urv_fetch: add im_rd_o output

This signal is active when an instruction is read from memory. This also indicates when the bus is not used (and therefore could be used by a scrubber)

urv_fetch: add im_rd_o output
This signal is active when an instruction is read from memory. This also indicates when the bus is not used (and therefore could be used by a scrubber)
edf32c1b · Tristan Gingold · 10f8dc29 · edf32c1b · edf32c1b · edf32c1b
Commit edf32c1b authored Dec 13, 2021 by Tristan Gingold
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Showing with 24 additions and 12 deletions

urv_cpu.v rtl/urv_cpu.v +2 -0

urv_fetch.v rtl/urv_fetch.v +15 -6

urv_pkg.vhd rtl/urv_pkg.vhd +7 -6

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--- a/rtl/urv_cpu.v
+++ b/rtl/urv_cpu.v
@@ -46,6 +46,7 @@ module urv_cpu
   // instruction mem I/F
   output [31:0] im_addr_o,
+   output        im_rd_o, 
   input [31:0]  im_data_i,
   input         im_valid_i,
@@ -167,6 +168,7 @@ module urv_cpu
      // instruction memory
      .im_addr_o(im_addr_o),
+      .im_rd_o(im_rd_o),
      .im_data_i(im_data_i),
      .im_valid_i(im_valid_i),

--- a/rtl/urv_fetch.v
+++ b/rtl/urv_fetch.v
@@ -35,6 +35,7 @@ module urv_fetch
 //  Instruction memory
 output [31:0]     im_addr_o,
+ output            im_rd_o, 
 input [31:0]      im_data_i,
 input             im_valid_i,
@@ -64,18 +65,22 @@ module urv_fetch
   reg [31:0]  pc_next;
   reg         dbg_mode;
   reg [2:0]   pipeline_cnt;
+   wire        frozen;
+   assign frozen = (f_stall_i 
+                    || dbg_mode || dbg_force_i || pipeline_cnt != 0);
   always@*
-     if( x_bra_i )
+     if (x_bra_i)
       pc_next <= x_pc_bra_i;
-     else if (!rst_d || f_stall_i || !im_valid_i
+     else if (rst_d || frozen || !im_valid_i)
-              || dbg_mode || dbg_force_i || pipeline_cnt != 0)
       pc_next <= pc;
     else
       pc_next <= pc + 4;
   // Start fetching the next instruction
   assign im_addr_o = pc_next;
+   assign im_rd_o = x_bra_i || !(frozen || rst_i);
   assign dbg_enabled_o = dbg_mode;
   assign dbg_insn_ready_o = pipeline_cnt == 4;
@@ -97,11 +102,11 @@ module urv_fetch
          //  The instruction won't be valid on the next cycle, as the
          //  instruction memory is registered.
-	  rst_d <= 0;
+	  rst_d <= 1;
       end
     else
       begin
-	  rst_d <= 1;
+	  rst_d <= 0;
 	  if (!f_stall_i)
            begin
@@ -111,6 +116,8 @@ module urv_fetch
               if(!dbg_mode
                  && (dbg_force_i || x_dbg_toggle_i || pipeline_cnt != 0))
                 begin
+                    //  Enter or entering in debug mode
                    //  Stall until the debug mode is set (pipeline flushed).
                    f_valid_o <= 0;
                    //  Ebreak enters directly in the debug mode.  As it is
@@ -125,6 +132,7 @@ module urv_fetch
                 end
               else if(dbg_mode)
                 begin
+                    //  In debug mode
                    if (x_dbg_toggle_i)
                      begin
                         //  Leave debug mode immediately.
@@ -147,7 +155,8 @@ module urv_fetch
                 begin
 	            f_ir_o <= im_data_i;
                    // A branch invalidate the current instruction.
-	            f_valid_o <= (rst_d && !x_bra_i);
+                    // Not valid on the first cycle
+	            f_valid_o <= (!rst_d && !x_bra_i);
 	         end
               else
                 begin

--- a/rtl/urv_pkg.vhd
+++ b/rtl/urv_pkg.vhd
@@ -15,16 +15,17 @@ package urv_pkg is
            rst_i   : in  std_logic;
            irq_i   : in  std_logic;
            -- instruction mem I/F
-            im_addr_o : out std_logic_vector (31 downto 0);
+            im_addr_o  : out std_logic_vector (31 downto 0);
-            im_data_i : in  std_logic_vector (31 downto 0);
+            im_rd_o    : out std_logic;
-            im_valid_i : in std_logic;
+            im_data_i  : in  std_logic_vector (31 downto 0);
+            im_valid_i : in  std_logic;
            -- data mem I/F
            -- The interface is pipelined: store/load are asserted for one cycle
            -- and then store_done/load_done is awaited.
-            dm_addr_o : out std_logic_vector (31 downto 0);
+            dm_addr_o        : out std_logic_vector (31 downto 0);
-            dm_data_s_o : out std_logic_vector (31 downto 0);
+            dm_data_s_o      : out std_logic_vector (31 downto 0);
-            dm_data_l_i : in std_logic_vector (31 downto 0);
+            dm_data_l_i      : in  std_logic_vector (31 downto 0);
            dm_data_select_o : out std_logic_vector (3 downto 0);
            dm_store_o : out std_logic;