Rename entity xoroshiro128 and change interface.

* Rename to rng_xoroshiro128plus
 * Change interface to valid/ready-based stream (not tested).

rtl/rng_xoroshiro128plus.vhdl

@@ -4,7 +4,7 @@
 --  Author: Joris van Rantwijk <joris@jorisvr.nl>
 --
 --  This is a 64-bit random number generator in synthesizable VHDL.
---  The generator produces 64 new random bits on every (enabled) clock cycle.
+--  The generator can produce 64 new random bits on every clock cycle.
 --
 --  The algorithm "xoroshiro128+" is by David Blackman and Sebastiano Vigna.
 --  See also http://xoroshiro.di.unimi.it/
@@ -14,8 +14,8 @@
 --  to initialize the generator at reset. The generator also supports
 --  re-seeding at run time.
 --
---  After reset, at least one clock cycle is needed before valid
---  random data appears on the output.
+--  After reset and after re-seeding, at least one clock cycle is needed
+--  before valid random data appears on the output.
 --
 --  NOTE: This is not a cryptographic random number generator.
 --
@@ -39,7 +39,7 @@ use ieee.std_logic_1164.all;
 use ieee.numeric_std.all;
 
 
-entity xoroshiro128plus is
+entity rng_xoroshiro128plus is
 
     generic (
         -- Default seed value.
@@ -53,29 +53,37 @@ entity xoroshiro128plus is
         -- Synchronous reset, active high.
         rst:        in  std_logic;
 
-        -- Clock enable, active high.
-        enable:     in  std_logic;
-
-        -- High to re-seed the generator (requires enable = '1').
+        -- High to request re-seeding of the generator.
         reseed:     in  std_logic;
 
         -- New seed value (must be valid when reseed = '1').
         newseed:    in  std_logic_vector(127 downto 0);
 
-        -- Output value.
-        -- A new value appears on every rising clock edge where enable = '1'.
-        output:     out std_logic_vector(63 downto 0) );
+        -- High when the user accepts the current random data word
+        -- and requests new random data for the next clock cycle.
+        out_ready:  in  std_logic;
+
+        -- High when valid random data is available on the output.
+        -- This signal is low during the first clock cycle after reset and
+        -- after re-seeding, and high in all other cases.
+        out_valid:  out std_logic;
+
+        -- Random output data (valid when out_valid = '1').
+        -- A new random word appears after every rising clock edge
+        -- where out_ready = '1'.
+        out_data:   out std_logic_vector(63 downto 0) );
 
 end entity;
 
 
-architecture xoroshiro128plus_arch of xoroshiro128plus is
+architecture xoroshiro128plus_arch of rng_xoroshiro128plus is
 
     -- Internal state of RNG.
     signal reg_state_s0:    std_logic_vector(63 downto 0) := init_seed(63 downto 0);
     signal reg_state_s1:    std_logic_vector(63 downto 0) := init_seed(127 downto 64);
 
     -- Output register.
+    signal reg_valid:       std_logic;
     signal reg_output:      std_logic_vector(63 downto 0) := (others => '0');
 
     -- Shift left.
@@ -105,16 +113,18 @@ architecture xoroshiro128plus_arch of xoroshiro128plus is
 begin
 
     -- Drive output signal.
-    output      <= reg_output;
+    out_valid   <= reg_valid;
+    out_data    <= reg_output;
 
     -- Synchronous process.
     process (clk) is
     begin
         if rising_edge(clk) then
 
-            if enable = '1' then
+            if out_ready = '1' or reg_valid = '0' then
 
                 -- Prepare output word.
+                reg_valid       <= '1';
                 reg_output      <= std_logic_vector(unsigned(reg_state_s0) +
                                                     unsigned(reg_state_s1));
 
@@ -128,18 +138,20 @@ begin
                 reg_state_s1    <= rotl(reg_state_s0, 36) xor
                                    rotl(reg_state_s1, 36);
 
+            end if;
+
             -- Re-seed function.
             if reseed = '1' then
                 reg_state_s0    <= newseed(63 downto 0);
                 reg_state_s1    <= newseed(127 downto 64);
-                end if;
-
+                reg_valid       <= '0';
             end if;
 
             -- Synchronous reset.
             if rst = '1' then
                 reg_state_s0    <= init_seed(63 downto 0);
                 reg_state_s1    <= init_seed(127 downto 64);
+                reg_valid       <= '0';
                 reg_output      <= (others => '0');
             end if;