VHDL Code for CRC-5 Checksum circuit for USB Error correct

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govandi999

Joined: 10 Jun 2005
Posts: 10
Helped: 3

13 May 2008 8:36 vhdl code for crc

We have to design a module which is a simple CRC-5 check sum circuit that will implement the polynomial (1 + x2 + x5). This polynomial is used for error correction in USB. The block diagram is as follows:

VHDL Code for CRC-5 Checksum circuit for USB Error correct

VHDL Code for CRC-5 Checksum circuit for USB Error correct

We have to design a circuit that takes the 32 bit input and after 32 cycles produces the 5 bit CRC. The operation of the circuit is explained below
1) Start ← 1
2) The Din is used to calculate the CRC for the next 32 clk cycles. At each cycle
Dout <= Din
3) The CRC is output during the next 5 cycles.

At each clock cycle the CRC is calculated as follows

NewCRC(0) := DIN xor OLDCRC(4);
NewCRC(1) := OLDCRC(0);
NewCRC(2) := DIN xor OLDCRC(1) xor OLDCRC(4);
NewCRC(3) := OLDCRC(2);
NewCRC(4) := OLDCRC(3);

We have to write a VHDL code for the CRC generator.

I have written the following code but there is some problem, can you please correct it.

library IEEE;
use IEEE.STD_LOGIC_1164.ALL;
use IEEE.STD_LOGIC_ARITH.ALL;
use IEEE.STD_LOGIC_UNSIGNED.ALL;

entity CRC5 is
port ( START : in std_logic;
CLK : in std_logic;
DIN : in std_logic;
DOUT : out std_logic
);
end CRC5;

architecture BEH of CRC5 is

signal temp : std_logic;
SIGNAL NewCRC : STD_LOGIC_VECTOR(4 DOWNTO 0);
SIGNAL OLDCRC : STD_LOGIC_VECTOR(4 DOWNTO 0);

begin

PROCESS (CLK)

VARIABLE COUNT : INTEGER := 0;

begin
IF (CLK'EVENT AND CLK ='1') THEN

IF (START ='1') THEN

temp <= Din;
Dout <= temp;

COUNT := COUNT +1;

IF (COUNT = 32) THEN

NewCRC(0) <= temp xor OLDCRC(4);
NewCRC(1) <= OLDCRC(0);
NewCRC(2) <= temp xor OLDCRC(1) xor OLDCRC(4);
NewCRC(3) <= OLDCRC(2);
NewCRC(4) <= OLDCRC(3);

end if;

end if;
end if;

end process;

DOUT <= NewCRC(0);
DOUT <= NewCRC(1);
DOUT <= NewCRC(2);
DOUT <= NewCRC(3);
DOUT <= NewCRC(4);

end BEH;

ieropsaltic

Joined: 25 Sep 2006
Posts: 205
Helped: 44

13 May 2008 10:07 vhdl crc

You use OLDCRC without putting values in them and they aren't inputs. So, you need first to assign values to them during operation. Would you post your algorithm in more detail so that we can help you more?

tkbits

Joined: 04 Dec 2004
Posts: 235
Helped: 36

14 May 2008 16:54 crc vhdl code

The way the code is written, you have several different values going to the single-bit DOUT simultaneously. At each clock cycle, the most significant bit of the CRC will be DOUT.

You have coded START as an enable signal. Is START a start signal or an enable signal ? If START is a start signal, I would expect it to load a seed value into NewCRC. START would then be set to 0 to enable clocking.

You're missing a way to transfer the NewCRC value to OLDCRC. As an alternative, after (CLK'EVENT AND CLK='1'), the signal assignment CRC(4) <= CRC(3); means "new" CRC(4) gets the value of "old" CRC(3).