doubt in this program plz tell why this error is coming and what modifications i have to do

[chaitanyakurmala]

this is a program for asynchronous FIFO such that it is operating in
between two clock domains transfering 8bit parallel data from one
domain to another.
there are 16 fifo buffers in between the two clock modules.the code is
written such that it is shifting the data and transfering always the
data in buffer1 to clockmodule 2.
the error coming is
Xst:787 - "D:/ise projects/fifo12/fifo1p.vhd" line 94: Index value <17>
is not in Range of array <fifo_buffer>.
i indicated the line in which error is coming in inverted columns and
also put stars between start and end of line
library IEEE;
use IEEE.STD_LOGIC_1164.ALL;
use IEEE.STD_LOGIC_ARITH.ALL;
use IEEE.STD_LOGIC_UNSIGNED.ALL;
use work.bus_package.all;

---- Uncomment the following library declaration if instantiating
---- any Xilinx primitives in this code.
--library UNISIM;
--use UNISIM.VComponents.all;

entity fifo1p is
Port ( chan_in : in channel;
chan_out : out channel := init_chan_values;
ack_in : out STD_LOGIC;
ack_out : in STD_LOGIC := '0';
clk_in : in STD_LOGIC;
clk_out : in STD_LOGIC);
end fifo1p;

architecture Behavioral of fifo1p is

-- FIFO delay is that portion of delay not due to clock mismatch
constant fifo_delay : time := 1.5 ns;
constant buffer_size : natural := 16;

type buffer_type is array (1 to buffer_size) of channel;

signal index_delayed : natural := 0;
--constant i:natural :=1;

begin

behavior: process (clk_in, clk_out)

variable fifo_index : natural := 0;
variable i : natural ;
variable fifo_buffer : buffer_type;
variable req_temp : std_logic := '0';

begin
--n := fifo_index;
-- Accept input
if clk_in'event and clk_in = '1' then
if chan_in.req = '1' and fifo_index < buffer_size then
fifo_index := fifo_index + 1;
index_delayed <= fifo_index after fifo_delay;
fifo_buffer(fifo_index) := chan_in;
end if;
end if;

-- Acknowlege input
if clk_in'event and clk_in = '0' then
if chan_in.req = '0' then
ack_in <= '0' after 0.5 ns;
elsif fifo_index < buffer_size then
ack_in <= '1' after 0.5 ns;
end if;
end if;

-- Check for output acknowlegements Send output data
if clk_out'event and clk_out = '1' then
if ack_out = '1' then
req_temp := '0';
if fifo_index > 1 then
fifo_index := fifo_index - 1;
end if;
index_delayed <= fifo_index;

-- for index in 1 to fifo_index loop
-- fifo_buffer(index) := fifo_buffer(index + 1);
--end loop;
i:=1;
L1:loop
*****"fifo_buffer(i) := fifo_buffer(i+1);"******
i := i+1;
if (i = fifo_index+1) then
exit L1;
end if;
end loop L1;
end if;
if fifo_index > 0 then
req_temp := '1';
chan_out <= fifo_buffer(1) after 0.5 ns;
end if;
end if;

chan_out.req <= req_temp after 0.5 ns;

end process behavior;

end Behavioral;

 

[alessandro basili]

When fifo_index is 15 it will still be incremented by one, because on
the chan_in.req = '1' the condition is true, so it will go to 16. It
means that fifo_index + 1 = 17, means that last time you addressed
fifo_buffer you will have fifo_buffer (16) := fifo_buffer (17), over the
constrained declaration.
Why don't you just use fifo_index as the index, without this i loop?