CMSIS-DSP/ComputeGraph/documentation/Generic.md

Generic and functions nodes

The generic and function nodes are the basic nodes that you use to create other kind of nodes in the graph.

There are 3 generic classes provided by the framework to be used to create new nodes :

• GenericSource
• GenericNode
• GenericSink

They are defined in cmsisdsp.cg.scheduler

There are 3 other classes that can be used to create new nodes from functions:

• Unary
• Binary
• Dsp

Generic Nodes

Any new kind of node must inherit from one of those classes. Those classes are providing the methods addInput and/or addOutput to define new IOs.

The method typeName from the parent class must be overridden.

A new kind of node is generally defined as:

class ProcessingNode(GenericNode):
    def __init__(self,name,theType,inLength,outLength):
        GenericNode.__init__(self,name)
        self.addInput("i",theType,inLength)
        self.addOutput("o",theType,outLength)

    @property
    def typeName(self):
        return "ProcessingNode"

See the simple example for more explanation about how to define a new node.

Methods

The constructor of the node is using the addInput and/or addOutput to define new IOs.

def addInput(self,name,theType,theLength):

• name is the name of the input. It will becomes a property of the Python object so it must not conflict with existing properties. If name is, for instance, "i" then it can be accessed with node.i in the code
• theType is the datatype of the IO. It must inherit from CGStaticType (see below for more details about defining the types)
• theLength is the amount of samples consumed by this IO at each execution of the node

def addOutput(self,name,theType,theLength):

• name is the name of the input. It will becomes a property of the Python object so it must not conflict with existing properties. If name is, for instance, "o" then it can be accessed with node.o in the code
• theType is the datatype of the IO. It must inherit from CGStaticType (see below for more details about defining the types)
• theLength is the amount of samples produced by this IO at each execution of the node

@property
def typeName(self):
    return "ProcessingNode"

This method defines the name of the C++ class implementing the wrapper for this node.

Datatypes

Datatypes for the IOs are inheriting from CGStaticType.

Currently there are 3 classes defined:

• CType for the standard CMSIS-DSP types
• CStructType for a C struct
• PythonClassType to create structured datatype for the Python scheduler

CType

You create such a type with CType(id) where id is one of the constant coming from the Python wrapper:

• F64
• F32
• F16
• Q31
• Q15
• Q7
• UINT32
• UINT16
• UINT8
• SINT32
• SINT16
• SINT8

For instance, to define a float32_t type for an IO you can use CType(F32)

CStructType

The constructor has the following definition

def __init__(self,name,size_in_bytes): 

• name is the name of the C struct
• size_in_bytes is the size of the struct. It should take into account padding. It is used in case of buffer sharing since the datatype of the shared buffer is int8_t. The Python script must be able to compute the size of those buffers and needs to know the size of the structure including padding.

PythonClassType

def __init__(self,python_name)

In Python, there is no struct. This datatype is mapped to an object. Object have reference type. Compute graph FIFOs are assuming a value type semantic.

As consequence, in Python side you should never copy those structs since it would copy the reference. You should instead copy the members of the struct and they should be scalar values.

Function and constant nodes

A Compute graph C++ wrapper is useful when the software components you use have a state that needs to be initialized in the C++ constructor, and preserved between successive calls to the run method of the wrapper.

Most CMSIS-DSP functions have no state. The compute graph framework is providing some ways to easily use functions in the graph without having to write a wrapper.

This feature is relying on the nodes:

• Unary
• Binary
• Dsp
• Constant

All of this is explained in detail in the simple example with CMSIS-DSP.