1 files changed, 67 insertions, 0 deletions

diff --git a/arbiter.v b/arbiter.v
new file mode 100644
index 0000000..0e83403
--- /dev/null
+++ b/arbiter.v
@@ -0,0 +1,67 @@
+/* Arbiter
+ * A module that grants access to a shared ressource (e.g a bus)
+ * to an arbitrary amount of drivers. A driver of the shared resource
+ * has a bit on the `request` line and fires it up when it wants to use
+ * the resource. The arbiter fires the corresponding `grant` bit when
+ * the driver is allowed to use the resource */
+module arbiter #(
+    parameter NUM_CLIENTS = 4
+) (
+    input clk,
+
+    // Request bits, each driver has its request line
+    input[NUM_CLIENTS-1:0] request,
+
+    // Grant bits, each driver has its grant line
+    output[NUM_CLIENTS-1:0] grant
+);
+    // The current round of the arbiter
+    reg[NUM_CLIENTS-1:0] round = 1;
+
+    // A circle of wires used by the arbiter to
+    // indicate that a driver that has the priority
+    // to use the resource yields it to the next driver
+    // because it doesn't need to use the resource
+    wire pass_on_rollover;
+    /* verilator lint_off UNOPTFLAT */
+    logic pass_on;
+    assign pass_on_rollover = pass_on; // NOTE: assign is applied on last assignment to wire
+    
+    // Logic bitfield used to generate the grant bus
+    logic[NUM_CLIENTS-1:0] grant_gen;
+    assign grant = grant_gen;
+
+    always @(posedge clk) begin
+        // Keep current grant as round, allowing a selected driver
+        // to freeze the arbiter during the entire duration of its
+        // resource utilization
+        round <= |grant ?
+            grant : { round[NUM_CLIENTS-2:0], round[NUM_CLIENTS-1] };
+    end
+
+    always_comb begin
+        pass_on = pass_on_rollover;
+
+        for (integer i = 0; i < NUM_CLIENTS; i++) begin
+            logic has_priority;
+
+            // Current driver has priority if it's its round
+            // or previous driver has passed on its
+            has_priority = round[i] || pass_on;
+
+            // By default, do not grant to driver
+            grant_gen[i] = 0;
+
+            // By default, do not pass on priority
+            // to next driver (current may not have it)
+            pass_on = 0;
+        
+            if (has_priority) begin
+                if (request[i])
+                    grant_gen[i] = 1;
+                else
+                    pass_on = 1;
+            end
+        end
+    end
+endmodule
\ No newline at end of file