5/17/2023 0 Comments Checksum calculator crestron![]() ![]() Those routines will all return the same result, differing only in arguments are ignored, and the initial CRC, i.e. applied to the previous CRC value, crc. crcany can be used to generate alternative code for big-endian and other word sizes.Ĭrc16dnp.h: // The _bit, _byte, and _word routines return the CRC of the len bytes at mem, Intel x86 and x86-64, and it assumes that uintmax_t is 64 bits. This code assumes a little-endian architecture for the word-wise calculation, e.g. Following is the code generated by crcany for that CRC definition. It has very good performance detecting up to 6-bit errors in a packet. One of the good 16-bit performers in Koopman's tables that is also in the catalog of CRCs used in practice is CRC-16/DNP. The classic CRCs, such as the CCITT/Kermit 16-bit CRC or the X.25 16-bit CRC are not the best performers. single digit number of errors per packet, and you want to maximize your error detection performance, you would need to look at the packet size you are applying the CRC to, assuming that that is constant or bounded, and look at the performance of the best polynomials in Philip Koopman's extensive research. If you have a relatively low bit error rate, e.g. In this case, you can pick any 16-bit CRC in the catalog, and you will get good performance. You do not seem to have a protocol definition with a specific CRC definition that you need to match. Byte-wise is still much faster than bit-wise, but the implementation is more easily portable over architectures. crcany generates both byte-wise routines and word-wise routines, the latter tuned to the architecture they are generated on. ![]() That function returns 0xbb3d for me when I pass in "123456789".Ĭrcany will generate efficient C code for any CRC, and includes a library of over one hundred known CRC definitions.Įfficient CRC code uses tables instead of bit-wise calculations. Out > bits_read) & 1 // item a) work from the least significant bits So, your function might look like: #define CRC16 0x8005 c) reverse the CRC bits (I'm guessing this bit is a carry over from hardware implementations).b) push the last 16 bits of the CRC out of the CRC register after you've finished with the input data.a) run the data bits through the CRC loop starting from the least significant bit instead of from the most significant bit. ![]() If you want to match the CRC16 with polynomial 0x8005 as shown on the CRC calculator page, you need to make the following changes to your CRC function: Keep in mind that CRC algorithms were designed to be implemented in hardware, so some of how bit ordering is handled may not make so much sense from a software point of view. For example, sometimes one implementation will work from the low order bits of the data bytes up, while sometimes they'll work from the high order bits down (as yours currently does).Īlso, you need to 'push out' the last bits of the CRC after you've run all the data bits through. There are several details you need to 'match up' with for a particular CRC implementation - even using the same polynomial there can be different results because of minor differences in how data bits are handled, using a particular initial value for the CRC (sometimes it's zero, sometimes 0xffff), and/or inverting the bits of the CRC. Can someone tell me where I might be going wrong? I've come to the conclusion that either my understanding of how to calculate a CRC16 is wrong, or the online calculator is wrong (the former seems more likely). I'm checking my output against this online CRC calculator. Uint16_t gen_crc16(const uint8_t *data, uint16_t size) The relevant code I've written is below (it can also be found here). I've created a function to calculate a CRC16 checksum, but it doesn't seem to be outputting correct values. Part of this code involves using a CRC16 checksum on the data to detect corruption from line noise. ![]() I'm working on a library to provide simple reliable communication over an RS232 or RS485 connection. ![]()
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