fpga: lib: Pipeline ctrlport_timer

This pipelines ctrlport_timer to eliminate the long combinational path
caused by the time comparisons. This change also removes the
PRECISION_BITS parameter and converts it to a signal named
time_ignore_bits.

1 files changed, 81 insertions, 24 deletions

diff --git a/fpga/usrp3/lib/rfnoc/utils/ctrlport_timer.v b/fpga/usrp3/lib/rfnoc/utils/ctrlport_timer.v
index 293ee6559..52f4a90ca 100644
--- a/fpga/usrp3/lib/rfnoc/utils/ctrlport_timer.v
+++ b/fpga/usrp3/lib/rfnoc/utils/ctrlport_timer.v
@@ -1,9 +1,10 @@
 //
-// Copyright 2018 Ettus Research, A National Instruments Company
+// Copyright 2021 Ettus Research, A National Instruments Brand
 //
 // SPDX-License-Identifier: LGPL-3.0-or-later
 //
 // Module: ctrlport_timer
+//
 // Description:
 //   The Control-Port timer module converts an asynchronous timed
 //   transaction into a synchronous blocking transaction. This
@@ -15,23 +16,23 @@
 //   updates every time the time strobe is asserted.
 //
 // Parameters:
-//   - PRECISION_BITS : The number of bits to ignore when performing a 
-//                      time comparison to determine execution time.
-//   - EXEC_LATE_CMDS : If a command is late, a TSERR response is sent.
-//                      If EXEC_LATE_CMDS = 1, then the late command will
-//                      be passed to the output regardless of the TSERR.
+//   - EXEC_LATE_CMDS : If EXEC_LATE_CMDS = 0 and a command is late, then a
+//                      TSERR response is returned. If EXEC_LATE_CMDS = 1, then
+//                      a late command will be passed to the output as if it
+//                      were on time.
 //
 // Signals:
-//   - time_now*      : The time_now signal is the current time and the stb
-//                      signal indicates that the time_now is valid.
-//   - s_ctrlport_*   : The slave Control-Port bus.
-//                      This must have the has_time and time signals.
-//   - m_ctrlport_*   : The master Control-Port bus.
-//                      This will not have the has_time and time signals.
+//   - time_now*        : The time_now signal is the current time and the stb
+//                        signal indicates that the time_now is valid.
+//   - time_ignore_bits : Number of least-significant bits to ignore when doing
+//                        time comparisons. This should be a constant.
+//   - s_ctrlport_*     : The slave Control-Port bus.
+//                        This must have the has_time and time signals.
+//   - m_ctrlport_*     : The master Control-Port bus.
+//                        This will not have the has_time and time signals.
 
 module ctrlport_timer #(
-  parameter       PRECISION_BITS = 0,
-  parameter [0:0] EXEC_LATE_CMDS = 0
+  parameter [0:0] EXEC_LATE_CMDS = 1
 )(
   // Clocks and Resets
   input  wire         clk,
@@ -39,6 +40,7 @@ module ctrlport_timer #(
   // Timestamp (synchronous to clk)
   input  wire [63:0]  time_now,
   input  wire         time_now_stb,
+  input  wire [3:0]   time_ignore_bits,
   // Control Port Master (Request)
   input  wire         s_ctrlport_req_wr,
   input  wire         s_ctrlport_req_rd,
@@ -68,11 +70,13 @@ module ctrlport_timer #(
 
   // Control triggers:
   // - pending: A command is waiting on the input port
-  // - ontime: The timed command is due for execution (on time)
+  // - has_time: The command is timed
+  // - on_time: The timed command is due for execution (on time)
   // - late: The timed command is late
   // - exec: Execute the command (pass it to the output)
   // - consume: Consume the input command
-  wire         pending, ontime, late, exec, consume;
+  reg          pending, has_time, on_time, late;
+  wire         exec, consume, valid;
   // Cached values for input command
   wire         cached_req_wr, cached_req_rd;
   wire [19:0]  cached_req_addr;
@@ -80,6 +84,7 @@ module ctrlport_timer #(
   wire [3:0]   cached_req_byte_en;
   wire         cached_req_has_time;
   wire [63:0]  cached_req_time;
+  wire         cached_req_valid;
 
   axi_fifo_flop #(.WIDTH(1+1+20+32+4+1+64)) req_cache_i (
     .clk(clk), .reset(rst), .clear(1'b0),
@@ -88,18 +93,70 @@ module ctrlport_timer #(
     .i_tvalid(s_ctrlport_req_wr | s_ctrlport_req_rd), .i_tready(),
     .o_tdata({cached_req_wr, cached_req_rd, cached_req_addr, cached_req_data,
               cached_req_byte_en, cached_req_has_time, cached_req_time}),
-    .o_tvalid(pending), .o_tready(consume),
+    .o_tvalid(cached_req_valid), .o_tready(consume),
     .occupied(), .space()
   );
 
-  // Command is on time
-  assign ontime = cached_req_has_time && pending && time_now_stb && 
-    (cached_req_time[63:PRECISION_BITS] == time_now[63:PRECISION_BITS]);
-  // Command is late
-  assign late = cached_req_has_time && pending && time_now_stb && 
-    (cached_req_time[63:PRECISION_BITS] < time_now[63:PRECISION_BITS]);
+  // Create a mask so that we can ignore the lower time_ignore_bits of the time.
+  wire [63:0] mask = (~64'd0 << time_ignore_bits);
+
+  // Pipeline registers
+  reg        has_time_0;
+  reg        has_time_1;
+  reg        valid_0;
+  reg        valid_1;
+  reg [63:0] time_0;
+  reg        time_stb_1, time_stb_0;
+  reg [63:0] req_time_0;
+  reg        on_time_1;
+  reg        late_1;
+
+  // This always block pipelines the time comparisons and other condition
+  // checks, so we don't have a long combinational path here. This adds some
+  // delay to the execution of timed commands, but this block is not intended
+  // for cycle-accurate register updates.
+  always @(posedge clk) begin
+    if (rst || consume) begin
+      has_time_0 <= 'bX;
+      time_stb_0 <= 'bX;
+      time_0     <= 'bX;
+      req_time_0 <= 'bX;
+      has_time_1 <= 'bX;
+      time_stb_1 <= 'bX;
+      on_time_1  <= 'bX;
+      late_1     <= 'bX;
+      has_time   <= 'bX;
+      on_time    <= 'bX;
+      // These need to be zeroed, to clear the pipeline:
+      valid_0    <= 1'b0;
+      valid_1    <= 1'b0;
+      pending    <= 1'b0;
+      late       <= 1'b0;
+    end else begin
+      // Stage 0
+      valid_0    <= cached_req_valid;
+      has_time_0 <= cached_req_has_time;
+      time_stb_0 <= time_now_stb;
+      time_0     <= time_now & mask;
+      req_time_0 <= cached_req_time & mask;
+
+      // Stage 1
+      valid_1    <= valid_0;
+      has_time_1 <= has_time_0;
+      time_stb_1 <= time_stb_0;
+      on_time_1  <= req_time_0 == time_0;
+      late_1     <= req_time_0 <  time_0;
+
+      // Stage 3
+      pending  <= valid_1;
+      has_time <= has_time_1;
+      on_time  <= valid_1 && has_time_1 && time_stb_1 && on_time_1;
+      late     <= valid_1 && has_time_1 && time_stb_1 && late_1;
+    end
+  end
+
   // Logic to pass cmd forward
-  assign exec = pending && (!cached_req_has_time || ontime || (EXEC_LATE_CMDS && late));
+  assign exec    = pending && (!has_time || on_time || (EXEC_LATE_CMDS && late));
   assign consume = exec || late;
 
   assign m_ctrlport_req_wr      = cached_req_wr & exec;