AXI4-Stream Protocol Overview

Protocol Structure

We send the command and responses via the AXI4-Stream protocol, a lightweight data transfer mechanism that uses a simple handshake between the source (master) and sink (slave). Data is sent in units called bursts, which are sequences of one or more data beats. Each beat is transferred when both sides agree: the source asserts TVALID to indicate valid data, and the sink asserts TREADY to signal readiness to accept it.

Within a burst:

• TVALID stays high while the source has data to send.
• TREADY may toggle depending on whether the sink can accept data.
• A transfer occurs only when both TVALID and TREADY are high in the same cycle.

This handshake ensures flow control: the source never overruns the sink, and the sink can apply backpressure by deasserting TREADY.

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Handshake States

TVALID	TREADY	State	Description
1	1	Transfer	Data beat accepted (handshake occurs)
1	0	Sink stall	Source has data, sink not ready
0	1	Source idle	Sink ready, source has no data
0	0	Quiescent	Both inactive, no transfer possible

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At the end of a burst, the source asserts TLAST to mark the final beat, signaling the sink that the burst is complete.

Implementing the Interfaces in the IP

The basic structure for the Vitis IP is as follows: The function has two AXI-Stream interfaces, cmd_stream and result_stream. There is an inifite loop that waits for each command packet, processes it, and sends a response to the output stream.

void simp_fun(
    hls::stream<stream_t>& cmd_stream,
    hls::stream<stream_t>& result_stream,
    ... // other AXI-Lite registers
) {
#pragma HLS INTERFACE axis port=cmd_stream
#pragma HLS INTERFACE axis port=result_stream
#pragma HLS INTERFACE ap_ctrl_chain port=return

    while (!cmd_stream.empty()) {
        Cmd cmd;

        // Read command from input FIFO using stream_read
        bool tlast_ok = cmd.stream_read<stream_t>(cmd_stream);

        // Proccess cmd and compute res
        // res = ...
        

        // Write result to output FIFO using stream_write
        res.stream_write<stream_t>(result_stream, true); // Assert TLAST on final word
    }
}

The functions stream_read and stream_write are methods of the Cmd and Result classes. The functions unpack and pack the data fields from the words in the structure.