AT38 Community Member asked a question. I am developing a serial comms protocol between two STM32Fs. I am using the hardware CRC module to generate checksums for some of the data. I am also writing a test script in python to aid development. Reading around, it seems that all I should need to do is reverse the bit order going in and reverse and invert the bits read out.
I have done this in my script and in the firmware. No joy. I have tried 7 variants on CRC32 in my script, using crccheck lib. I have systematically inverted and reversed bits, byte wise, word wise, and swapped endianness before, after and both, and compared against inverted, reversed and endianness swapped checksums with each of the above 8 algorithms, even fed the CRC modules byte by byte combinations.
No fricking joy. Does anyone have a proven concrete software implementation of the F4's hardware CRC module, either in c-like or python, that I can use to validate the firmware's output and complete my test script? Or if not, check that your own F4 produces the same output for the source data? I've posted dozens of working examples over the years. Awesome, thankyou. One way how to check if your CRC calculation is not completely failed is to append the result to the checksummed data and perform the CRC again; the result should be 0 - this is what Clive did in hist last example.
I was able to prove my CRC function was in error, and then located the fault in fairly short order. It was a daft typo mucking up the last word of the input data. Yesterday was a no brain day. Sometimes, those are the hardest to find.Released: Apr 3, View statistics for this project via Libraries. The crccheck. For the class names simply remove all dashes and slashes from the above names and apply CamelCase, e. Other CRC can be calculated by using the general class crccheck.
It only takes a minute to sign up. While there may be faster options than CRC, if you use them then you are likely to end up sacrificing some degree of error detection capability. Depending on what your error detection requirements are, an alternative may be to use CRC code optimised to your application instead.
One option to help with this is pycrc which is a tool written in python 1 which can generate C source code for dozens of combinations of crc model and algorithm. This allows you to optimise speed and size for your own application by selecting and benchmarking different combinations. It supports the crc-8 modelbut also supports crc-5crc and crc amongst others.
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As for algorithmsit supports bit-by-bitbit-by-bit-fast and table-driven. You can even do funky things like specify using dual nibble lookups with a 16 byte look-up table rather than single byte look-up, with byte look-up table.Learn Python - Full Course for Beginners [Tutorial]
Given your memory and speed constraints, this option may well be the best compromise between speed and code size. The only way to be sure would be to benchmark it though. The pycrc git repository is on githubas is its issue trackerbut it can also be downloaded from sourceforge.
Simple one bit parity basically XORing the data over itself over and over is about as fast as one can get. You do lose a lot of the error checking of a CRC though.
A really good paper comparing the performance of various checksums and CRCs in an embedded context:. The Effectiveness of Checksums for Embedded Networks. The Fletcher checksum has lower computational cost than the Adler checksum and, contrary to popular belief, is also more effective in most situations. There is generally no reason to continue the common practice of using an XOR checksum in new designs, because it has the same software computational cost as an addition-based checksum but is only about half as effective at detecting errors.
The Adler checksum should be sufficient for checking for transmission distortions.Force default printer
It's used by the Zlib compression library, and was adopted by the Java 3D Mobile Graphics Standard to provide a fast but effective data integrity check. From the wikipedia page :. An Adler checksum is obtained by calculating two bit checksums A and B and concatenating their bits into a bit integer. A is the sum of all bytes in the string plus one, and B is the sum of the individual values of A from each step. At the beginning of an Adler run, A is initialized to 1, B to 0.
The bytes are stored in network order big endianB occupying the two most significant bytes. I'm not aware of anything that's as effective at error detection as a CRC and faster - if there were, people would be using it instead. If you want to improve the speed of your component, you might need to look at changing your overall technique to seperate out the transfer component from the validation component. If you have these as two independant items on different threads then you can get your full speed of transfer and only resend failed packets.
You should have an estimation of actual error rate to tune your algorithm selection and parameters for block size. Sign up to join this community. The best answers are voted up and rise to the top. What is a faster alternative to a CRC? Ask Question. Asked 8 years, 11 months ago. Active 1 year, 3 months ago. Viewed 17k times. What are some alternative error detection algorithms to check out that would be faster?
FigBug FigBug 2, 3 3 gold badges 15 15 silver badges 20 20 bronze badges. How is the CRC itself implemented?This documentation normally shows Python 2. Python 3. The CRC function can be called multiple times. On subsequent calls, pass the CRC value previously calculated as a second parameter:. Byte strings are acceptable. You may calculate a CRC for an object that implements the buffer protocol:. The class provides an interface similar to the Python hashlibmd5 and sha modules. The parameters are the same as those for the factory function mkCrcFun.
Crc objects contain the following constant values:. Crc objects support the following methods:. The Crc. Enter search terms or a module, class or function name.
Cyclic redundancy check
Navigation index modules next previous crcmod v1. Note This documentation normally shows Python 2. Parameters: poly — The generator polynomial to use in calculating the CRC.
The value is specified as a Python integer or long integer. The bits in this integer are the coefficients of the polynomial. The only polynomials allowed are those that generate 8, 16, 24, 32, or 64 bit CRCs. This initial value should be the initial shift register value, reversed if it uses a reversed algorithm, and then XORed with the final XOR value. That is equivalent to the CRC result the algorithm should return for a zero-length string.
Defaults to all bits set because that starting value will take leading zero bytes into account. Starting with zero will ignore all leading zero bytes. Defaults to True because the bit reversed algorithms are more efficient. Used by some CRC algorithms. Defaults to zero. Returns: CRC calculation function Return type: function The function that is returned is as follows:.
Returns: Calculated CRC value. This depends on the width of the CRC polynomial. This value is updated after each call to update. The CRC value is set to the initial value. If a string is provided in the optional arg parameter, it is passed to the update method. The CRC value is copied from the current value. This allows multiple CRC calculations using a common initial string. Parameters: functionName — String specifying the name of the function.For applications that require data compression, the functions in this module allow compression and decompression, using the zlib library.
There are known incompatibilities between the Python module and versions of the zlib library earlier than 1. For reading and writing. Computes an Adler checksum of data. An Adler checksum is almost as reliable as a CRC32 but can be computed much more quickly. If value is present, it is used as the starting value of the checksum; otherwise, a fixed default value is used.
This allows computing a running checksum over the concatenation of several inputs. The algorithm is not cryptographically strong, and should not be used for authentication or digital signatures. Since the algorithm is designed for use as a checksum algorithm, it is not suitable for use as a general hash algorithm. If you are only using the checksum in packed binary format this is not necessary as the return value is the correct 32bit binary representation regardless of sign.
Changed in version 2. In older versions the value is signed on some platforms and unsigned on others. Changed in version 3. Compresses the data in stringreturning a string contained compressed data.
The default value is 6. Raises the error exception if any error occurs. It can take several ranges of values.Corosync documentation
The default is Larger values produce better compression at the expense of greater memory usage. The resulting output will include a zlib-specific header and trailer. Valid values range from 1 to 9. Higher values using more memory, but are faster and produce smaller output. The default is 8. In older versions the value would be signed on some platforms and unsigned on others.Myphonak app
Decompresses the data in stringreturning a string containing the uncompressed data. The wbits parameter depends on the format of stringand is discussed further below. If bufsize is given, it is used as the initial size of the output buffer. It is similar to the parameter for compressobjbut accepts more ranges of values:. The input must include a zlib header and trailer. Only supported since zlib 1. The input must be a raw stream with no header or trailer.
The input must include a gzip header and trailer. When decompressing a stream, the window size must not be smaller than the size originally used to compress the stream; using a too-small value may result in an error exception.
The default wbits value is 15, which corresponds to the largest window size and requires a zlib header and trailer to be included. The default size is It has the same meaning as described for decompress. Compress stringreturning a string containing compressed data for at least part of the data in string.
This data should be concatenated to the output produced by any preceding calls to the compress method. Some input may be kept in internal buffers for later processing. All pending input is processed, and a string containing the remaining compressed output is returned.A cyclic redundancy check CRC is an error-detecting code commonly used in digital networks and storage devices to detect accidental changes to raw data.
Blocks of data entering these systems get a short check value attached, based on the remainder of a polynomial division of their contents. On retrieval, the calculation is repeated and, in the event the check values do not match, corrective action can be taken against data corruption.
CRCs can be used for error correction see bitfilters. CRCs are so called because the check data verification value is a redundancy it expands the message without adding information and the algorithm is based on cyclic codes. CRCs are popular because they are simple to implement in binary hardwareeasy to analyze mathematically, and particularly good at detecting common errors caused by noise in transmission channels.
Because the check value has a fixed length, the function that generates it is occasionally used as a hash function. The CRC was invented by W. Wesley Peterson in ; the bit CRC function, used in Ethernet and many other standards, is the work of several researchers and was published in CRCs are based on the theory of cyclic error-correcting codes.
The use of systematic cyclic codes, which encode messages by adding a fixed-length check value, for the purpose of error detection in communication networks, was first proposed by W. Wesley Peterson in This is important because burst errors are common transmission errors in many communication channelsincluding magnetic and optical storage devices.
Specification of a CRC code requires definition of a so-called generator polynomial. This polynomial becomes the divisor in a polynomial long divisionwhich takes the message as the dividend and in which the quotient is discarded and the remainder becomes the result.
The important caveat is that the polynomial coefficients are calculated according to the arithmetic of a finite fieldso the addition operation can always be performed bitwise-parallel there is no carry between digits.
The two elements are usually called 0 and 1, comfortably matching computer architecture. For a given nmultiple CRCs are possible, each with a different polynomial. A CRC-enabled device calculates a short, fixed-length binary sequence, known as the check value or CRCfor each block of data to be sent or stored and appends it to the data, forming a codeword.
When a codeword is received or read, the device either compares its check value with one freshly calculated from the data block, or equivalently, performs a CRC on the whole codeword and compares the resulting check value with an expected residue constant.
The device may take corrective action, such as rereading the block or requesting that it be sent again. Otherwise, the data is assumed to be error-free though, with some small probability, it may contain undetected errors; this is inherent in the nature of error-checking. CRCs are specifically designed to protect against common types of errors on communication channels, where they can provide quick and reasonable assurance of the integrity of messages delivered.
However, they are not suitable for protecting against intentional alteration of data. Firstly, as there is no authentication, an attacker can edit a message and recompute the CRC without the substitution being detected. When stored alongside the data, CRCs and cryptographic hash functions by themselves do not protect against intentional modification of data. Any application that requires protection against such attacks must use cryptographic authentication mechanisms, such as message authentication codes or digital signatures which are commonly based on cryptographic hash functions.
Secondly, unlike cryptographic hash functions, CRC is an easily reversible function, which makes it unsuitable for use in digital signatures. Thirdly, CRC is a linear function with a property that .
Here's my code:.Cardpay solutions
This calculates the CRC for each line, but its output e. PS2: Old PS is deprecated - therefore deleted - because of the suggestion in the comment.
Thank you. I don't get, how I missed this, but it was really good. A modified version of kobor42's answer, with performance improved by a factor by reading fixed size chunks instead of "lines":. To show any integer's lowest 32 bits as 8 hexadecimal digits, without sign, you can "mask" the value by bit-and'ing it with a mask made of 32 bits all at value 1, then apply formatting.
It's quite irrelevant whether the integer you are thus formatting comes from zlib. It doesn't matter what you set it to, you will get the same hash for the same file setting it too low might make your code slow, too high might use too much memory.
A modified and more compact version of CrouZ's answer, with a slightly improved performance, using a for loop and file buffering:. It is faster because for loops are faster than while loops sources: here and here.
And zlib. Learn more. Asked 10 years, 8 months ago. Active 2 months ago. Viewed 38k times. Here's my code:! Jason Sundram Active Oldest Votes. Bastian 10k 1 1 gold badge 28 28 silver badges 38 38 bronze badges. If you set prev to 0 instead then you don't need to worry about an exception. Granted, the bigger the file the more memory the program needs; depends on the trade-off you want, memory for speed, or speed for memory.
A way to speed up the calculation considerably factor while keeping the memory usage low is to read fixed size chunks instead of reading "lines" from the binary file. Added a separate answer for this. Paulo Freitas Paulo Freitas CrouZ CrouZ 1, 15 15 silver badges 14 14 bronze badges. This is an underappreciated answer. Alex Martelli Alex Martelli k gold badges silver badges bronze badges. Good point re: formatting, but it looks like his code also doesn't compute what he wants it to.
There are really two problems here: 1 Compute the CRC of a file. But it was nice to see another way to do it, I've never used format before.
Python 3. The result is a 32 bit integer. The CRC checksum of an empty file is 0. Boris Boris 4, 5 5 gold badges 37 37 silver badges 48 48 bronze badges. A modified and more compact version of CrouZ's answer, with a slightly improved performance, using a for loop and file buffering: def forLoopCrc fpath : """With for loop and buffer.Cat s30 frp
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