fpga: x400: Fix SPI trigger clock crossing

1 files changed, 37 insertions, 8 deletions

diff --git a/fpga/usrp3/top/x400/x4xx_gpio_spi.v b/fpga/usrp3/top/x400/x4xx_gpio_spi.v
index 25591d570..e9d3c0fc0 100644
--- a/fpga/usrp3/top/x400/x4xx_gpio_spi.v
+++ b/fpga/usrp3/top/x400/x4xx_gpio_spi.v
@@ -369,19 +369,48 @@ module x4xx_gpio_spi #(
     assign readback_stb_extended = readback_stb;
 
   `else // X410
-    //  Register set_stb for use in 2x domain.
+
+  // We only trigger one cycle of set_stb_2x per state change. This way the latency
+  // is deterministic from the first change and aligned to the correct address,
+  // without the need to pipeline the signal.
+  //               ┐     ┌─────┐     ┌─────┐     ┌─────┐     ┌─────┐     ┌─────┐
+  //clk          : └─────┘     └─────┘     └─────┘     └─────┘     └─────┘     └───
+  //               xxxxxx/          \/          \/          \xxxxxxxxxxxxxxxxxxxxxx
+  //state        : xxxxxx\   ctrl   /\   div    /\   data   /xxxxxxxxxxxxxxxxxxxxxx
+  //               xxxxxxxxxxxxxxxxxx/          \/          \/          \xxxxxxxxxx
+  //state_dlyd   : xxxxxxxxxxxxxxxxxx\   ctrl   /\   div    /\   data   /xxxxxxxxxx
+  //               ┐                 ┌───────────────────────────────────┐
+  //set_stb      : └─────────────────┘                                   └─────────
+  //               xxxxxxxxxxxxxxxxxx/          \/          \/          \xxxxxxxxxx
+  //set_addr     : xxxxxxxxxxxxxxxxxx\   0x1    /\   0x0    /\   0x2    /xxxxxxxxxx
+  //                ──┐  ┌──┐  ┌──┐  ┌──┐  ┌──┐  ┌──┐  ┌──┐  ┌──┐  ┌──┐  ┌──┐  ┌──┐
+  //clk_2x       :    └──┘  └──┘  └──┘  └──┘  └──┘  └──┘  └──┘  └──┘  └──┘  └──┘  └
+  //               xxxxxxxxxxxxxxxxxxxxxxxx/          \/          \/          \xxxx
+  //state_dlyd_2x: xxxxxxxxxxxxxxxxxxxxxxxx\   ctrl   /\   div    /\   data   /xxxx
+  //                                       ┌─────┐     ┌─────┐     ┌─────┐
+  //set_stb_2x   : ────────────────────────┘     └─────┘     └─────┘     └─────────
+  //                                                                     ┌─────┐
+  //trigger_spi  : ──────────────────────────────────────────────────────┘     └───
+  //
+
     reg set_stb_2x = 1'b0;
-    reg ctrlport_clk_phase = 1'b1;
+    reg [2:0] state_dlyd, state_dlyd_2x = IDLE;
+
+    always @ (posedge ctrlport_clk) begin
+      if (ctrlport_rst) begin
+        state_dlyd <= IDLE;
+      end else begin
+        state_dlyd <= state;
+      end
+    end
 
     always @ (posedge ctrlport_clk_2x) begin
       if (ctrlport_rst) begin
-        ctrlport_clk_phase <= 1'b1;
-        set_stb_2x         <= 1'b0;
+        state_dlyd_2x <= IDLE;
+        set_stb_2x <= 1'b0;
       end else begin
-        // Assert strobe only during a single 2x cycle of the
-        // 1x pulse, when 1x clock is low.
-        set_stb_2x         <= ctrlport_clk_phase & set_stb;
-        ctrlport_clk_phase <= ~ctrlport_clk_phase;
+        state_dlyd_2x <= state_dlyd;
+        set_stb_2x <= set_stb && (state_dlyd_2x != state_dlyd);
       end
     end