This documentation describes the process of creating an own SPI driver for CODESYS, based on the SPI Template, which can be found in the Code Repository. The driver consists of a "Device Description" as well as a CODESYS Library. All essential settings in the Library or the Device Description are repeated in this documentation. For more detailed informations about the Device Descriptions, please check the general I/O driver documentation.
You can use a copy of the Device Description SPI_Template.devdesc.xml as a starting point. You need to adapt the following sections of the device description for your needs.
The template of the device description can be downloaded with an ordinary SVN client (like Tortoise SVN) or directly from the repository browser, from the Code Repository.
The version can be freely defined. Just, the format is fix. And you should take care that you never release two different device descriptions with the same Type, ID and Version. As this might produce conflicts in the repositories of other users.
The template library can be checked out with CODESYS SVN or alternatively with CODESYS SVN Forge. Both can be obtained from the CODESYS Store. As well as the device description, it is placed in the code repository.
It is a standard CODESYS Library, which needs the standard information to behave as expected to the user. We describe all bits, which need to be changed in the following documentation.
The function block can be renamed. But make sure, that you change the "FBInstance" parameter in the device description accordingly. It has to be extended from "SPI", which is part of the "Raspberry Pi Peripherals" library.
In this example, the output of this function block is the output of the SPI driver, and the value which will be used by the user in his application. So the value can be used like this in the application:
rMyTemperature := SPITemplate.rValue;
All methods, which are overloading in your function block, need to call the super method, so that the method of the base SPI function block is still executed.
AfterReadInputs can process any inputs from the SPI device and update the output parameters of the FB. So for example "rValue" should be updated in this function.
BeforeWriteOutputs can be used in a similar way to write the outputs to the SPI device. The output is read from the "input" of the function block.
Initialize is a special method which can be used to read configuration parameters. When the application is loaded, this function is called, and the corresponding "Connector" (s. Device Description documentation) is passed to it. You can then use IoStandard.ConfigGetParameter() to access the configuration parameters.
In SPI you always have to write out the same number of bytes, which you plan to receive. The method "transfer" is actually doing the transfer on the SPI bus. The addressing code is stolen from the MCP3008 chip. There you have one start bit (in bytes 0), and then a "command", which defines the ADC we want to read.
For more informations about the MCP3008, you may check out these pages from Adafruit.
As the data is shifted out on the SPI bus, and then received, you can use the same buffer for transmit and receive (like we did in the example above).
In the "Project Information" you have to enter few names and identifiers:
As the Company, you can use "Open Source" if it is a plain open source driver (corresponds to the VendorID 0003 of the Device Description).
The Title of the library is the name, which the user selects in the library repository when he adds your library manually.
The Version can be freely defined. But it usually makes sense to keep this version in sync with the "Device Description"
Now, we switch to the more advanced properties of our library.
The property DefaultNamespace defines the namespace prefix, which one has to use to access POUs of your library. We recommend, that you use the same as the library name, but w/o spaces.
The property Placeholder is important to set. But deviations from the library name are for more advanced use cases. So just enter the same as you entered in Title
The rest of the properties can be usually ignored.