From d193ca5905c1293d3b8be3a1014dacc634ffef75 Mon Sep 17 00:00:00 2001
From: Alex Forencich <alex@alexforencich.com>
Date: Sun, 16 Nov 2014 01:58:17 -0800
Subject: [PATCH] Add LSB_PRIORITY parameter

---
 rtl/arbiter.v          | 22 +++++++++++++++++-----
 rtl/priority_encoder.v | 22 +++++++++++++++++-----
 2 files changed, 34 insertions(+), 10 deletions(-)

diff --git a/rtl/arbiter.v b/rtl/arbiter.v
index 219b4661..d6e06303 100644
--- a/rtl/arbiter.v
+++ b/rtl/arbiter.v
@@ -35,7 +35,9 @@ module arbiter #
     // arbitration type: "PRIORITY" or "ROUND_ROBIN"
     parameter TYPE = "PRIORITY",
     // block type: "NONE", "REQUEST", "ACKNOWLEDGE"
-    parameter BLOCK = "NONE"
+    parameter BLOCK = "NONE",
+    // LSB priority: "LOW", "HIGH"
+    parameter LSB_PRIORITY = "LOW"
 )
 (
     input  wire                     clk,
@@ -62,7 +64,8 @@ wire [$clog2(PORTS)-1:0] request_index;
 wire [PORTS-1:0] request_mask;
 
 priority_encoder #(
-    .WIDTH(PORTS)
+    .WIDTH(PORTS),
+    .LSB_PRIORITY(LSB_PRIORITY)
 )
 priority_encoder_inst (
     .input_unencoded(request),
@@ -78,7 +81,8 @@ wire [$clog2(PORTS)-1:0] masked_request_index;
 wire [PORTS-1:0] masked_request_mask;
 
 priority_encoder #(
-    .WIDTH(PORTS)
+    .WIDTH(PORTS),
+    .LSB_PRIORITY(LSB_PRIORITY)
 )
 priority_encoder_masked (
     .input_unencoded(request & mask_reg),
@@ -113,12 +117,20 @@ always @* begin
                 grant_valid_next = 1;
                 grant_next = masked_request_mask;
                 grant_encoded_next = masked_request_index;
-                mask_next = {PORTS{1'b1}} >> (PORTS - masked_request_index);
+                if (LSB_PRIORITY == "LOW") begin
+                    mask_next = {PORTS{1'b1}} >> (PORTS - masked_request_index);
+                end else begin
+                    mask_next = {PORTS{1'b1}} << (masked_request_index + 1);
+                end
             end else begin
                 grant_valid_next = 1;
                 grant_next = request_mask;
                 grant_encoded_next = request_index;
-                mask_next = {PORTS{1'b1}} >> (PORTS - request_index);
+                if (LSB_PRIORITY == "LOW") begin
+                    mask_next = {PORTS{1'b1}} >> (PORTS - request_index);
+                end else begin
+                    mask_next = {PORTS{1'b1}} << (request_index + 1);
+                end
             end
         end
     end
diff --git a/rtl/priority_encoder.v b/rtl/priority_encoder.v
index 03af53fe..cb380c76 100644
--- a/rtl/priority_encoder.v
+++ b/rtl/priority_encoder.v
@@ -31,7 +31,9 @@ THE SOFTWARE.
  */
 module priority_encoder #
 (
-    parameter WIDTH = 4
+    parameter WIDTH = 4,
+    // LSB priority: "LOW", "HIGH"
+    parameter LSB_PRIORITY = "LOW"
 )
 (
     input  wire [WIDTH-1:0]         input_unencoded,
@@ -48,14 +50,19 @@ generate
     if (WIDTH == 2) begin
         // two inputs - just an OR gate
         assign output_valid = |input_unencoded;
-        assign output_encoded = input_unencoded[1];
+        if (LSB_PRIORITY == "LOW") begin
+            assign output_encoded = input_unencoded[1];
+        end else begin
+            assign output_encoded = ~input_unencoded[0];
+        end
     end else begin
         // more than two inputs - split into two parts and recurse
         // also pad input to correct power-of-two width
         wire [$clog2(W2)-1:0] out1, out2;
         wire valid1, valid2;
         priority_encoder #(
-            .WIDTH(W2)
+            .WIDTH(W2),
+            .LSB_PRIORITY(LSB_PRIORITY)
         )
         priority_encoder_inst1 (
             .input_unencoded(input_unencoded[W2-1:0]),
@@ -63,7 +70,8 @@ generate
             .output_encoded(out1)
         );
         priority_encoder #(
-            .WIDTH(W2)
+            .WIDTH(W2),
+            .LSB_PRIORITY(LSB_PRIORITY)
         )
         priority_encoder_inst2 (
             .input_unencoded({{W1-WIDTH{1'b0}}, input_unencoded[WIDTH-1:W2]}),
@@ -72,7 +80,11 @@ generate
         );
         // multiplexer to select part
         assign output_valid = valid1 | valid2;
-        assign output_encoded = valid2 ? {1'b1, out2} : {1'b0, out1};
+        if (LSB_PRIORITY == "LOW") begin
+            assign output_encoded = valid2 ? {1'b1, out2} : {1'b0, out1};
+        end else begin
+            assign output_encoded = valid1 ? {1'b0, out1} : {1'b1, out2};
+        end
     end
 endgenerate