker, and assembler)..  The main difference between the two sets of tools is the debugger and code editor.  For the modified demo in this note, RVMDK will be the tool set used.

The modifications made to the OS port demonstration in this application note can also be used as general guidelines on how to modify other OS port demonstrations so that they can run on a different development platform, with the same or similar processor.

This application note contains the following sections:

·                     Obtaining the Necessary Materials describes the required materials and where to get them.  This includes the Keil MCBSTM32 evaluation board (which includes an evaluation version of RVMDK, the ULINK-ME JTAG interface, and other necessary cables), serial cable(s) for testing purposes, and the FreeRTOS source and LM3S102 demonstration code.

·                     Connecting and Configuring the Hardware and Software explains how to connect and configure the necessary materials.

·                     Modifying the LMS3102 FreeRTOS Port Demo to Run on the MCBSTM32 describes modifications to the existing LM3S102 FreeRTOS port demo that enables it to run on the MCBSTM32.

·                     New Task/Co-routine Creation and Exercises provides some ideas for exercises that can be added on to the modified port demonstration.

2        Obtaining the Necessary Materials

This section describes the necessary materials for modifying and running the FreeRTOS demo on the Keil MCBSTM32 evaluation board.

2.1      PC and Keil MCBSTM32 Evaluation Kit

The Keil MCBSTM32 is based on the STMicroelectronics Cortex-M3 family of ARM devices, and enables you to create and test working programs for this advanced architecture.  Specifically, the board includes the STM32F103RB microcontroller, one serial interface, one analog input (via potentiometer), an SD Card interface, a CAN interface, an LCD, a USB interface, and eight LEDs.

RVMDK supports out-of-box ARM7, ARM9, and Cortex-M3 technology-based microcontrollers from many different vendors, and includes a code editor, compilation/link tools, a debugger, and other utilities.

The MCBSTM32 evaluation kit includes the MCBSTM32 evaluation board, a ULINK-ME USB-JTAG interface, and an evaluation version of RVMDK.  The kit can be ordered directly from Keil at academic pricing, or through a local Keil distributor. 

Note: The evaluation version of RVMDK will enable the building and debugging of the FreeRTOS port demonstrations referenced in this application note, and will be used throughout.  See the section that follows for more information.

2.2      FreeRTOS Source Code and LM3S102 Keil/RVDS Demo Application

FreeRTOS is a royalty-free, open-source, real-time operating system kernel.  There are many ports and demos available for various processor architectures and development tools from

Depending on the architecture and chip being used, it might be easier to modify an existing port demonstration based on the processor/board itself rather than the software tools.  This depends on what ports are already available and if the port layer itself must be changed, as opposed to just changing a demo application.  For example, although STM32 ports/demos already exist from, they are not targeted at RVMDK,  These existing port/demo project files were built specifically for other toolsets, and might have differing assembler/compiler syntax.  All of the existing port/demos can be found in the Port/Demo list at

There is an STM32 port for the IAR tools, but the FreeRTOS port layer for RVMDK is required (and happens to be available in another port),   However, the demonstration files can be compiled and assembled with RVMDK as is, although a new Keil project file must be created with all of these files. Fortunately, the Cortex-M3 port layer of FreeRTOS is the same over different chip implementations such as an ST part vs. a Luminary part.  For these reasons, you might think it would be easiest to create a new RVMDK project with the RVMDK FreeRTOS port layer and the demo files from the STM32 IAR port demo.  That’s probably correct, assuming that you are using a full version (not evaluation version) of RVMDK.  The problem is the way memory is allocated by the STM32 IAR FreeRTOS port.  This can cause the code size to go over the 16K limitation of the evaluation version of RVMDK.  For more information, see User Documentation à Configuration à Memory Management from the Menu at

However, there are other Cortex-M3-based ports and demos targeted at RVMDK that can be used as a work around.  These can be found in the Port/Demo list under the Menu.  The LM3S102 Keil/RVDS port’s kernel allocates heap memory a bit differently. Instead of declaring a fixed-sized array of memory, a pointer is set at the end of RAM and accesses are made from offsets of the pointer as if it was an array.  This is in fact more consistent with traditional heap allocation, and the kernel can allocate RAM up to the point where the heap clashes with the compiler allocated memory.  With this method, the heap does not show up as a large block of RAM and is not included in the actual code size. Because of this, the base port used in this note will be the LM3S102 Keil/RVDS port.

Instructions on how to download and install the FreeRTOS source code and demo applications can be found on the website as well.

Note: All of the source and demos are downloaded together in one package, and it is not necessary to download only specific files.

2.3      RS232 DB9 Loopback Connector

Both the existing and modified demos include an interrupt-driven USART test where a co-routine transmits characters that are then received by a task, so a loopback serial connector will be needed for testing purposes.  A loopback connector can be made from a standard male-to-female serial cable by simply shorting pins 2 and 3 on the female side with a paperclip. The male connector is for the MCBSTM32 USART port.  More information about this and how to test a homemade loopback connector can be found at

If the USART transmission portion must be verified, for example, if you want to actually see the characters being transmitted on the host PC, and the host PC does not have a serial port, a converter will be needed.  Parallel-to-serial and USB-to-serial converters can be built or purchased

3        Connecting and Configuring the Hardware and Software

This section describes how to connect the MCBSTM32 and JTAG interface to a host PC running RVMDK.  It also describes how to open the FreeRTOS base demo application for editing in RVMDK.

3.1      Setting up the Hardware

Use the following steps to setup your hardware:

1.       Connect the ULINK-ME directly to the 20- pin JTAG connector on the MCBSTM32. Power the ULINK-ME by connecting the appropriate end of the included USB cable into it, and the other end into an open USB port on the host PC.


Figure 1.     Hardware connections between ULINK-ME and MCBSTM32

If you are using a ULINK2 interface, it powers and connects the same way.  If you are using a different JTAG interface, see the appropriate documentation for information on how to connect and power it.

2.       Do the same with the included USB cable to power the MCBSTM32.  When you power the board, the code that is already in flash memory (perhaps the included demo application) should run.

3.       Fit the male side of one of the above described RS232 DB9 cables into the serial port of the MCBSTM32 labelled COM.

3.2      Starting RVMDK and Opening the Demo Project

Use the following steps to start RVMDK and open the project:

1.       If RVMDK is not already installed on the host PC, insert the RVDMK evaluation CD into the CD-ROM drive of the PC and follow the on-screen instructions to complete the installation.  If you do not have the CD, you can download the tools from

2.       The editor portion of RVMDK is called the Keil uVision3 (this is also the same name as the debugger).  Launch uVision3 from its installed location or shortcut on the PC.  RVMDK will startup initially in the uVision3 IDE.  This is the editor and project management portion of the tools.

3.       To open the LM3S102 demo uVision3 project file, click Project à Open Project from the uVision3 IDE menu options, and open the project file located at \…\FreeRTOS\Demo\CORTEX_LM3S102_KEIL\FreeRTOS.uV2.  At this point, it is possible to compile, debug, and run this port and demo via the simulator.  To do this, click on the Options for Target icon Image, click on the Debug tab, and ensure that Use Simulator is selected.


Figure 2.     Options for Target Debug dialogue with “Use Simulator” selected

For information on how to compile, debug, and run applications with RVMDK, see the RealView Compilation Tools for uVision documentation, and the uVision User Guide.

For details about FreeRTOS source code and specific demo applications, see

4        Modifying the LM3S102 FreeRTOS Port Demo to Run on the MCBSTM32

This section describes the modifications required to run the existing LM3S102 port demo on the MCBSTM32.  Although these modifications are specific to these two platforms, this section can also be used as general guidelines on how to modify other existing OS ports to run on different hardware platforms based on the same core architecture.

The LM3S102 demo application includes an interrupt-driven UART test where a co-routine transmits characters that are then received by a task via a loopback serial connector.  LEDs on-board are used to indicate status, and some are controlled by a co-routine that does nothing but flash some LEDs.  The LCD display is also used to display a message.  The demo for the MCBSTM32 will have the same functionality, but of course, this can be extended.  For more information, see the LM3S102 Keil/RVDS port demo application explanation at

4.1      Changing Target Processor Information

The first steps of the modification process include renaming some of the project folders, in addition to changing some of the target and project settings within uVision3.

4.1.1       Renaming the Project Folder Name

To avoid confusion, either change the name of the \…\FreeRTOS\Demo\CORTEX_LM3S102_KEIL folder, or create a new folder at that level and copy everything into it.  For this application note, the CORTEX_LMS3102_KEIL folder was simply renamed to CORTEX_STM32F103RB_KEIL because the LMS3102 port demo was no longer needed.

Note: The project must be closed in order to change the directory name.

4.1.2       Changing the Target Processor

The next step is to change the target processor information because it is currently set to LMS3S1xx.  Re-open the project from the new folder, and click Components, Environment, and Books button Image.  Under Project Targets, highlight LM3S1xx and delete it by clicking the delete icon Image.  Click the insert icon Image and input the new target name STM32F103RB.  Click OK. Now the STM32F103RB target should be the only one in the list of targets.

Up to this point, nothing has actually changed other than the names of the folder and target.  To actually change the target processor in the uVision3 IDE, click on the Options for Target icon Image, click the Device tab, and change the selected device from the Luminary Micro LM3S101 to the STMicroelectronics STM32F103RB.  After the choice is highlighted, the specs and peripherals for this microcontroller are listed.


Figure 3.     STM32F103RB Options for Target Device selection

Click OK to select the device and exit the Options for Target.

4.1.3       Modifying the Compiler Include Paths

The next step is to modify a few of the current compiler include paths to have the relevant library functions. To do this, click on the C/C++ tab under Options for Target, then click on the Include Paths button Image, and change the \…\Keil\ARM\RV31\LIB\Luminary path to \…\Keil\ARM\RV31\LIB\ST, and change the \…\CORTEX_LM3S102_KEIL path to \…\CORTEX_STM32F103RB_KEIL.

4.1.4       Changing the Preprocessor Symbol

The current LM3S102 preprocessor symbol must be changed.  To do this, click on the C/C++ tab under Options for Target, and change the name of the defined preprocessor symbol RVDS_ARMCM3_LM3S102 to RVDS_ARMCM3_STM32F103RB.  The preprocessor symbols are defined in a file that is part of the source directory called portable.h. This determines the relevant macro file that will be used for a particular port.  The macro file used for all Cortex-M3 ports is the same, so simply change the line that reads:

#ifdef RVDS_ARMCM3_LM3S102


#ifdef RVDS_ARMCM3_STM32F103RB

in portable.h.  This step is mostly to keep naming consistency.

Note: RVDS is a completely different set of tools from RVMDK; however, they use the exact same compilation tools. Therefore, many similarities exist in files used by both RVDS and RVMDK for FreeRTOS ports and demos.

4.2      Adding and Removing Existing Target-specific Files

The next step of the modification process is to add or modify the necessary library, initialization, and startup files for the new target processor, and remove any unnecessary files.

4.2.1       Library Files

At this point, a build of the project using the build button Image should display two major errors in the code.  There are two references to the LM3S102 library file LM3Sxxx.h in main.c and pdc.cLM3Sxxx.h was part of the library compiler include path that we changed in the last section, so change the line #include "LM3Sxxx.h" to #include "stm32f10x_lib.h".

To reflect the new library file that defines peripheral register addresses, register bit definitions, and peripheral function prototypes, we also must add a firmware library file. This file defines many useful subroutines for writing software on the STM32F103RB.  Keil provides this file that is configured for RVMDK, and  can be found in \...\Keil\ARM\RV31\LIB\ST\.  Copy and paste the file STM32F10xR.LIB into \…\FreeRTOS\Demo\CORTEX_STM32F103RB_KEIL\include\

For simplicity, we are also adding an even higher level of abstraction by using MCBSTM32-specific library files for the USART and LCD interfaces.  Keil provided these files, and you can find them in most of the MCBSTM32 project folders, such as \…\Keil\ARM\Boards\Keil\MCBSTM32\Blinky\.  Copy and paste the files serial.c, LCD_4bit.c, and LCD.h from an MCBSTM32 project folder into \…\FreeRTOS\Demo\CORTEX_STM32F103RB_KEIL\include\.  We will use routines from these files later on.

4.2.2       Initialization Files

The file pdc.c is a peripheral control file specific to the LM3S102, so remove that file from the project by right-clicking on the file from the Project Workspace tree in the uVision3 IDE and selecting Remove File ‘pdc.c.  Because this file and pdc.h are no longer needed, delete these two files from \…\FreeRTOS\Demo\CORTEX_STM32F103RB_KEIL\include\.  New initialization source and header files for the MCBSTM32 are to be included in the project to initialize its own peripherals.  Again, Keil provided these files, and you can find them in most of the MCBSTM32 project folders, such as \…\Keil\ARM\Boards\Keil\MCBSTM32\Blinky\.  Copy and paste the files STM32_Init.c, STM32_Init.h, and STM32_Reg from an MCBSTM32 project folder into \…\FreeRTOS\Demo\CORTEX_STM32F103RB_KEIL\include\, and replace the line #include "pdc.h" with #include "STM32_Init.c" in main.c.  Also, replace #include "pdc.h" with #include "STM32_Init.h" in partest.c.

4.2.3       Startup Files and Exception Vector Table

The last major file swap involves the startup files that are responsible for some specific chip and peripheral initialization, stack and heap setup, core exception modes and registers setup, in addition to initializing the exception vector table.  Using the same steps as above, remove, delete, and replace \…\FreeRTOS\Demo\CORTEX_STM32F103RB_KEIL\init\Startup.s with \…\Keil\ARM\Startup\ST\STM32F10x.s in \…\FreeRTOS\Demo\CORTEX_STM32F103RB_KEIL\init\.  Because the startup code is the first code that the processor executes, it calls the main() function, and is never referenced by other files. STM32F10x.s must be manually added to the Demo folder in the Project Workspace by right-clicking the folder and selecting Add Files to Group ‘Demo’.

The current vector table in STM32F10x.s only implements dummy handlers. These files are for the programmer to modify (depending on the system).  The FreeRTOS source and demo application make use of four different exception types and provide the exception handlers.  These handlers are vPortSVCHandler, xPortPendSVHandler, xPortSysTickHandler, and vUART_ISR.  The dummy handler calls in STM32F10x.s must be replaced with calls to these FreeRTOS handlers. 


Figure 4.     STM32F10x.s vector table

The dummy handlers below the vector can now be removed or commented out.

Again, the code for these handlers is written in other parts of the application.  Most notably, the FreeRTOS scheduler is started in the xPortPendSVHandler handler, and all context switches are performed in this handler as well.  Tasks that are blocked (waiting) are awakened by the tick interrupt.  On the Cortex-M3, the tick interrupt is generated by the core SysTick timer (which is actually part of the Cortex-M3 core).

You can find more information on these particular handlers at

4.3      Modifying the Source Files

Now that most of the “house-cleaning” steps are out of the way, you can try actual code modifications and build tests. 

4.3.1       Hardware Setup/Initialization

Rebuilding the project from this point using the rebuild all button Image will turn up a multitude of errors, most of them (if not all) in main.c.  These errors occur because of the LM3S102 functions and identifiers that were defined in the library and initialization files that we removed in the last section.

The function prvSetupHardware() calls functions that setup specific peripherals of the LM3S102, and those functions can all be removed and replaced with a single call to the stm32_Init() function found in STM32_init.c.  The function Stm32_Init() initializes some of the STM32F103RB peripherals, including the clocks, timers, GPIOs, and USART interfaces.

Because we removed the only call to vParTestInitialise(), the definition of this function can be completely removed as well (from ParTest.c).

The function vSerialInit() and its prototype in main.c can also be removed completely because it is specific to the LM3S102 and is no longer needed.

More initialization is still needed, including initializing the interrupt controller of the STM32F103RB.  Fortunately, this needed code has already been written, and can be found in the existing STM32F103 FreeRTOS port demo for the IAR tools found in the file serial.c in \..\FreeRTOS\Demo\CORTEX_STM32F103_IAR\serial (keep this file handy because more will be needed from it later).  We will need most of the code in the function xSerialPortInitMinimal(), but some of the unsupported features of it must be stripped away and changed. 

The new function that needs to be added to main.c vSerialPortInitMinimal() is shown here:


Figure 5.     More needed initialization via vSerialPortInitMinimal()

Now simply call this function from prvSetupHardware() with the needed baud rate as shown here:


Figure 6.     Calling the initialization functions from prvSetupHardware()

Make sure that mainBAUD_RATE is set to 11520 at the top of main.c.  This is the new baud rate for the STM32F103RB’s USART.

4.3.2       Getting the Status LEDs to Work 

Since FreeRTOS makes use of on-board LEDs to provide status and feedback information, it’s important to get the LEDs working before running the OS itself.  Generic instructions on how to do this for any platform can be found at under Demo App Introduction -> Modifying a demo.  A dummy main() is provided to test the LEDs, and should be used temporarily instead of the current main() function.  However, instead of calling vParTestInitialise() to initialize the IO, make a call to prvSetupHardware().  Also, the system is currently setup to periodically cause an interrupt (the details are not important now), and FreeRTOS isn’t actually going to be running yet to handle this, so delete the crude delay loop, as we will single-step through the code to test the LEDs.  Also, comment out the entire original main() function for now.

The functions that blink the LEDs (vParTestToggleLED() and vParTestSetLED())  are defined in ParTest.c, and must be modified for the new hardware.  These functions configure the ucOutputValue variable depending on what LED needs to be lit or toggled. A call to the LM3S102’s peripheral control write function (PDCWrite()) is then made, passing that variable.  First we must include the library files to make use of the new hardware by adding the line #include "stm32f10x_lib.h" to ParTest.c.  Then, delete the calls to PDCWrite(), and replace them each with the line:

GPIOB->ODR = (GPIOB->ODR & 0xFFFF00FF) | (ucOutputValue << 8);

The function prvPDCWrite() itself (and its declaration) in main.c should also be removed.

The file Stm32f10x_map.h is provided by RVMDK and contains all of the STM32F103RB’s peripheral registers’ definitions and memory mappings.  ST uses C structures for this particular chip to address these registers.  The structure component GPIOB->ODR refers to GPIOB’s port output data register.  The 8 LEDs on the MCBSTM32 are connected to GPIOB pins [15:8], and can be lit/toggled by writing to these bits of GPIOB->ODR.

For more information about the STM32F103RB’s peripherals and peripheral registers, see the STM32F103xx Reference Manual.

At this point, a build of the project will still turn up many errors regarding more references to LM3S102-specific hardware.  Comment out each of these errors, since we only want to confirm that we have the LEDs working at this time.  In the uVision3 IDE, double-click each error to bring the cursor to the error line, and comment out each one.  Ignore the warning messages for now.

Note: Make sure to use the latest version of the Cortex-M3 FreeRTOS port layer, as it had been updated during the writing of this application note, and older versions will require different modifications than the ones in this note.

To confirm that the LEDs are working, all of the hardware must be connected as described in Section 3 above, and uVision3 must be running in hardware mode (instead of simulation mode).  To ensure uVision3 is in hardware mode, click on the Options for Target icon Image, click on the Debug tab, and ensure that Use: ULINK Cortex Debugger is selected (not Use Simulator).

Once the build is error free and an executable image is created, click the Download to Flash Memory icon Image.  The flash download attempt will fail!  To this point, we have not changed the flash download and execution address to reflect the STM32F103RB’s memory map.  The flash memory of the STM32F103RB starts at address 0x08000000 and should be used as the R/O Base under the Linker tab under Options for Target.  The R/O Base sets the address for constants and code containing the RO (Read-Only) output section.  The R/O Base also sets the initial program entry address.  Also, be sure to remove any Misc controls linker options that might be there (such as –first Reset_Handler) that override the Linker Control String specifically for the LM3S102.  The Linker tab under Options for Target should look like this: 


Figure 7.     Options for Target Linker dialogue for STM32F103RB

For more information on ARM compilation tools output sections, see the RealView Compilation Tools for uVision Essentials Guide and the RealView Compilation Tools for uVision Developer Guide.

Another attempt at a flash download will turn up another problem.  uVision3 uses specific, built-in flash download algorithms depending on the chip.  Under the Utilities tab in the Options for Target, click on the ULINK Cortex Debugger Settings, ensuring that the Flash Download tab is selected. Click on the LM3Sxxx 8kB Flash programming algorithm, then click the Remove button. Click the Add button, scroll down and select the STM32F10x 128kB Flash algorithm, click Add, and finally click OK twice to apply the changes.

Now building the code and downloading the executable to flash should work correctly, and the uVision3 output window should verify this.  Click the Start/Stop Debug Session icon Image.  The uVision3 debugger will come up with the Program Counter pointing to the first line in main().

Single-step through the code using the Step Over button Image.  Each step over the calls to vParTestToggleLED() should toggle the corresponding LED on the MCBSTM32.

4.3.3       LED Co-routines

Now that we know the LEDs work, restore the original main() function and delete the dummy main() function.  At this point we want to create the LED co-routines with:


The co-routines use FreeRTOS to blink 5 LEDs at different rates.  Make sure to comment out the UART transmission co-routine, the LCD task, as well as the UART receive task (it will take a bit more work to get these running).

Rebuild the project using the rebuild all button Image, download the executable to flash, and press the RESET push-button on the MCBSTM32 to run the code out of flash.  The first five LEDs (PB8PB12) should all continuously blink at different rates.  These LEDs are under the control of the flash co-routines, each flashing at a specific frequency, with PB8 being the fastest and PB12 being the slowest.

Information about FreeRTOS tasks and co-routines can be found at

4.3.4       Creating the Queues for the USART

Memory queues are used in FreeRTOS port demos to implement USART functionality.

For this demo modification, create two separate queues in memory for the Tx and Rx sides of the USART communication instead of using the single xCommsQueue that was created in the first line in main().  To do this, replace the line:

xCommsQueue = xQueueCreate(mainRX_QUEUE_LEN, sizeof( portCHAR));

with the lines:

xRxedChars = xQueueCreate(mainRX_QUEUE_LEN, (unsigned portBASE_TYPE) sizeof(signed portCHAR));


xCharsForTx = xQueueCreate(mainRX_QUEUE_LEN + 1, (unsigned portBASE_TYPE) sizeof(signed portCHAR));

The use of two queues instead of one implements a slightly different functionality by transmitting characters over one queue and interrupts reading/receiving characters from the other queue.  The characters are then sent over the USART.  The LMS3102’s version that uses only one queue uses the queue to only receive characters, with transmitted characters sent directly from the USART Tx buffer by an interrupt.

These queues are created with xQueueCreate (which takes in the size of the queue in bytes and the data size for each element), and must be declared at the top of main.c with the lines:

static xQueueHandle xRxedChars;
static xQueueHandle xCharsForTx;

Also, be sure to remove the old declaration for xCommsQueue to avoid warnings about it being declared and not used.

There is one additional step to be done in order to get the two-queue model working.  To create the extra queue, we must increase the heap size slightly, otherwise the scheduler won’t start (because the idle task won’t be created, but that is beyond the scope of this note).  To do this, set the constant configTOTAL_HEAP_SIZE to 2468 instead of 1468 in FreeRTOSCOnfig.h.  This is an application-specific hardware configuration file for FreeRTOS.

The parameter configTOTAL_HEAP_SIZE might need to be increased even more if new tasks and co-routines are created as per Section 5 of this note.

4.3.5       USART Tx Co-routine

Now that the queues have been created, we can bring up the USART transmission co-routine that initiates the transmission of sequential characters.  This requires editing some code that is specific to the LM3S102.  The creation of the co-routine is called in main() with:

xCoRoutineCreate(vSerialTxCoRoutine, mainTX_CO_ROUTINE_PRIORITY, mainTX_CO_ROUTINE_INDEX);

Some STM32F103RB USART1 settings must be changed in STM32_Init.c.  The USART configuration definitions are around line 2150 of that file.  Change the declaration #define __USART1_ DATABITS from 0x0 to 0x8 and change the declaration #define __USART1_ STOPBITS from 0x0 to 0x1.

 In general, the definitions in this file are left for the programmer to define depending on the specific hardware.

More modifications must be made.  In main.c, all instances of UART0_BASE must be replaced with USART1_BASE (defined in STM32F10x_map.h) to reflect the memory-mapped address of the STM32F103RB’s USART interface.  Similarly, there are obsolete instances of UART_INT_TX, which is the transmit interrupt status bit.  Replace all instances of this with the STM32F103RB’s equivalent, USART_IT_TXE.

The routine vSerialTxCoRoutine()in main.c is the USART transmission co-routine, and needs a few modifications to work on the MCBSTM32.  This part of the existing LMS103 code disables UART transmission interrupts before sending a sequential character, then re-enables the UART transmission interrupts: 


Figure 8.     Part of the LMS3102’s vSerialTxCoRoutine()









This part of the code should be replaced with the following code:


Figure 9.     Part of the new STM32F103RB’s vSerialTxCoRoutine()

The definition of ser_putchar() is found in serial.c and the definition of USART_ITConfig() is contained in the firmware library file STM32F10xR.LIB.  Both of these files must be manually added to the Project Workspace in the uVision3 IDE.

We only want to transmit and receive ASCII characters ‘0’ to ‘z’, so we use the if statement to restart cNextChar.

More information about the routines found in STM32F10xR.LIB can be found in the ARM-based 32-bit MCU STM32F101xx and STM32F103xx Firmware Library document from STMicroelectronics.

The LM3S102 port at the time of writing this application note is coded so that nothing but the ‘0’ character is sent out the serial port.  It’s more interesting to see all of the sequential characters, so within vSerialTxCoRoutine(), move the line

cNextChar = mainFIRST_TX_CHAR;

from inside of the infinite for(;;) loop to outside of it right below the line

crSTART( xHandle );

To verify that the USART transmission under control of FreeRTOS is actually working, a ‘dumb’ terminal program must be used, such as Windows HyperTerminal.  If it’s not already, connect the serial cable from the MCBSTM32 to a working and verified serial port (or another port using a converter) on the host PC.

In Windows XP, start HyperTerminal by clicking Start à All Programs à Accessories à Communications à HyperTerminal.  Enter a New Connection Name (perhaps “USART Tx”), click on the Connect using drop-down menu and select the correct COM port, and click OK.  The Port Settings for the COM port are shown, and enter the settings that match those of the STM32F103RB’s STM32_Init.c file,shown below.


Figure 10.     Serial COM port properties for PC in Windows HyperTerminal

Reprogram the flash of the MCBSTM32 with the updated executable and press the RESET push-button on the board.  Sequential characters should show up on the terminal window starting at ‘0’, as shown in Fig. 11.


Figure 11.     STM32F103RB’s USART transmit results in HyperTerminal

The sixth LED on the MCBSTM32 (marked PB13) should also be flashing to indicate successful USART character transmissions.

4.3.6       LCD Task

Going in sequential order in main(), the next part of the demo to modify is the LCD task vLCDTask.  This is much simpler than it looks due to the level of abstraction used in the LCD functions in LCD_4bit.c.

You can get as creative as you want with the LCD functionality, but for a simple test, just try using the following instead of the LM3S102’s version of vLCDTask:


Figure 12.     New vLCDTask()

This code simply flashes the text given to lcd_print() so that it all fits on the MCBSTM32’s LCD screen.  Notice that we no longer need prvWriteString() for this simple test, so it and its declaration at the top of main.c can be removed.

Be sure to include LCD_4bit.c in uVision3’s Project Workspace in order to make use of the new library functions if it’s not there already, and declare the functions somewhere at the top of main.c (otherwise warnings will be generated at build time about these LCD library functions being declared implicitly.  Also, make sure to uncomment the LCD task creation line in main():


Again, reprogram the flash of  the MCBSTM32 with the new executable, and reset the board to confirm that the LCD is working.

4.3.7       USART Rx Task

The next part of the demo to be modified is the USART Rx task vCommsRxTask().  Only a very small modification needs to be done here, and that is to change the name of the queue that is used for this task.  Just replace the instances of xCommsQueue in the calls to xQueueReceive() with xRxedCharsxQueueReceive() is a generic function which receives data from a specified queue.

For more details on the queues and queue functions used by FreeRTOS, see

4.3.8       USART Interrupt Service Routine

The final part of the demo to be changed is the USART Interrupt Service Routine vUART_ISR().  In Section 4.2.3 of this note, this ISR call was added to the Cortex-M3’s vector table.

Similar to other changes described in this note, the code for this routine can be found in the existing STM32F103 FreeRTOS port demo for the IAR tools, located in the file serial.c in \..\FreeRTOS\Demo\CORTEX_STM32F103_IAR\serial.  This is the same file from which we pulled initialization code in Section 4.3.1 of this note

The routine vUARTInterruptHandler() is the code we need.  Use the body of this routine in vUART_ISR() in main.cvUART_ISR() in serial.c should now look like this:


Figure 13.     New vUART_ISR()

Again, many of the functions here are defined by the FreeRTOS port layer and the ARM-based 32-bit MCU STM32F101xx and STM32F103xx Firmware Library document from STMicroelectronics.

Now if you haven’t done so already, uncomment the USART Rx task creation line in main():


4.3.9       Testing the Modified Demo

If all of the steps have been performed correctly to this point, a re-build of the project should turn up no errors or warnings.

Re-build the project using the re-build all button Image, download the executable to flash, and press the RESET push-button on the MCBSTM32 to run the code out of flash.  The first seven LEDs (PB8PB14) should all continuously blink at different rates, with PB8 blinking the fastest and PB14 blinking the slowest. 

Again, the first five LEDs are under the control of the flash co-routines, each flashing at a particular frequency, with PB8 being the fastest and PB12 being the slowest.  PB13 will flash each time a character is transmitted on the serial port.  PB14 will flash each time a character is received and validated on the serial port though the loopback connector.

PB15 is used to indicate that an error has been detected and should remain off, unless of course the loopback connector is removed.  Try it and see!

The LCD will display a message, depending on what was done in Section 4.3.6.

5        New Task/Co-routine Creation and Exercises

Now that we have a working port demo for the MCBSTM32, you can add new tasks to be used by FreeRTOS.  Be careful not to go over the 16K code limitation of the RVMDK evaluation tools.

General information about tasks and adding new ones can be found in the documentation at

5.1      More LED Task Functionality

Try creating a task that blinks a single LED by passing in the particular LED like so:

void LEDTask( void *uxLED )

Use a crude delay mechanism to create a particular blink rate.  When this single task is created and the scheduler is started, watch the blink rate.  What happens to the blink rate if you create an identical task for a different LED and run both simultaneously?  Why?

What happens when the priority of one of these tasks is changed, and why?

Instead of using a crude delay mechanism, call vTaskDelay().  What happens now, and why?

5.2      Analogue-To-Digital Conversion

For a slightly more difficult example, try using the tick hook functionality to periodically trigger an analogue-to-digital conversion.  Create a new co-routine that receives the result from the STMF103RB ADC’s ADC_IRQHandler and prints the result to the LCD.

For this, crhook.c must be added from \...\FreeRTOS\Demo\Common\Minimal.

6        Conclusion

With the steps outlined in this application note, you should be able to get FreeRTOS up and running on the MCBSTM32 with an extendable demo application, using the license-free RVMDK evaluation tools from ARM/Keil.  This is useful in seeing FreeRTOS run in real hardware (using one of ARM’s latest processor cores: the Cortex-M3) with a completely free set of tools.

Copyright © 2008 ARM Limited. All rights reserved. ARM DAI 0210A