M_AXI Resources - 2021.2 English

Vitis High-Level Synthesis User Guide (UG1399)

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2021.2 English

The AXI Master Adapter converts the customized AXI commands from the HLS scheduler to standard AXI AMBA protocol and sends them to the external memory. The MAXI adapter uses resources such as FIFO to store the requests/Data and ack. Here is the summary of the modules and the resource they consume:

  • Write Module: The bus write modules performs the write operations.
    • FIFO_wreq: This FIFO module stores the future write requests. When the AW channel is available a new write request to global memory will be popped out of this FIFO.
    • buff_wdata: This FIFO stores the future write data that needs to be sent to the global memory. When the W channel is available and AXI protocol conditions are met, the write data of size= burst_length will be popped out of this FIFO and sent to the global memory.
    • FIFO_resp: This module is responsible for controlling the number of pipelined outstanding requests sent to the global memory.
  • Read Module: These modules perform the read operations. It uses the following resources
    • FIFO_rreq: This FIFO module stores the future write requests. When the AR channel is free a read request to global memory will be popped out of this FIFO.
    • buff_rdata: This FIFO stores the read data that are received from the global memory.

The device resource consumption of the M_AXI adapter is a sum of all the write modules (size of the FIFO_wreq module, buff_wdata, and size of FIFO_ resp) and the sum of all read modules. In general, the size of the FIFO is calculated as = Width * Depth. When you refer to a 1KB FIFO storage it can be one of the configurations such as 32*32, 8*64 etc, which are selected according to the design specification. Similarly, the adapter FIFO storage can be globally configured for the design using the following options of the config_interface command:

    • -m_axi_latency
    • -m_axi__max_read/write_burst_length
    • -m_axi_num_read/write_outstanding
    • -m_axi_addr64
    Tip: You can also use similar options on the INTERFACE pragma or directive to configure specific m_axi interfaces.

    These configuration options control the width and depth of the FIFO as shown below.

    • Size of the FIFO_wreq/rreq module = ~~~ (width( config_interface m_axi_addr64[=true|false])) * Depth( config_interface m_axi_latency )). This FIFO will be implemented as a shift register after vivado.
    • Size of the buff_wdata module = ~~~ (width ( port width after hls synthesis) * Depth (config_interface m_axi_num_read/write_outstanding* config_interface m_axi_max_read/write_burst_length )).

      Tip: This FIFO by default will be implemented as BRAM, but it can be controlled by the user when using the following setting: config_interface -maxi_buffer_impl <auto|lutram|bram|uram>.
    • Size of the FIFO_resp module = width(2) * depth (config_interface -m_axi_num_read/write_outstanding-1).