Buffer Port Connection for Multirate Processing - 2024.1 English

AI Engine-ML Kernel and Graph Programming Guide (UG1603)

Document ID
UG1603
Release Date
2024-06-06
Version
2024.1 English

In some designs the size of the output buffer port of a kernel can be different than the size of the input buffer port of the next kernel. In that case, the connection declaration will contain the size of the output port and the size of the input buffer port. For example:

connect net0 (kernel0.out[0], kernel1.in[0]);
dimensions(kernel0.out[0]) = {128};
dimensions(kernel1.in[0]) = {192};

In the above example, kernel0 writes 128 samples and kernel1 expects 192 samples will be written to memory. In such scenarios, the AI Engine compiler performs multirate analysis. In this case, the compiler will specify that kernel1 should run twice while kernel0 will run three times. You can specify the repetition count for these kernels in the graph manually, as follows:

repetition_count(kernel0) = 3;
repetition_count(kernel1) = 2;

Just as you can multicast an output buffer port to multiple input buffer ports in the graph (automatic DMA insertion mechanism), you can also perform multirate processing in this specific use case:

connect net0 ( kernel0.out[0] , kernel1.in[0] );
connect net1 ( kernel0.out[0] , kernel2.in[0] );
dimensions(kernel0.out[0]) = {128};
dimensions(kernel1.in[0])  = {64};
dimensions(kernel2.in[0])  = {192};

In this example, the AI Engine compiler automatically detects that kernel0 should run three times, kernel1 should run six times, and kernel2 should run twice for one graph iteration, graph.run(1). These repetition counts can also be specified manually in the graph.

The following is a graphical example with a rational up-converter (7/5) followed by a single rate kernel. Input frame length is 350 for both, but the output frame length of the up-converter is 490.
connect ( datain , upconv.in[0] );
connect ( upconv.out[0] , singlerate.in[0] );
connect ( singlerate.out[0] , dataout );

// If the connection is buffer based dimensions can be specified
// in the graph
dimensions(upconv.in[0]) = {350};
dimensions(upconv.out[0])  = {490};
dimensions(singlerate.in[0]) = {350};
dimensions(singlerate.out[0])  = {350};

// If the connections are stream based, repetitions count must be specified
repetition_count(upconv) = 5;      // LCM(350,490)/490 = 5
repetition_count(singlerate) = 7;  // LCM(350,490)/350 = 7
Figure 1. Up-Converter Followed by a Single Rate Kernel

In the preceding figure, the up-conversion kernel must be run five times, and the single rate kernel must be run seven times to produce and consume the same number of samples between the two kernels.

In this example the output frame length of the up-converter is larger than the length of the input frame of the single rate kernel. This means that the up-converter will write over the ping and pong buffers (or other more complex schemes) in a single iteration.