CRC Generator (crc_generator_b)

Appends a trailing cyclic redundancy check (CRC) to sample data between two flush or discontinuity messages.

Design

On reception of a flush or discontinuity opcode message, a CRC will be generated for any sample data received since the last flush or discontinuity message.

A CRC is an error-detecting algorithm commonly used to detect errors during the exchange of data. The CRC value is the remainder of a polynomial division algorithm on the input data stream.

An example of a common CRC type is CRC-8-WCDMA. This is used for error detection in mobile networks. It has a polynomial shown by (20), which can also be defined as 0x9B.

(20)\[x^8+x^7+x^4+x^3+x+1\]

Using the CRC-8-WCDMA example the with the input 0000 0001 0000 0010 0000 0011 0000 0100, the resulting output is 0000 0001 0000 0010 0000 0011 0000 0100 1101 0110. Where the left most bit is input into the CRC generator component first. Compared to the input the output message size has increased by 8 bits to 40 bits due to the addition of a trailing CRC. The appended CRC is 1101 0110 (0xD6).

The polynomial_taps property sets the divisor of the polynomial long division algorithm. It is set at build time.

The seed property defines the initial value of the CRC at the start of a message. It is settable during run time.

Depending on the application the CRC result may need to be reflected for a different endianness. Property output_reflect provides the option to enable or disable this. It is set at build time.

Some CRC’s require a final XOR on the CRC result. The final XOR value is set by the property final_xor. It is settable during run time.

Interface

<?xml version="1.0"?>
<componentspec>
  <property name="polynomial_taps" type="uchar" arraylength="64" writable="false" parameter="true" default="16,15,2"
    description="CRC polynomial taps."/>
  <property name="output_reflect" type="bool" writable="false" parameter="true" default="false"
    description="Endian of the CRC result, false is CRC most significant bit first and true is CRC least significant bit first."/>
  <property name="seed" type="ulonglong" writable="true" default="0" description="Initial seed for the CRC."/>
  <property name="final_xor" type="ulonglong" writable="true" default="0" description="Value to XOR the CRC result with."/>
  <port name="input" producer="false" protocol="bool_timed_sample-prot"/>
  <port name="output" producer="true" protocol="bool_timed_sample-prot"/>
</componentspec>

Ports

Inputs:

  • input: Primary input samples port.

    • Protocol: bool_timed_sample-prot

    • Optional: False

Outputs:

  • output: Primary output samples port.

    • Protocol: bool_timed_sample-prot

    • Optional: False

Opcode Handling

Discontinuity and flush opcode messages mark the end of a message and cause the component to append a CRC to any sample data received since the previous discontinuity or flush message. The component applies backpressure whilst outputting the CRC. When the component has finished generating the CRC it is reset, the discontinuity or flush opcode is forwarded to the output, and backpressure is removed.

All other message opcode types are passed through this component without any effect.

Properties

  • polynomial_taps: CRC polynomial taps. An array of up to 64 unsigned characters, meaning a maximum of 64 taps can be specified. If the polynomial is \(x^8+x^2+x+1\) then polynomial_taps should be set to “8,2,1”. There is no need to specify the \(x^0\) term, this is implied.

    • Type: uchar

      • Array dimensions: ['64']

    • Access:

      • Parameter: True

      • Writable: False

      • Initial: False

      • Volatile: False

    • Default value: 16,15,2

  • output_reflect: Endian of the CRC result, false is CRC most significant bit first and true is CRC least significant bit first.

    • Type: bool

    • Access:

      • Parameter: True

      • Writable: False

      • Initial: False

      • Volatile: False

    • Default value: false

  • seed: Initial seed for the CRC.

    • Type: ulonglong

    • Access:

      • Parameter: False

      • Writable: True

      • Initial: False

      • Volatile: False

    • Default value: 0

  • final_xor: Value to XOR the CRC result with.

    • Type: ulonglong

    • Access:

      • Parameter: False

      • Writable: True

      • Initial: False

      • Volatile: False

    • Default value: 0

A table listing the default polynomials the component is built for is shown below.

Polynomial

polynomial_taps

output_reflect

\(x^8 + x^2 + x + 1\)

8,2,1
true and
false

\(x^8 + x^4 + x^3 + x^2 + 1\)

8,4,3,2
true and
false

\(x^8 + x^5 + x^3 + x^2 + x + 1\)

8,5,3,2,1

false

\(x^8 + x^5 + x^4 + 1\)

8,5,4

true

\(x^8 + x^5 + x^4 + x^3 + 1\)

8,5,4,3

true

\(x^8 + x^7 + x^4 + x^3 + x + 1\)

8,7,4,3,1

true

\(x^8 + x^7 + x^6 + x^4 + x^2 + 1\)

8,7,6,4,2

false

\(x^{16} + x^{10} + x^8 + x^7 + x^3 + 1\)

16,10,8,7,3

false

\(x^{16} + x^{12} + x^5 + 1\)

16,12,5
true and
false
\(x^{16} + x^{13} + x^{12} + x^{11} + x^{10} + x^8 +\)
\(x^6 + x^5 + x^2 + 1\)
16,13,12,11,10,8,
6,5,2
true and
false

\(x^{16} + x^{15} + x^2 + 1\)

16,15,2
true and
false
\(x^{16} + x^{15} + x^{11} + x^9 + x^8 + x^7 +\)
\(x^5 + x^4 + x^2 + x + 1\)
16,15,11,9,8,7,
5,4,2,1

false
\(x^{16} + x^{15} + x^{13} + x^7 + x^4 + x^2 +\)
\(x + 1\)
16,15,13,7,4,2,
1

false
\(x^{16} + x^{15} + x^{14} + x^{11} + x^6 + x^5 +\)
\(x^2 + x + 1\)
16,15,14,11,6,5,
2,1

false

\(x^{32} + x ^7 + x^5 + x^3 + x^2 + x + 1\)

32,7,5,3,2,1

false
\(x^{32} + x^{26} + x^{23} + x^{22} + x^{16} + x^{12} +\)
\(x^{11} + x^{10} + x^8 + x^7 + x^5 + x^4 +\)
\(x^2 + x + 1\)
32,26,23,22,16,12,
11,10,8,7,5,4,
2,1
true and
false
\(x^{32} + x^{28} + x^{27} + x^{26} + x^{25} + x^{23} +\)
\(x^{22} + x^{20} + x^{19} + x^{18} + x^{14} + x^{13} +\)
\(x^{11} + x^{10} + x^9 + x^8 + x^6 + 1\)
32,28,27,26,25,23,
22,20,19,18,14,13,
11,10,9,8,6

true
\(x^{32} + x^{31} + x^{24} + x^{22} + x^{16} + x^{14} +\)
\(+ x^8 + x^7 + x^5 + x^3 + x + 1\)
32,31,24,22,16,14,
8,7,5,3,1

false
\(x^{32} + x^{31} + x^{29} + x^{27} + x^{21} + x^{20} +\)
\(x^{17} + x^{16} + x^{15} + x^{12} + x^{11} + x^5 +\)
\(x^3 + x + 1\)
32,31,29,27,21,20,
17,16,15,12,11,5,
3,1

true

\(x^{64} + x^4 + x^3 + x + 1\)

64,4,3,1

true
\(x^{64} + x^{62} + x^{57} + x^{55} + x^{54} + x^{53} +\)
\(x^{52} + x^{47} + x^{46} + x^{45} + x^{40} + x^{39} +\)
\(x^{38} + x^{37} + x^{35} + x^{33} + x^{32} + x^{31} +\)
\(x^{29} + x^{27} + x^{24} + x^{23} + x^{22} + x^{21} +\)
\(x^{19} + x^{17} + x^{13} + x^{12} + x^{10} + x^9 +\)
\(x^7 + x^4 + x + 1\)
64,62,57,55,54,53,
52,47,46,45,40,39,
38,37,35,33,32,31,
29,27,24,23,22,21,
19,17,13,12,10,9,
7,4,1
true and
false

Implementations

  • crc_generator_b (HDL)

    Galois linear feedback shift register (LFSR) HDL implementation.

  • crc_generator_b (RCC)

Example Application

<?xml version="1.0"?>
<application done="file_write">
  <instance component="ocpi.core.file_read" connect="crc_generator_b">
    <property name="filename" value="input.bin"/>
  </instance>
  <instance component="ocpi.comp.sdr.communication.crc_generator_b" connect="file_write">
    <property name="polynomial_taps" value="16,15,2"/>
    <property name="seed" value="0"/>
    <property name="output_reflect" value="false"/>
    <property name="final_xor" value="0"/>
  </instance>
  <instance component="ocpi.core.file_write">
    <property name="filename" value="output.bin"/>
  </instance>
</application>

Dependencies

The dependencies to other elements in OpenCPI are:

There is also a dependency on:

  • ieee.std_logic_1164

  • ieee.numeric_std

  • ieee.math_real

Limitations

Limitations of crc_generator_b are:

  • CRC sizes above 64 are not supported.

Testing

Tested platforms: None

Component testing not completed.