IntroductionJust like the FreeRTOS kernel itself, FreeRTOS+IO is configured using a header file. Whereas the FreeRTOS configuration header file is called FreeRTOSConfig.h, the FreeRTOS+IO configuration header file is called FreeRTOSIOConfig.h.
The settings in the kernel’s configuration file (FreeRTOSConfig.h) do not greatly impact the size of the executable binary created when the code is compiled. This is because, for the most part, the settings it contains are used to include or exclude entire functions. The default* behaviour of most linkers is to remove any unreferenced functions from the executable binary anyway. The settings in the FreeRTOS+IO configuration file (FreeRTOSIOConfig.h), on the other hand, do greatly impact code size. This is because they are used to optionally include or exclude code within functions, rather than optionally include or exclude the entire function itself. It is important to ensure FreeRTOSIOConfig.h is configured optimally if flash/ROM space is at a premium.
* GCC is the exception to this, where specific compile time options are required to request that unreferenced code is removed.
An example FreeRTOSIOConfig.h header fileThe details of the parameters contained in a FreeRTOSIOConfig.h header file are dependent on the board support package being used. Below is an example, and below that an explanation.
Global transfer mode optionsIn the example above, the top group of options are used to include or exclude specific transfer modes from FreeRTOS+IO. The polling mode is always available, so there is no option specific to that.
|ioconfigUSE_ZERO_COPY_TX||Must be set to 1 if you are going to use the interrupt driven zero copy transfer mode. Otherwise, set to zero to ensure the best run time and code size efficiency.|
|ioconfigUSE_CIRCULAR_BUFFER_RX||Must be set to 1 if you are going to use the interrupt driven circular buffer transfer mode. Otherwise, set to zero to ensure the best run time and code size efficiency.|
|ioconfigUSE_TX_CHAR_QUEUE||Must be set to 1 if you are going to use the interrupt driven character queue transfer mode to write data. Otherwise, set to zero to ensure the best run time and code size efficiency.|
|ioconfigUSE_RX_CHAR_QUEUE||Must be set to 1 if you are going to use the interrupt driven character queue transfer mode to read data. Otherwise, set to zero to ensure the best run time and code size efficiency.|
Peripheral specific optionsThe board support package defines which peripherals are available. For each peripheral there is a global include or exclude option. For example, to include the SPI peripheral, set configINCLUDE_SPI to 1. To exclude the SPI peripheral, set configINCLUDE_SPI to 0.
Each peripheral then has a sub-option for each transfer mode supported by the peripheral. For example, if you want to use the SPI port in interrupt driven zero copy mode set ioconfigUSE_SPI_ZERO_COPY_TX to 1, otherwise set ioconfigUSE_SPI_ZERO_COPY_TX to 0.
Therefore, to continue the example, to use the SPI port in interrupt driven zero copy mode three settings are required:
- ioconfigUSE_ZERO_COPY_TX must be set to 1 to enable the transfer mode in FreeRTOS+IO.
- ioconfigINCLUDE_SPI must be set to 1 to enable SPI support.
- ioconfigUSE_SPI_ZERO_COPY_TX must be set to 1 to specifically enable the use of the zero copy transfer mode on the SPI port.