Minor code cleanup for MT19937 and improve testbench.

* Optimized seeding strategy now synthesizes for Spartan 6 at 300 MHz.

rtl/rng_mt19937.vhdl

@@ -15,8 +15,7 @@
 --  to initialize the generator at reset. The generator also supports
 --  re-seeded at run time.
 --
--- TODO : rewrite this thing about initialization
---  After reset, and after re-seeding, the generator needs 625 clock
+--  After reset, and after re-seeding, the generator needs 4 * 624 clock
 --  cycles to initialize its internal state. During this time, the generator
 --  is unable to provide correct output.
 --
@@ -185,7 +184,7 @@ begin
             -- Enable state machine on next cycle
             --  a) every 1st out of 4 cycles during reseeding, and
             --  b) on-demand for new output.
-            reg_enable  <= reg_reseedstate(3) or
+            reg_enable  <= (reg_reseeding and reg_reseedstate(3)) or
                            (not reg_reseeding and
                             (out_ready or not reg_valid));
 
@@ -196,17 +195,17 @@ begin
 
             -- Reseed state 2: Multiply by constant.
             if force_const_mul then
-                -- Multiply by 37.
+                -- Compute 37 * Mprev.
                 reg_seed_b  <= std_logic_vector(
                       unsigned(reg_seed_a)
                     + shift_left(unsigned(reg_seed_a), 2)
                     + shift_left(unsigned(reg_seed_a), 5));
-                -- Multiply by (2**19 - 2**15).
+                -- Compute (2**19 - 2**15) * Mprev.
                 reg_seed_b2 <= std_logic_vector(
                       shift_left(unsigned(reg_seed_a), 19)
                     - shift_left(unsigned(reg_seed_a), 15));
             else
-                -- Multiply by 1812433253.
+                -- Compute 1812433253 * Mprev.
                 -- Let synthesizer choose a multiplier implementation.
                 reg_seed_b  <= std_logic_vector(
                     mulconst(unsigned(reg_seed_a)));
@@ -214,12 +213,14 @@ begin
 
             -- Reseed state 3: Continue multiplication by constant.
             if force_const_mul then
+                -- Compute (37 + 2**6 * 37 + 2**19 - 2**15) * Mprev.
                 -- Finalize multiplication by 1812433253 =
                 -- (37 + 2**6*37 - 2**15 + 2**19 - 2**26*37)
                 reg_seed_c  <= std_logic_vector(
                       unsigned(reg_seed_b)
                     + shift_left(unsigned(reg_seed_b), 6)
                     + unsigned(reg_seed_b2));
+                -- Compute (2**32 - 2**26 * 37) * Mprev + reseed_cnt.
                 reg_seed_c2 <= std_logic_vector(
                       unsigned(reg_reseed_cnt)
                     - shift_left(unsigned(reg_seed_b), 26));
@@ -229,11 +230,14 @@ begin
 
             -- Reseed state 4: Prepare next element of initial state.
             if reg_reseeding = '1' then
-                -- Add result of multiplication to reseed counter.
                 if force_const_mul then
+                    -- Compute   (37 + 2**6 * 37 + 2**19 - 2**15) * Mprev
+                    --         + (2**32 - 2**26 * 37) * Mprev + reseed_cnt
+                    --         = 1812433253 * Mprev + reseed_cnt.
                     reg_seed_d  <= std_logic_vector(unsigned(reg_seed_c) +
                                                     unsigned(reg_seed_c2));
                 else
+                    -- Compute 1812433253 * Mprev + reseed_cnt.
                     reg_seed_d  <= std_logic_vector(unsigned(reg_seed_c) +
                                                     unsigned(reg_reseed_cnt));
                 end if;

sim/tb_mt19937.vhdl

@@ -58,6 +58,7 @@ begin
     process is
         file outf1: text is out "sim_mt19937_seed1.dat";
         file outf2: text is out "sim_mt19937_seed2.dat";
+        file outf3: text is out "sim_mt19937_seed3.dat";
         variable lin: line;
         variable nskip: integer;
         variable v: std_logic_vector(31 downto 0);
@@ -174,7 +175,58 @@ begin
 
         end loop;
 
-        -- End simulation.
+        -- Re-seed generator.
+        report "Re-seed generator";
+        s_reseed    <= '1';
+        s_newseed   <= x"0f5a3c57";
+        s_ready     <= '0';
+        wait until falling_edge(clk);
+        s_reseed    <= '0';
+        s_newseed   <= (others => '0');
+
+        -- Give generator more than enough time to complete initialization.
+        for i in 0 to 4*624 + 500 loop
+            wait until falling_edge(clk);
+        end loop;
+
+        s_ready     <= '1';
+
+        -- Produce numbers
+        for i in 0 to 999 loop
+
+            -- Check that output is valid.
+            assert s_valid = '1' report "Output not VALID";
+
+            -- Write output to file.
+            write(lin, "0x" & to_hex_string(s_data));
+            writeline(outf3, lin);
+
+            -- Sometimes skip cycles.
+            if i mod 5 = 2 then
+                nskip := 1;
+                if i mod 3 = 0 then
+                    nskip := nskip + 1;
+                end if;
+                if i mod 11 = 0 then
+                    nskip := nskip + 1;
+                end if;
+
+                v := s_data;
+                s_ready <= '0';
+                for t in 1 to nskip loop
+                    wait until falling_edge(clk);
+                    assert s_valid = '1' report "Output not valid";
+                    assert s_data = v report "Output changed while not ready";
+                end loop;
+                s_ready <= '1';
+            end if;
+
+            -- Go to next cycle.
+            wait until falling_edge(clk);
+
+        end loop;
+
+         -- End simulation.
         report "End testbench";
 
         clock_active    <= false;