Merge FPGA repository back into UHD repository

The FPGA codebase was removed from the UHD repository in 2014 to reduce
the size of the repository. However, over the last half-decade, the
split between the repositories has proven more burdensome than it has
been helpful. By merging the FPGA code back, it will be possible to
create atomic commits that touch both FPGA and UHD codebases. Continuous
integration testing is also simplified by merging the repositories,
because it was previously difficult to automatically derive the correct
UHD branch when testing a feature branch on the FPGA repository.

This commit also updates the license files and paths therein.

We are therefore merging the repositories again. Future development for
FPGA code will happen in the same repository as the UHD host code and
MPM code.

== Original Codebase and Rebasing ==

The original FPGA repository will be hosted for the foreseeable future
at its original local location: https://github.com/EttusResearch/fpga/

It can be used for bisecting, reference, and a more detailed history.

The final commit from said repository to be merged here is
05003794e2da61cabf64dd278c45685a7abad7ec. This commit is tagged as
v4.0.0.0-pre-uhd-merge.

If you have changes in the FPGA repository that you want to rebase onto
the UHD repository, simply run the following commands:

- Create a directory to store patches (this should be an empty
  directory):

    mkdir ~/patches

- Now make sure that your FPGA codebase is based on the same state as
  the code that was merged:

    cd src/fpga # Or wherever your FPGA code is stored
    git rebase v4.0.0.0-pre-uhd-merge

  Note: The rebase command may look slightly different depending on what
  exactly you're trying to rebase.

- Create a patch set for your changes versus v4.0.0.0-pre-uhd-merge:

    git format-patch v4.0.0.0-pre-uhd-merge -o ~/patches

  Note: Make sure that only patches are stored in your output directory.
  It should otherwise be empty. Make sure that you picked the correct
  range of commits, and only commits you wanted to rebase were exported
  as patch files.

- Go to the UHD repository and apply the patches:

    cd src/uhd # Or wherever your UHD repository is stored
    git am --directory fpga ~/patches/*
    rm -rf ~/patches # This is for cleanup

== Contributors ==

The following people have contributed mainly to these files (this list
is not complete):

Co-authored-by: Alex Williams <alex.williams@ni.com>
Co-authored-by: Andrej Rode <andrej.rode@ettus.com>
Co-authored-by: Ashish Chaudhari <ashish@ettus.com>
Co-authored-by: Ben Hilburn <ben.hilburn@ettus.com>
Co-authored-by: Ciro Nishiguchi <ciro.nishiguchi@ni.com>
Co-authored-by: Daniel Jepson <daniel.jepson@ni.com>
Co-authored-by: Derek Kozel <derek.kozel@ettus.com>
Co-authored-by: EJ Kreinar <ej@he360.com>
Co-authored-by: Humberto Jimenez <humberto.jimenez@ni.com>
Co-authored-by: Ian Buckley <ian.buckley@gmail.com>
Co-authored-by: Jörg Hofrichter <joerg.hofrichter@ni.com>
Co-authored-by: Jon Kiser <jon.kiser@ni.com>
Co-authored-by: Josh Blum <josh@joshknows.com>
Co-authored-by: Jonathon Pendlum <jonathan.pendlum@ettus.com>
Co-authored-by: Martin Braun <martin.braun@ettus.com>
Co-authored-by: Matt Ettus <matt@ettus.com>
Co-authored-by: Michael West <michael.west@ettus.com>
Co-authored-by: Moritz Fischer <moritz.fischer@ettus.com>
Co-authored-by: Nick Foster <nick@ettus.com>
Co-authored-by: Nicolas Cuervo <nicolas.cuervo@ettus.com>
Co-authored-by: Paul Butler <paul.butler@ni.com>
Co-authored-by: Paul David <paul.david@ettus.com>
Co-authored-by: Ryan Marlow <ryan.marlow@ettus.com>
Co-authored-by: Sugandha Gupta <sugandha.gupta@ettus.com>
Co-authored-by: Sylvain Munaut <tnt@246tNt.com>
Co-authored-by: Trung Tran <trung.tran@ettus.com>
Co-authored-by: Vidush Vishwanath <vidush.vishwanath@ettus.com>
Co-authored-by: Wade Fife <wade.fife@ettus.com>

1 files changed, 70 insertions, 0 deletions

diff --git a/fpga/usrp3/lib/control/pulse_synchronizer.v b/fpga/usrp3/lib/control/pulse_synchronizer.v
new file mode 100644
index 000000000..af3460878
--- /dev/null
+++ b/fpga/usrp3/lib/control/pulse_synchronizer.v
@@ -0,0 +1,70 @@
+//
+// Copyright 2018 Ettus Research, A National Instruments Company
+//
+// SPDX-License-Identifier: LGPL-3.0-or-later
+//
+// Module: pulse_synchronizer
+// Description:
+// - Synchronizes a single-cycle pulse or an edge from one
+//   clock domain to another
+// - Clocks A and B can be asynchronous
+//
+
+module pulse_synchronizer #(
+  parameter MODE   = "PULSE", // Capture mode {PULSE, POSEDGE, NEGEDGE}
+  parameter STAGES = 2        // Number of synchronizer stages
+) (
+  input   clk_a,    // Clock A
+  input   rst_a,    // Reset in clock domain A
+  input   pulse_a,  // Pulse in clock domain A to synchronize
+  output  busy_a,   // Synchronizer is busy (pulse_a ignored when asserted)
+  input   clk_b,    // Clock B
+  output  pulse_b   // Pulse in clock domain B
+);
+  // Trigger logic based on the capture mode
+  wire trigger;
+  generate if (MODE == "POSEDGE") begin
+    reg pulse_a_del_pe = 1'b0;
+    always @ (posedge clk_a)
+      pulse_a_del_pe <= rst_a ? 1'b0 : pulse_a;
+    assign trigger = pulse_a & ~pulse_a_del_pe;
+  end else if (MODE == "NEGEDGE") begin
+    reg pulse_a_del_ne = 1'b1;
+    always @ (posedge clk_a)
+      pulse_a_del_ne <= rst_a ? 1'b1 : pulse_a;
+    assign trigger = ~pulse_a & pulse_a_del_ne;
+  end else begin
+    assign trigger = pulse_a;
+  end endgenerate
+
+  // Translate pulse/edge to a level and synchronize that into the B domain
+  reg pulse_toggle_a = 1'b0;
+  always @(posedge clk_a) begin
+    pulse_toggle_a <= rst_a ? 1'b0 : (pulse_toggle_a ^ (trigger & ~busy_a));
+  end
+
+  wire pulse_toggle_b;
+  reg  pulse_toggle_b_del = 1'b0;
+  wire handshake_toggle_a;
+
+  synchronizer #(
+    .STAGES(STAGES), .INITIAL_VAL(0)
+  ) toggle_sync_i (
+    .clk(clk_b), .rst(1'b0), .in(pulse_toggle_a), .out(pulse_toggle_b)
+  );
+
+  // Handshake toggle signal back into the A domain to deassert busy
+  synchronizer #(
+    .STAGES(STAGES), .INITIAL_VAL(0)
+  ) handshake_sync_i (
+    .clk(clk_a), .rst(1'b0), .in(pulse_toggle_b_del), .out(handshake_toggle_a)
+  );
+
+  always @(posedge clk_b) begin
+    pulse_toggle_b_del <= pulse_toggle_b;
+  end
+
+  assign pulse_b = pulse_toggle_b_del ^ pulse_toggle_b;
+  assign busy_a = pulse_toggle_a ^ handshake_toggle_a;
+
+endmodule
\ No newline at end of file