There are currently four FreeRTOS ports for Luminary Micro Stellaris M3 based embedded microcontrollers – one that uses the ARM Keil tools, one for Rowley CrossWorks, one using the IAR tool suite, and one that uses GCC. This page relates only to the ARM Keil based port for which two demos are provided. The demo presented on this page is targeted at the DK-LMS102 development board. See also the LM3S811 Keil RVDS demo application.
The LM3S102 is a low cost, low pin count device. It has 2KBytes of RAM and 8KBytes of ROM on chip. An ideal candidate to demonstrate the co-routine functionality included from FreeRTOS V4.0.0 onwards.
Upgrading to FreeRTOS V5.0.3: FreeRTOS V5.0.3 introduced the configMAX_SYSCALL_INTERRUPT_PRIORITY configuration option to the ARM Cortex-M3 port. See the RTOS kernel configuration documentation for full information on this feature.
Upgrading to FreeRTOS V4.8.0: Prior to V4.8.0 the FreeRTOS kernel did not make use of the SVCall interrupt. From V4.8.0 onwards it does. Therefore, to upgrade an older project to the V4.8.0 standard, a small edit to the startup code is required. To do this, simply install vPortSVCHandler() in the SVCall position within the interrupt vector table (contained in the startup source file). The demo projects included in the FreeRTOS download have already been updated so these can be used as an example.
IMPORTANT! Notes on using the Luminary Micro LM3S102 portPlease read all the following points before using this RTOS port.
My application does not run, what could be wrong?
Source Code OrganizationThe FreeRTOS download contains the source code for all the FreeRTOS ports so contains many more files than used by this demo. See the Source Code Organization section for a description of the downloaded files and information on creating a new project.
The Keil/RVDS demo application project for the Stellaris LM3S102 port is called FreeRTOS.Uv2 and can be located in the FreeRTOS/Demo/CORTEX_LM3S102_KEIL directory.
The Demo ApplicationThe FreeRTOS source code download includes two demos for this port which include both fully preemptive tasks and co-routines. Demo-1 creates 3 tasks (including the idle task) and 6 co-routines. Demo-2 creates 2 tasks and 7 co-routines. Due to the ROM and RAM constraints the standard demo tasks are not used.
Demo application hardware setupAll the DK-LMS102 jumpers can remain in their default positions.
The demo application includes an interrupt driven UART test where a co-routine transmits characters that are then received by a task. For correct operation of this functionality a loopback connector must be fitted to the SER0 connector of the DK-LMS102 prototyping board (pins 2 and 3 must be connected together on the 9Way connector).
The demo application uses the LEDs built into the prototyping board so no other hardware setup is required.
FunctionalityWhen executing correctly the Demo-1 application will behave as follows:
The Demo-2 application has similar functionality but tests different features of the RTOS port. To switch to Demo-2 simply copy the files from the FreeRTOS/Demo/CORTEX_LM3S102_KEIL/Demo2 into the FreeRTOS/Demo/CORTEX_LM3S102_KEIL directory.
Building and executing the demo application
RTOS port specific configurationConfiguration items specific to this port are contained in FreeRTOS/Demo/CORTEX_LM3S102_KEIL/FreeRTOSConfig.h. The constants defined in this file can be edited to suit your application. In particular – the definition configTICK_RATE_HZ is used to set the frequency of the RTOS tick. The supplied value of 1000Hz is useful for testing the RTOS kernel functionality but is faster than most applications require. Lowering this value will improve efficiency.
Also note configKERNEL_INTERRUPT_PRIORITY and configMAX_SYSCALL_INTERRUPT_PRIORITY. See the RTOS kernel configuration documentation for full information on these configuration constants. Attention please!: Remember that ARM Cortex-M3 cores use numerically low priority numbers to represent HIGH priority interrupts, which can seem counter-intuitive and is easy to forget! If you wish to assign an interrupt a low priority do NOT assign it a priority of 0 (or other low numeric value) as this can result in the interrupt actually having the highest priority in the system – and therefore potentially make your system crash if this priority is above configMAX_SYSCALL_INTERRUPT_PRIORITY.
The lowest priority on a ARM Cortex-M3 core is in fact 255 – however different ARM Cortex-M3 vendors implement a different number of priority bits and supply library functions that expect priorities to be specified in different ways. Use the supplied examples as a reference.
Each port #defines ‘BaseType_t’ to equal the most efficient data type for that processor. This port defines BaseType_t to be of type long.
Note that vPortEndScheduler() has not been implemented.
Interrupt service routinesThe interrupt vector table is contained within FreeRTOS/Demo/CORTEX_LM3S102_KEIL/init/startup.s and can be populated as required. In the demo applications the vector table remains in flash.
Unlike most ports, interrupt service routines that cause a context switch have no special requirements and can be written as per the compiler documentation. The macro portEND_SWITCHING_ISR() can be used to request a context switch from within an ISR. This mechanism is demonstrated by the UART ISR defined within main.c (see the function vUART_ISR()).
Switching between the pre-emptive and co-operative RTOS kernelsSet the definition configUSE_PREEMPTION within FreeRTOS/Demo/CORTEX_LM3S102_KEIL/FreeRTOSConfig.h to 1 to use pre-emption or 0 to use co-operative. The demo application will only execute correctly with configUSE_PREEMPTION set to 0 if configIDLE_SHOULD_YIELD is set to 1.
Compiler optionsAs with all the ports, it is essential that the correct compiler options are used. The best way to ensure this is to base your application on the provided demo application files.
Memory allocationSource/Portable/MemMang/heap_1.c is included in the ARM Cortex-M3 demo application project to provide the memory allocation required by the RTOS kernel. Please refer to the Memory Management section of the API documentation for full information.
Serial port driverIt should also be noted that the serial drivers are written to test some of the real time kernel features – and they are not intended to represent an optimized solution.
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