SPI

Ingo

Preface

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.

Device Description

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.

Device Identification

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.

  • Set the Type to 501 for all SPI devices
<DeviceDescription>
  ...
  <Device>
    ...
    <DeviceIdentification>
      ...
      <Type>501</Type>
      ...
    </DeviceIdentification>
    ...
  </Device>
  ...
</DeviceDescription>
  • The ID consists of the "Vendor ID" (0003 for Open Source) and the "Device ID". As the Device ID needs to be unique, you have to register it in the Device Database.
<DeviceDescription>
  ...
  <Device>
    ...
    <DeviceIdentification>
      ...
      <Id>0003 0001</Id>
      ...
    </DeviceIdentification>
    ...
  </Device>
  ...
</DeviceDescription>
  • 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.
<DeviceDescription>
  ...
  <Device>
    ...
    <DeviceIdentification>
      ...
      <Version>1.0.0.0</Version>
      ...
    </DeviceIdentification>
    ...
  </Device>
  ...
</DeviceDescription>

Connector

The connector describes the node in the device tree, which the user sees after he added your device. The I/O driver will search for this connector.

  • The moduleType is very important, that the I/O driver can find the corresponding device in the configuration. For SPI drivers, it has to be 500.
<DeviceDescription>
  ...
  <Device>
    ...
    <Connector moduleType="500" interface="Raspberry.SPI" ...>
      ...

      ...
    </Connector>
    ...
  </Device>
  ...
</DeviceDescription>
  • The interface has to be set to "Raspberry.SPI". Note, that currently only the Raspberry Pi is supported. When this limitation is removed, there will be another interface name.
<DeviceDescription>
  ...
  <Device>
    ...
    <Connector moduleType="500" interface="Raspberry.SPI" ...>
      ...

      ...
    </Connector>
    ...
  </Device>
  ...
</DeviceDescription>

Driver Info

This section specifies some parameters of the driver, and in our case, also which library should be used.

  • The RequiredLib tag specifies exactly which library has to be inserted to your project, when this driver is added to your device tree.
<DeviceDescription>
  ...
  <Device>
    ...
    <Connector moduleType="500" interface="Raspberry.SPI" ...>
      ...
      <DriverInfo needsBusCycle="false">
      ...
        <RequiredLib libname="SPI Template" vendor="Open Source" version="1.0.0.0" identifier="SPI Template">
        ...
        </RequiredLib>
        ...
        </Parameter>
      ...
    </Connector>
    ...
  </Device>
  ...
</DeviceDescription>
  • The FBInstance as well as its sub-tags defines which Function Block out of this library should be called.
    For an SPI driver all function calls from the template are important.
<DeviceDescription>
  ...
  <Device>
    ...
    <Connector moduleType="500" interface="Raspberry.SPI" ...>
      ...
      <DriverInfo needsBusCycle="false">
      ...
        <RequiredLib libname="SPI Template" vendor="Open Source" version="1.0.0.0" identifier="SPI Template">
        ...
          <FBInstance basename="$(DeviceName)" fbname="SPITemplate">
          ...
            <Initialize methodName="Initialize" />
            <CyclicCall methodname="AfterReadInputs" task="#buscycletask" whentocall="afterReadInputs" />
            <CyclicCall methodname="BeforeWriteOutputs" task="#buscycletask" whentocall="beforeWriteOutputs" />
          ...
          </FBInstance>
        ...
        </RequiredLib>
        ...
        </Parameter>
      ...
    </Connector>
    ...
  </Device>
  ...
</DeviceDescription>

Host Parameter Set

The host parameter set defines all your configuration parameters, as well as I/O channels. In the current SPI interface, you can use configuration parameters, but no I/O channels.

The configuration parameters can be read in the library within the function "Initialize".

<DeviceDescription>
  ...
  <Device>
    ...
    <Connector moduleType="500" interface="Raspberry.SPI" ...>
      ...
      <HostParameterSet>
      ...
        <Parameter ...>
        ...
        </Parameter>
      ...
    </Connector>
    ...
  </Device>
  ...
</DeviceDescription>

For more information about the HostParameterSet and datatypes, please check the general I/O driver documentation.

Library

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.

Function Block

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.

FUNCTION_BLOCK SPITemplate EXTENDS spi
VAR_INPUT
END_VAR
VAR_OUTPUT
    dwRaw : DWORD;
    rValue : REAL;
END_VAR

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;

Methods

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.
SUPER^.AfterReadInputs();

IF _iState = 10 THEN

    FOR usiChannel := 0 TO 0 DO
        aby[0] := 1;
        aby[1] := 16#80 + SHL(usiChannel AND 7, 4);
        aby[2] := 0;
        aby[3] := 0;

        IF NOT transfer(pabyTxBuffer:=ADR(aby) , pabyRxBuffer:=ADR(aby) , udiLen:=3 , uiDelayus:=0) THEN
            _iState := 1000;
        END_IF

        CASE usiChannel OF
            0:      
                dwRaw := aby[3];
                dwRaw := SHL(dwRaw,8) OR aby[2];
                dwRaw := SHL(dwRaw,8) OR aby[1];
                dwRaw := SHL(dwRaw,8) OR aby[0];

                rValue := DWORD_TO_REAL(SHR(dwRaw, usiBitWidth));
                rValue := rValue * rResolution;
        END_CASE
    END_FOR     
END_IF
  • 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.

SUPER^.Initialize(wModuleType, dwInstance, pConnector);
pParam := ConfigGetParameter(_pConnector, 1000);
IF pParam <> 0 THEN
    pusiBitWidth := IoStandard.ConfigGetParameterValuePointer(pParam, ADR(udiResult));
    usiBitWidth := pusiBitWidth^;
END_IF

pParam := ConfigGetParameter(_pConnector, 1001);
IF pParam <> 0 THEN
    prResolution := IoStandard.ConfigGetParameterValuePointer(pParam, ADR(udiResult));
    rResolution := prResolution^;
END_IF

SPI Transfer

You might have noticed the few lines above:

        by[0] := 1;
        aby[1] := 16#80 + SHL(usiChannel AND 7, 4);
        aby[2] := 0;
        aby[3] := 0;

        IF NOT transfer(pabyTxBuffer:=ADR(aby) , pabyRxBuffer:=ADR(aby) , udiLen:=3 , uiDelayus:=0) THEN
            _iState := 1000;
        END_IF

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).

Project Information

In the "Project Information" you have to enter few names and identifiers:

Summary

  • 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"

Properties

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.


Related

Documentation: IndexMain