starting files and example programs

1 files changed, 75 insertions, 0 deletions

diff --git a/PIC/fifo.vhd b/PIC/fifo.vhd
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+++ b/PIC/fifo.vhd
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+library ieee;
+use ieee.std_logic_1164.all;
+use ieee.numeric_std.all;
+
+-- First-word fall-through (FWFT) FIFO.
+-- dout always presents the current head combinatorially (no read latency).
+-- rd_en advances the read pointer; the next head is visible on the same cycle.
+-- DEPTH must be a power of 2; ADDR_BITS = log2(DEPTH).
+
+entity fifo is
+  generic (
+    DEPTH     : positive := 16;
+    ADDR_BITS : positive := 4;
+    WIDTH     : positive := 8
+  );
+  port (
+    clk   : in  std_logic;
+    wr_en : in  std_logic;
+    rd_en : in  std_logic;
+    din   : in  std_logic_vector(WIDTH - 1 downto 0);
+    dout  : out std_logic_vector(WIDTH - 1 downto 0);
+    full  : out std_logic;
+    empty : out std_logic
+  );
+end entity fifo;
+
+architecture rtl of fifo is
+
+  -- MSB pointer trick: pointers are ADDR_BITS+1 wide.
+  -- Empty: read_ptr = write_ptr
+  -- Full:  (read_ptr XOR write_ptr) = DEPTH  (MSBs differ, lower bits equal)
+  signal read_ptr  : unsigned(ADDR_BITS downto 0) := (others => '0');
+  signal write_ptr : unsigned(ADDR_BITS downto 0) := (others => '0');
+
+  type mem_t is
+    array (0 to DEPTH - 1)
+    of std_logic_vector(WIDTH - 1 downto 0);
+
+  signal mem : mem_t := (others => (others => '0'));
+
+  signal empty_i : std_logic;
+  signal full_i  : std_logic;
+
+begin
+
+  empty_i <= '1' when read_ptr = write_ptr else '0';
+  full_i  <= '1' when (read_ptr xor write_ptr) = to_unsigned(DEPTH, ADDR_BITS + 1)
+             else '0';
+
+  empty <= empty_i;
+  full  <= full_i;
+
+  -- FWFT: head is always visible without asserting rd_en
+  dout <= mem(to_integer(read_ptr(ADDR_BITS - 1 downto 0)));
+
+  enqueue : process(clk)
+  begin
+    if rising_edge(clk) then
+      if wr_en = '1' and full_i = '0' then
+        mem(to_integer(write_ptr(ADDR_BITS - 1 downto 0))) <= din;
+        write_ptr <= write_ptr + 1;
+      end if;
+    end if;
+  end process enqueue;
+
+  dequeue : process(clk)
+  begin
+    if rising_edge(clk) then
+      if rd_en = '1' and empty_i = '0' then
+        read_ptr <= read_ptr + 1;
+      end if;
+    end if;
+  end process dequeue;
+
+end architecture rtl;