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Can queues be used to drive RTL?

shareefj

This may well be a case of my not being able to see the wood for the trees or generally doing something stupid, but I thought I'd ask anyway.

I'm trying to interface a bus driver with some DPI code and thought that using a queue would be the best way.  What I have on the DPI side is a function that is called when new data is available to be pushed onto the queue.  On the RTL side, I've got a module with a FIFO interface and data is popped off the queue as it is loaded into the receiving downstream flop.  So something like the following:

bit [31:0] queue[$];

function push_back;
  input bit [31:0] value;
  queue.push_back(value);
endfunction

always_ff@(posedge clk) begin
  if(fifo_pop)
    queue.pop_front();
end

assign din = queue[0];

always_ff@(posedge clk) begin
  if (fifo_pop)
    capture_r <= din;
end

What I'm seeing is that the queue is updated before din has been captured in the receiving flop.  Is that expected and if so, what should I be doing here?  I've not done a load of DPI work before and haven't seen too many examples of how to cleanly drive an interface like this.

Thanks.

Parents

  • Just as a side note, modifying the above example such that the pop and assignment happen on the same line in the same always block works.  I just can't do that in my real code due to hierarchy.  Putting in a simple one entry FIFO that bridges between DPI and RTL seems to work though.

    always_ff @(posedge clk) begin
      if (fifo_pop)
        capture_r <= queue.pop_front();
    end
Reply

  • Just as a side note, modifying the above example such that the pop and assignment happen on the same line in the same always block works.  I just can't do that in my real code due to hierarchy.  Putting in a simple one entry FIFO that bridges between DPI and RTL seems to work though.

    always_ff @(posedge clk) begin
      if (fifo_pop)
        capture_r <= queue.pop_front();
    end
Children
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