prim: Add arbiter module and testbench

Signed-off-by: Alex Forencich <alex@alexforencich.com>

rtl/prim/taxi_arbiter.sv

@@ -0,0 +1,155 @@
+// SPDX-License-Identifier: CERN-OHL-S-2.0
+/*
+
+Copyright (c) 2014-2025 FPGA Ninja, LLC
+
+Authors:
+- Alex Forencich
+
+*/
+
+`resetall
+`timescale 1ns / 1ps
+`default_nettype none
+
+/*
+ * Arbiter module
+ */
+module taxi_arbiter #
+(
+    parameter PORTS = 4,
+    // select round robin arbitration
+    parameter logic ARB_ROUND_ROBIN = 1'b1,
+    // blocking arbiter enable
+    parameter logic ARB_BLOCK = 1'b1,
+    // block on acknowledge assert when nonzero, request deassert when 0
+    parameter logic ARB_BLOCK_ACK = 1'b0,
+    // LSB priority selection
+    parameter logic LSB_HIGH_PRIO = 1'b0
+)
+(
+    input  wire logic                      clk,
+    input  wire logic                      rst,
+
+    input  wire logic [PORTS-1:0]          req,
+    input  wire logic [PORTS-1:0]          ack,
+
+    output wire logic                      grant_valid,
+    output wire logic [PORTS-1:0]          grant,
+    output wire logic [$clog2(PORTS)-1:0]  grant_index
+);
+
+localparam CL_PORTS = $clog2(PORTS);
+
+logic [PORTS-1:0] grant_reg = 'd0, grant_next;
+logic grant_valid_reg = 1'b0, grant_valid_next;
+logic [CL_PORTS-1:0] grant_index_reg = 'd0, grant_index_next;
+
+assign grant_valid = grant_valid_reg;
+assign grant = grant_reg;
+assign grant_index = grant_index_reg;
+
+wire req_valid;
+wire [CL_PORTS-1:0] req_index;
+wire [PORTS-1:0] req_mask;
+
+taxi_penc #(
+    .WIDTH(PORTS),
+    .LSB_HIGH_PRIO(LSB_HIGH_PRIO)
+)
+penc_inst (
+    .input_mask(req),
+    .output_valid(req_valid),
+    .output_index(req_index),
+    .output_mask(req_mask)
+);
+
+logic [PORTS-1:0] mask_reg = 'd0, mask_next;
+
+wire masked_req_valid;
+wire [CL_PORTS-1:0] masked_req_index;
+wire [PORTS-1:0] masked_req_mask;
+
+if (ARB_ROUND_ROBIN) begin
+
+    taxi_penc #(
+        .WIDTH(PORTS),
+        .LSB_HIGH_PRIO(LSB_HIGH_PRIO)
+    )
+    penc_masked (
+        .input_mask(req & mask_reg),
+        .output_valid(masked_req_valid),
+        .output_index(masked_req_index),
+        .output_mask(masked_req_mask)
+    );
+
+end else begin
+
+    assign masked_req_valid = 1'b0;
+    assign masked_req_index = '0;
+    assign masked_req_mask = '0;
+
+end
+
+always_comb begin
+    grant_next = 'd0;
+    grant_valid_next = 1'b0;
+    grant_index_next = 'd0;
+    mask_next = mask_reg;
+
+    if (ARB_BLOCK && !ARB_BLOCK_ACK && ((grant_reg & req) != 0)) begin
+        // granted req still asserted; hold it
+        grant_valid_next = grant_valid_reg;
+        grant_next = grant_reg;
+        grant_index_next = grant_index_reg;
+    end else if (ARB_BLOCK && ARB_BLOCK_ACK && grant_valid && ((grant_reg & ack) == 0)) begin
+        // granted req not yet acknowledged; hold it
+        grant_valid_next = grant_valid_reg;
+        grant_next = grant_reg;
+        grant_index_next = grant_index_reg;
+    end else if (req_valid) begin
+        if (ARB_ROUND_ROBIN) begin
+            if (masked_req_valid) begin
+                grant_valid_next = 1'b1;
+                grant_next = masked_req_mask;
+                grant_index_next = masked_req_index;
+                if (LSB_HIGH_PRIO) begin
+                    mask_next = {PORTS{1'b1}} << (masked_req_index + 1);
+                end else begin
+                    mask_next = {PORTS{1'b1}} >> ((CL_PORTS+1)'(PORTS) - masked_req_index);
+                end
+            end else begin
+                grant_valid_next = 1;
+                grant_next = req_mask;
+                grant_index_next = req_index;
+                if (LSB_HIGH_PRIO) begin
+                    mask_next = {PORTS{1'b1}} << (req_index + 1);
+                end else begin
+                    mask_next = {PORTS{1'b1}} >> ((CL_PORTS+1)'(PORTS) - req_index);
+                end
+            end
+        end else begin
+            grant_valid_next = 1'b1;
+            grant_next = req_mask;
+            grant_index_next = req_index;
+        end
+    end
+end
+
+always_ff @(posedge clk) begin
+    grant_reg <= grant_next;
+    grant_valid_reg <= grant_valid_next;
+    grant_index_reg <= grant_index_next;
+    mask_reg <= mask_next;
+
+    if (rst) begin
+        grant_reg <= 'd0;
+        grant_valid_reg <= 1'b0;
+        grant_index_reg <= 'd0;
+        mask_reg <= 'd0;
+    end
+end
+
+endmodule
+
+`resetall