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VHDL IP Stack

CRC test in a stand-alone dedicated test-bench.

1. One thing I find very confusing is to figure out byte order and bit order, when calculating ETHERNET CRC. So it was in this case.


2. I looked a bit in the Internet for some frames, which their CRC frame is calculated. One such site is : CRC example.


3. I created a test-bench to inject the data. The test-bench instantiate the CRC from the VHDL IP stack project. I used a simple test-bench: no random, no multiple frame transmit.


4. The data of the frame is captured into fields as shown below:
   ------------------------frame data------------------------
   --MAC header
   constant mac_dest : std_logic_vector (47 downto 0) := x"0010a47bea80";
   constant mac_srce : std_logic_vector (47 downto 0) := x"001234567890";
   constant mac_type : std_logic_vector (15 downto 0) := x"0800";
   --IP ver 4 header
   constant ipheader : std_logic_vector (95 downto 0) := x"4500002eb3fe000080110540";
   constant ip_srcad : std_logic_vector (31 downto 0) := x"c0a8002c";
   constant ip_dstad : std_logic_vector (31 downto 0) := x"c0a80004";
   
   --ICMP header
   constant icmptype : std_logic_vector ( 7 downto 0) := x"04";
   constant icmpcode : std_logic_vector ( 7 downto 0) := x"00";
   constant icmpcsum : std_logic_vector (15 downto 0) := x"0400";--checksum
   constant icmpdata : std_logic_vector (207 downto 0) := x"001a2de8000102030405060708090a0b0c0d0e0f101100000000";
   
   constant pkt_dlen : integer := 512; --length in bits
   constant pkt_nlen : integer := pkt_dlen/4;--length in nibbles
   --expected result from WEB 0xE6C53DB2
   ------------------------frame data------------------------
   
   Note: When putting 00000000 at the frame end the CRC output is available. If, however, the CRC value, E6C53DB2, is used , we get 00000000 at CRC output.


5. Some waveforms are in the following link:  waves
   Two cases are shown. Frame end padded with zeros. This is usually done at the transmit side to calculate a valid CRC. Frame with a correct CRC appended at the end as usually is the case in the receive side. A CRC of zero, indicates a frame a good frame reception.


6. The code is available in the following page: code




7. When I worked on the CRC, I looked for numerical examples as much as a diver looks for oxygen, when he is under water. So here is another ICMP frame with a valid CRC: yet another example

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