Importing Stream-Based Kernels - 2025.2 English - UG1483

Vitis Model Composer User Guide (UG1483)
Document ID
UG1483
Release Date
2025-11-20
Version
2025.2 English

As explained in Data Accessing Mechanisms, stream-based kernels access data streams in a sample-by-sample fashion. Vitis Model Composer supports the following stream-based input and output data types as interfaces to the AIE Kernel block.

  • input_stream<TYPE>
  • output_stream<TYPE>
Table 1. Stream-Based Input and Output Data Types
<Type> Complexity Signedness
int8 Real Signed
int16 Real Signed
int32 Real Signed
int64 Real Signed
uint8 Real Unsigned
uint16 Real Unsigned
uint32 Real Unsigned
uint64 Real Unsigned
cint16 Complex Signed
cint32 Complex Signed
float Real N/A
cfloat Complex N/A
bfloat16 Real N/A
accfloat Real N/A
caccfloat Complex N/A

As an example, to import a simple kernel with a stream-based interface. The following simple.h header file declares the simple_comp function with one input stream and one output stream.

#ifndef __COMPLEX_KERNEL_H__
#define __COMPLEX_KERNEL_H__
#include <adf.h>
  void simple_comp(input_stream<cint16> * in, output_stream<cint16> * out);
#endif //__COMPLEX_KERNEL_H__

simple.cc defines the function.

#include "simple.h"
void simple_comp(input_stream<cint16> * in, output_stream<cint16> * out) {
  cint16 c1, c2;
 
  for (unsigned i=0; i<NUM_SAMPLES; i++) {
  c1 = readincr(in);
  c2.real = c1.real+c1.imag;
  c2.imag = c1.real-c1.imag;
  writeincr(out, c2);
}
}
Note: See the AI Engine Tools and Flows User Guide (UG1076) for details of the readincr() and writeincr() APIs.

Although the function arguments for buffer-based and stream-based kernels are different, the procedure for importing the stream-based kernel is the same.

After a successful import, the Function tab GUI displays automatically. You can quickly review the function definition and ports as shown in the following figure.

Figure 1. Function Tab

To import the kernel, you must specify the signal size of each output port. The following table provides details on this parameter.

Table 2. Stream Port Parameters
Parameter Name Description
Signal size This parameter represents the size of the output signal and must be set to a value greater than or equal to the maximum number of samples that are produced during any invocation of the kernel.

In the General tab, the Import button changes to Update, enabling further updates of block parameters.

Model Composer also supports cascade stream connections between two AI Engine processors.

An AI Engine kernel can use cascade stream connections as follows:

  • input_cascade<TYPE>
  • output_cascade<TYPE>

Each AI Engine architecture supports different cascade data types.

Table 3. Cascade Stream-Based Input and Output Data Types (AIE1)
<Type> Complexity Signedness
int8 Real Signed
uint8 Real Unsigned
int16 Real Signed
cint16 Complex Signed
int32 Real Signed
cint32 Complex Signed
float Real N/A
cfloat Complex N/A
acc48 Real N/A
acc80 Real N/A
cacc48 Complex N/A
cacc80 Complex N/A
accfloat Real N/A
caccfloat Complex N/A
Table 4. Cascade Stream-Based Input and Output Data Types (AIE-ML)
<Type> Complexity Signedness
int8 Real Signed
uint8 Real Unsigned
int16 Real Signed
uint16 Real Unsigned
cint16 Complex Signed
int32 Real Signed
uint32 Real Unsigned
cint32 Complex Signed
bfloat16 Real N/A
acc32 Real N/A
acc64 Real N/A
cacc64 Complex N/A
accfloat Real N/A
caccfloat Complex N/A

In Model Composer, a cascade stream port with an accumulator data type is represented as a fixed point signal (for example, x_sfix48). This can be either complex or real.

Consider the following example where a cascade output stream of one kernel connects to the cascade input stream of another kernel.

#ifndef __CASCADE_KERNELS_H__
#define __CASCADE_KERNELS_H__
void f_osacc48(input_buffer<int32> &i_hi, 
               input_buffer<int16> &i_lo, 
               output_cascade<acc48> *o1);
#endif
The kernel function f_osacc48 has two input windows: i_hi and i_lo, and one cascade stream output: o1.
Note: This kernel function includes both window-based ports and stream-based ports.

After importing this kernel function, the AIE Kernel block is as shown in the following figure.

Figure 2. AIE Kernel after Import

Consider another kernel function f_isacc48, which has one cascade stream input: i1, and two output windows: o_hi and o_lo.


#ifndef __CASCADE_KERNELS_H__
#define __CASCADE_KERNELS_H__
void f_isacc48(input_cascade<acc48> *i1,
               output_buffer<int32> &o_hi,
               output_buffer<int16> &o_lo);
#endif

After importing the second kernel function, the AIE Kernel block is as shown in the following figure.

Figure 3. AIE Kernel Block (Second Kernel Function) Generated by Your Tool

Now you can connect the two kernels to form a cascade connection using:

  • The cascade stream output of block f_osacc48.
  • The cascade stream input of block f_isaccc48.

The following figure shows this.

Figure 4. Connected Kernels (Cascade Connection) Generated by Your Tool