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137 lines
5.1 KiB
Markdown
137 lines
5.1 KiB
Markdown
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[](https://github.com/marketplace/actions/arduino_ci)
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[](https://github.com/RobTillaart/CRC/actions/workflows/arduino-lint.yml)
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[](https://github.com/RobTillaart/CRC/actions/workflows/jsoncheck.yml)
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[](https://github.com/RobTillaart/CRC/blob/master/LICENSE)
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[](https://github.com/RobTillaart/CRC/releases)
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# CRC
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Arduino library with CRC8, CRC16, CRC32 and CRC64 functions.
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## Description
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Goal of this library is to have a flexible and portable set of generic
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CRC functions and classes.
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The CRCx classes have a number of added values. Most important is that
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they allow one to verify intermediate CRC values. This is useful if one
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sends a "train of packets" which include a CRC so far. This detects both
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errors in one packet but also missing packets, or injected packages.
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Another trick one can do is change the polynome or the reverse flag during
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the process. This makes it harder to imitate.
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Furthermore the class allows to add values in single steps and continue too.
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Finally the class version gives more readable code (imho) as the parameters
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are explicitly set.
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**Note** the classes have same names as the static functions, except the class
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is UPPER case. So CRC8 is a class and **crc8()** is the function.
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Deeper tech info -https://en.wikipedia.org/wiki/Cyclic_redundancy_check
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and many other websites.
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## Interface CRC classes
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These interfaces are very similar for CRC8, CRC16, CRC32 and CRC64 class.
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The only difference is the data type for polynome, start- and end-mask,
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and the returned CRC.
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Use **\#include "CRC8.h"**
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- **CRC8()** Constructor
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- **void reset()** set all internals to constructor defaults.
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- **void restart()** reset internal CRC and count only; reuse values for other
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e.g polynome, XOR masks and reverse flags.
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- **void setPolynome(polynome)** set polynome, note reset sets a default polynome.
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- **void setStartXOR(start)** set start-mask, default 0.
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- **void setEndXOR(end)** set end-mask, default 0.
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- **void setReverseIn(bool reverseIn)** reverse the bit pattern of input data (MSB vs LSB).
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- **void setReverseOut(bool reverseOut)** reverse the bit pattern of CRC (MSB vs LSB).
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- **void add(value)** add a single value to CRC calculation.
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- **void add(array, uint32_t length)** add an array of values to the CRC.
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In case of a warning/error use casting to (uint8_t \*).
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- **uint8_t getCRC()** returns CRC calculated so far. This allows to check the CRC of
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a really large stream at intermediate moments, e.g. to link multiple packets.
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- **uint32_t count()** returns number of values added so far. Default 0.
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### Example snippet
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A minimal usage only needs:
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- the constructor, the add() function and the getCRC() function.
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```cpp
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#include "CRC32.h"
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...
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CRC32 crc;
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...
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while (Serial.available())
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{
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int c = Serial.read();
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crc.add(c);
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}
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Serial.println(crc.getCRC());
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```
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## Interface static functions
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Use **\#include "CRC.h"**
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Most functions have a default polynome, same start and end masks, and default there is no reversing.
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However these parameters allow one to tweak the CRC in all aspects known.
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In all the examples encountered the reverse flags were set both to false or both to true.
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For flexibility both parameters are kept available.
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- **uint8_t crc8(array, length, polynome = 0xD5, start = 0, end = 0, reverseIn = false, reverseOut = false)** idem with default polynome.
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- **uint16_t crc16(array, length, polynome = 0xA001, start = 0, end = 0, reverseIn = false, reverseOut = false)** idem with default polynome.
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- **uint16_t crc16-CCITT(array, length)** fixed polynome **0x1021**, non zero start / end masks.
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- **uint32_t crc32(array, length, polynome = 0x04C11DB7, start = 0, end = 0, reverseIn = false, reverseOut = false)** idem with default polynome.
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- **uint64_t crc64(array, length, polynome, start, end, reverseIn, reverseOut)** - experimental version, no reference found except on Wikipedia.
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Note these functions are limited to one call per block of data. For more flexibility use the classes.
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## Operational
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See examples.
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## Links
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- https://en.wikipedia.org/wiki/Cyclic_redundancy_check - generic background.
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- https://crccalc.com/ - online CRC calculator to verify.
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## Future
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- extend examples.
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- table versions for performance? (performance - memory discussion)
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- example showing multiple packages of data linked by their CRC.
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- stream version - 4 classes class?
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- setCRC(value) to be able to pick up where one left ?
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- getters, getPolynome() etc?
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-
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#### Exotic CRC's ?
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- **CRC1()** // parity :)
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- **CRC4(array, length, polynome, start, end, reverseIn, reverseOut)** nibbles?
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- default polynome 0x03
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#### Magic \#defines for "common" polynomes? verify ?
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- \#define CRC_ISO64 0x000000000000001B
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- \#define CRC_ECMA64 0x42F0E1EBA9EA3693
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