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There is a FIFO block that has Avalon interface compatible with Qsys that can be used in Qsys systems. However, in my case there is an external block that generates data that is to be read by a Nios II. The external block has a FIFO interface, thus the signals are basically q, empty and rdreq. I could try to use a PIO block to connect to the external FIFO and write some simple drivers to aid in communication. However, is there a better way to do this? Is it possible to somehow communiate with this external FIFO as if it was in the Qsys block with Avalon MM or Avalon streaming interface?

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FIFOs are typically used with streaming type interfaces. You can actually convert your FIFO signals into an Avalon ST interface quite easily. They map as follows:

  • Valid = ~empty
  • Ready = rdreq
  • Data = q

You don't actually need any other signals to make Avalon ST. The only weird one above is making sure to invert the empty signal.

Once in Qsys you will need to convert from Avalon ST to Avalon MM of which there are a few options. You could for example use a DMA controller to read data from the FIFO into on-chip memory that Nios can then read (e.g. the Scatter-Gather DMA Controller in Qsys).


Alternatively thinking about it, it might be easier to simply build a module to create an Avalon-MM interface. Something like:

module fifo_to_mm (
    input         clk,
    input         rst,

    input  [31:0] q,
    output        rdreq,
    input         empty,

    input         read,
    input         address,
    output [31:0] readdata
);

assign rdreq = read && (address == 1'b0);
assign readdata = address ? {31'b0, empty} : q;

endmodule

In that example, if you read address 0, it will read the next word from the FIFO. If you read address 1, it will tell you whether or not the FIFO has any data available.

I would make a TCL wrapper around this module (you can use the "File->New Component" tool in Qsys to help you).

  • Add a clock and reset interface. You should connect these to the same clock/reset that you feed out for your FIFO.
  • Add the FIFO signals into a conduit which you export from your Qsys system.
  • Add the Avalon-MM signals to an Av-MM slave interface which is configured as:
    • Bits Per Symbol: 8
    • Read wait: 0
    • Read latency: 1 or 2 (*)
    • Address units: WORDS

You should then be able to hook this straight up to Nios, and Qsys will take care of the interconnect fabric for you.


(*) read latency depends on your FIFO latency - i.e. number of cycles between asserting the rdreq signal and the q signal being updated.

If you want, you can register the readdata signal to improve Fmax, in which case add 1 to the read latency.

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  • \$\begingroup\$ OK, I have reached the stage where I must do this. Looking into the many peripherals available in Qsys, I am not sure which one to use. \$\endgroup\$ – quantum231 Jun 10 '18 at 22:58
  • \$\begingroup\$ The above signals are how I would manifest the FIFO as an Avalon ST interface device in my Qsys system. However, how do I make the Nios II read from it, since the Nios II reads/wires Avalon MM locations using an address. \$\endgroup\$ – quantum231 Jun 10 '18 at 23:12
  • \$\begingroup\$ @quantum231 see my update. \$\endgroup\$ – Tom Carpenter Jun 10 '18 at 23:32
  • \$\begingroup\$ Tom, thanks for this, I am working on it right now. I have one more question. The FIFO in question has 8 bit data width. Although I can resize it to 32 bits by appending '0' bits, I was wondering, is the Avalon MM readdata required to be 32 bits? \$\endgroup\$ – quantum231 Jun 11 '18 at 18:40
  • \$\begingroup\$ @quantum231 for Nios it is helpful. If you size it to 8-bit, Qsys will insert a load of extra logic to allow Nios to perform reads/writes of the 8-bit value in a byte-addressed mode. However give Nios can address 2^29 different 32-bit values, you might as well pad to 32-bit and save the extra logic. \$\endgroup\$ – Tom Carpenter Jun 11 '18 at 18:57

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