Trying to verify Verilog/VHDL designs with formal methods and tools
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library ieee;
use ieee.std_logic_1164.all;
use ieee.numeric_std.all;
entity fifo is
generic (
Formal : boolean := true;
Depth : positive := 16;
Width : positive := 16
);
port (
Reset_n_i : in std_logic;
Clk_i : in std_logic;
-- write
Wen_i : in std_logic;
Din_i : in std_logic_vector(Width-1 downto 0);
Full_o : out std_logic;
Werror_o : out std_logic;
-- read
Ren_i : in std_logic;
Dout_o : out std_logic_vector(Width-1 downto 0);
Empty_o : out std_logic;
Rerror_o : out std_logic
);
end entity fifo;
architecture rtl of fifo is
subtype t_fifo_pnt is natural range 0 to Depth-1;
signal s_write_pnt : t_fifo_pnt;
signal s_read_pnt : t_fifo_pnt;
type t_fifo_mem is array (t_fifo_pnt'low to t_fifo_pnt'high) of std_logic_vector(Din_i'range);
signal s_fifo_mem : t_fifo_mem;
function incr_pnt (data : t_fifo_pnt) return t_fifo_pnt is
begin
if (data = t_fifo_mem'high) then
return 0;
end if;
return data + 1;
end function incr_pnt;
begin
WriteP : process (Reset_n_i, Clk_i) is
begin
if (Reset_n_i = '0') then
s_write_pnt <= 0;
Werror_o <= '0';
elsif (rising_edge(Clk_i)) then
Werror_o <= Wen_i and Full_o;
if (Wen_i = '1' and Full_o = '0') then
s_fifo_mem(s_write_pnt) <= Din_i;
s_write_pnt <= incr_pnt(s_write_pnt);
end if;
end if;
end process WriteP;
ReadP : process (Reset_n_i, Clk_i) is
begin
if (Reset_n_i = '0') then
s_read_pnt <= 0;
Rerror_o <= '0';
elsif (rising_edge(Clk_i)) then
Rerror_o <= Ren_i and Empty_o;
if (Ren_i = '1' and Empty_o = '0') then
Dout_o <= s_fifo_mem(s_read_pnt);
s_read_pnt <= incr_pnt(s_read_pnt);
end if;
end if;
end process ReadP;
FlagsP : process (Reset_n_i, Clk_i) is
begin
if (Reset_n_i = '0') then
Full_o <= '0';
Empty_o <= '1';
elsif (rising_edge(Clk_i)) then
if (Wen_i = '1' and Ren_i = '0') then
if ((s_write_pnt = s_read_pnt - 1) or
(s_write_pnt = t_fifo_mem'high and s_read_pnt = t_fifo_mem'low)) then
Full_o <= '1';
end if;
Empty_o <= '0';
end if;
if (Ren_i = '1' and Wen_i = '0') then
if ((s_read_pnt = s_write_pnt - 1) or
(s_read_pnt = t_fifo_mem'high and s_write_pnt = t_fifo_mem'low)) then
Empty_o <= '1';
end if;
Full_o <= '0';
end if;
end if;
end process FlagsP;
FormalG : if Formal generate
default clock is rising_edge(Clk_i);
-- Initial reset
restrict {not Reset_n_i[*3]; Reset_n_i[+]}[*1];
-- Inputs are low during reset for simplicity
assume always not Reset_n_i -> not Wen_i and not Ren_i;
-- Asynchronous (unclocked) Reset asserts
FULL_RESET : process (all) is
begin
if (not Reset_n_i) then
RESET_FULL : assert not Full_o;
RESET_EMPTY : assert Empty_o;
RESET_WERROR : assert not Werror_o;
RESET_RERROR : assert not Rerror_o;
RESET_WRITE_PNT : assert s_write_pnt = 0;
RESET_READ_PNT : assert s_read_pnt = 0;
end if;
end process;
-- No write pointer change when writing into full fifo
WRITE_PNT_STABLE_WHEN_FULL : assert always
Wen_i and Full_o ->
next stable(s_write_pnt);
-- No read pointer change when reading from empty fifo
READ_PNT_STABLE_WHEN_EMPTY : assert always
Ren_i and Empty_o ->
next stable(s_read_pnt);
-- Full when write and no read and write pointer ran up to read pointer
FULL : assert always
Wen_i and not Ren_i and
(s_write_pnt = s_read_pnt - 1 or s_write_pnt = t_fifo_pnt'high and s_read_pnt = t_fifo_pnt'low) ->
next Full_o;
-- Not full when read and no write
NOT_FULL : assert always
not Wen_i and Ren_i ->
next not Full_o;
-- Empty when read and no write and read pointer ran up to write pointer
EMPTY : assert always
not Wen_i and Ren_i and
(s_read_pnt = s_write_pnt - 1 or s_read_pnt = t_fifo_pnt'high and s_write_pnt = t_fifo_pnt'low) ->
next Empty_o;
-- Not empty when write and no read
NOT_EMPTY : assert always
Wen_i and not Ren_i ->
next not Empty_o;
-- Write error when writing into full fifo
WERROR : assert always
Wen_i and Full_o ->
next Werror_o;
-- No write error when writing into not full fifo
NO_WERROR : assert always
Wen_i and not Full_o ->
next not Werror_o;
-- Read error when reading from empty fifo
RERROR : assert always
Ren_i and Empty_o ->
next Rerror_o;
-- No read error when reading from not empty fifo
NO_RERROR : assert always
Ren_i and not Empty_o ->
next not Rerror_o;
-- Write pointer increment when writing into not full fifo
-- and write pointer isn't at end value
WRITE_PNT_INCR : assert always
Wen_i and not Full_o and s_write_pnt /= t_fifo_pnt'high ->
next s_write_pnt = prev(s_write_pnt) + 1;
-- Write pointer wraparound when writing into not full fifo
-- and write pointer is at end value
WRITE_PNT_WRAP : assert always
Wen_i and not Full_o and s_write_pnt = t_fifo_pnt'high ->
next s_write_pnt = 0;
-- Read pointer increment when reading from not empty fifo
-- and read pointer isn't at end value
READ_PNT_INCR : assert always
Ren_i and not Empty_o and s_read_pnt /= t_fifo_pnt'high ->
next s_read_pnt = prev(s_read_pnt) + 1;
-- Read pointer wraparound when reading from not empty fifo
-- and read pointer is at end value
READ_PNT_WRAP : assert always
Ren_i and not Empty_o and s_read_pnt = t_fifo_pnt'high ->
next s_read_pnt = 0;
-- Correct input data stored after valid write access
DIN_VALID : assert always
Wen_i and not Full_o ->
next s_fifo_mem(s_write_pnt - 1) = prev(Din_i);
-- Correct output data after valid read access
DOUT_VALID : assert always
Ren_i and not Empty_o ->
next Dout_o = s_fifo_mem(s_read_pnt - 1);
end generate FormalG;
end architecture rtl;