AceCRC

This library contains a number of CRC algorithms that were generated from pycrc (https://pycrc.org) and programmatically converted to Arduino format to use C++ namespaces to avoid name collisions and PROGMEM flash memory for lookup tables to save static RAM. From the list of pycrc supported algorithms, this library supports:

CRC-8
CRC-16-CCITT
CRC-16-MODBUS
CRC-32

For each algorithm, 4 different implementations were generated:

bit: bit-by-bit
- brute-force loops to calculate the polynomial divisions
- smallest code size, but slowest
nibble: 4-bit lookup table in flash memory (PROGMEM)
- generates a lookup table of 16 elements
- larger code size, but faster
nibblem: 4-bit lookup table in static memory
- same as nibble, but using static memory for lookup table
- the "m" stands for "static memory" as opposed to "flash memory"
- 2-7% faster than nibble on AVR
- 1.9X-2.7X faster than nibble on ESP8266
- no difference for all other processors
byte: 8-bit lookup table in flash memory
- generates a lookup table of 256 elements
- largest code size, but fastest

The pycrc program generates *.h and *.c files containing C99 code with the following definitions:

crc_t crc_init(void);
crc_t crc_update(crc_t crc, const void *data, size_t data_len);
crc_t crc_finalize(crc_t crc);

This library converts the C99 code in the following way:

each algorithm and variant is wrapped its own C++ namespace to avoid name collision
- ace_crc::crc8_bit
- ace_crc::crc8_nibble
- ace_crc::crc8_nibblem
- ace_crc::crc8_byte
- ace_crc::crc16ccitt_bit
- ace_crc::crc16ccitt_nibble
- ace_crc::crc16ccitt_nibblem
- ace_crc::crc16ccitt_byte
- ace_crc::crc16modbus_bit
- ace_crc::crc16modbus_nibble
- ace_crc::crc16modbus_nibblem
- ace_crc::crc16modbus_byte
- ace_crc::crc32_bit
- ace_crc::crc32_nibble
- ace_crc::crc32_nibblem
- ace_crc::crc32_byte
a convenience function crc_t crc_calculate(const void *data, size_t data_len) is inserted into the header file of each namespace
- calculates the CRC in one-shot
the crc_table lookup table is moved into flash memory using PROGMEM
- the static RAM usage of all CRC routines becomes zero (other than a few stack variables)
- (Note that on platforms that do not support flash memory storage of data, PROGMEM is a no-op, and the various pgm_read_{xxx}() routines become just normal memory accessors.)
the static keyword is removed from header files
- not needed in C++
- it was preventing the generation of doxygen docs for those functions
the #define CRC_ALGO_{XXX} macro is converted into a const uint8_t
- becomes part of its enclosing namespace, preventing name collision
the typedef for crc_t is changed from uint_fast8_t, uint_fast16_t, and uint_fast32_t to uint8_t, uint16_t, and uint32_t respectively
- affects only 32-bit processors, and only the crc8, crc16ccitt, and crc16modbus algorithms
- see section Integer Sizes below for more information

TL;DR: Use crc32_nibble in most cases, except on ESP8266 where you should use crc32_nibblem.

Version: 1.1.1 (2023-07-12)

Changelog: CHANGELOG.md

HelloCRC
Installation
- Dependencies
Documentation
Usage
Resource Consumption
- Memory Benchmarks
- CPU Benchmarks
Recommendations
Background and Motivation
- Other CRC Libraries
Bugs and Limitations
System Requirements
License
Feedback and Support
Authors

HelloCRC

Here is the sample program from examples/HelloCRC that uses the CRC-16-CCITT algorithm using a 4-bit lookup table (16 elements):

#include <Arduino.h>
#include <AceCRC.h>

using namespace ace_crc::crc16ccitt_nibble;

static const char CHECK_STRING[] = "123456789";
static const size_t LENGTH = sizeof(CHECK_STRING) - 1;

void setup() {
  Serial.begin(115200);
  while (!Serial); // Wait - Leonardo/Micro

  crc_t crc = crc_init();
  crc = crc_update(crc, CHECK_STRING, LENGTH);
  crc = crc_finalize(crc);
  Serial.print("0x");
  Serial.println((unsigned long) crc, 16);

  crc = crc_calculate(CHECK_STRING, LENGTH);
  Serial.print("0x");
  Serial.println((unsigned long) crc, 16);
}

void loop() {
}

This prints the hexadecimal numbers

0xE5CC
0xE5CC

as expected.

Installation

The latest stable release is available in the Arduino IDE Library Manager. Search for "AceCRC" and click install.

The development version can be installed by cloning the git repo:

AceCRC (https://github.com/bxparks/AceCRC)

You can copy over the contents to the ./libraries directory used by the Arduino IDE. (The result is a directory named ./libraries/AceCRC). Or you can create symlinks from ./libraries to these directories.

The develop branch contains the latest development. The master branch contains the stable releases.

Dependencies

This library has no external dependencies for client use. But the following dependencies are required for development and testing purposes:

To regenerate and rebuild the source code, you need pycrc (https://pycrc.org/).
To run the unit tests under tests, you need AUnit (https://github.com/bxparks/AUnit)
To run the unit tests under Linux or MacOS, you need EpoxyDuino (https://github.com/bxparks/EpoxyDuino)

Documentation

README.md - this file
Doxygen docs hosted on GitHub Pages
examples/benchmarks - for memory and CPU consumption number for each algorithm on various microcontrollers.

Usage

Headers and Namespaces

Only a single header file AceCRC.h is required to use this library.

#include <AceCRC.h>

Then select the namespace of the algorithm that you want to use. For example, to use the version of crc32 which uses a 4-bit table (16 entries), which are stored in static memory instead of flash memory, use the following:

using namespace ace_crc::crc32_nibblem;

To use the 8-bit table, which are stored in flash memory, use:

using namespace ace_crc::crc32_byte;

Core CRC Functions

All algorithms and their variants are placed in separate C++ namespaces so they do not collide, and you can use multiple CRC algorithms in a single program without worrying about name collision.

The function names and their signatures from the underlying generated code from pycrc remain unchanged. For reference, the principle functions are:

crc_t crc_init(void);
crc_t crc_update(crc_t crc, const void *data, size_t data_len);
crc_t crc_finalize(crc_t crc);

See the examples/HelloCRC example code to see how these functions are used. The crc_update() function can be called multiple times with additional data, before calling crc_finalize().

This library adds the following convenience function to each header file in each namespace:

crc_t crc_calculate(const void *data, size_t data_len);

This function replaces the three separate calls to crc_init(), crc_update(), crc_finalize() with a single call.

Integer Sizes

By default, the pycrc program generates C99 code which contains one of the following definitions of the crc_t type:

typedef uint_fast8_t crc_t;
typedef uint_fast16_t crc_t;
typedef uint_fast32_t crc_t;

These are converted by this library to these instead:

typedef uint8_t crc_t;
typedef uint16_t crc_t;
typedef uint32_t crc_t;

On 8-bit processors, uint_fast8_t is identical to uint8_t, uint_fast16_t is identical to uint16_t, and uint_fast32_t is identical to uint32_t.

On 32-bit processors (e.g. SAMD, ESP8266, ESP32), uint_fast8_t and uint_fast16_t are both defined to be uint32_t, presumably because the 32-bit integer type is faster for most operations (but not always). The main effect of these definitions is to increase the size of the crc_table for the CRC-8 and CRC-16-CCITT algorithms by a factor of 4x or 2x, compared to what they could be.

After collecting the CPU and memory consumption results of various algorithms on different microcontrollers in examples/benchmarks, I found that using the smaller uint8_t or uint16_t did not affect the speed of the algorithms very much (2-14%). Some were got slightly slower, but some actually got slightly faster using the exact uint8_t and uint16_t types, which are supposed to be slower.

The bigger difference is the sizes of the internal crc_table which become a lot smaller when the crc_t becomes smaller. It also seemed potentially confusing for the end user if sizeof(crc_t) returned 4 instead of 1 or 2 when using the CRC-8 or CRC-16-CCITT algorithms.

I concluded that it was better in the Arduino microcontroller environments to make the crc_t type correspond to the exact sized integer types (uint8_t, uint16_t, uint32_t).

The size_t is used as the type of the data_len parameter of the crc_update() or the crc_calculate() function. On 8-bit processors size_t is 2 bytes, and on 32-bit processors size_t is 4 bytes. To calculate the CRC of an array that's longer than 64 kiB long on 8-bit processors, it must be broken down into chunks smaller than 64 kiB and processed through the crc_update() function multiple times. I suspect that this situation will happen only rarely on 8-bit processors which tend to deal with fairly small amounts of data.

Resource Consumption

Memory Benchmarks

I wrote a bunch of scripts in examples/benchmarks/MemoryBenchmark to automatically gather the flash and static RAM consumption of various CRC algorithms on various microcontrollers. The results are summarized in the README.md in that directory. All algorithms (except for xxx_nibblem variants) consume no static RAM, because their lookup tables are located in flash using PROGMEM.

Here are rough flash memory consumption for each algorithm:

crc8_bit: 64-130 bytes
crc8_nibble: 80-150 bytes
crc8_nibblem: 80-150 bytes
crc8_byte: 290-360 bytes
crc16ccitt_bit: 80-140 bytes
crc16ccitt_nibble: 100-190 bytes
crc16ccitt_nibblem: 100-190 bytes
crc16ccitt_byte: 560-630 bytes
crc16modbus_bit: 120-200 bytes
crc16modbus_nibble: 100-180 bytes
crc16modbus_nibblem: 100-180 bytes
crc16modbus_byte: 560-630 bytes
crc32_bit: 110-180 bytes
crc32_nibble: 140-210 bytes
crc32_nibblem: 140-210 bytes
crc32_byte: 1080-1140 bytes

CPU Benchmarks

The CPU performance of each CRC algorithm and variant is given in examples/benchmarks/CpuBenchmark as units of microseconds per kiB (1024 bytes).

For 8-bit processors (e.g. Nano, Micro), the numbers are roughly:

"bit": 7800-18000 micros/kiB
"nibble": 5300-7600 micros/kiB
"nibblem": 4900-7200 micros/kiB
"byte": 900-2200 micros/kiB

For 32-bit processors (e.g. SAMD, ESP8266, ESP32), the numbers are in the range of:

"bit": 400-2800 micros/kiB
"nibble": 110-700 micros/kiB
"nibblem": 110-700 micros/kiB
"byte": 50-400 micros/kiB

Recommendations

TL;DR: Use crc32_nibble in most cases, except on ESP8266 where you should use crc32_nibblem.

The benchmark numbers from CpuBenchmark and MemoryBenchmark are combined into a single place in examples/benchmarks for convenience. Here are 2 samples:

Arduino Nano

+--------------------------------------------------------------+
| CRC algorithm                   |  flash/  ram |  micros/kiB |
|---------------------------------+--------------+-------------|
| crc8_bit                        |     64/    0 |        7808 |
| crc8_nibble                     |    130/    0 |        7356 |
| crc8_nibblem                    |    134/   16 |        7224 |
| crc8_byte                       |    292/    0 |         916 |
|---------------------------------+--------------+-------------|
| crc16ccitt_bit                  |     84/    0 |       11888 |
| crc16ccitt_nibble               |    134/    0 |        5296 |
| crc16ccitt_nibblem              |    138/   32 |        5040 |
| crc16ccitt_byte                 |    562/    0 |        1496 |
|---------------------------------+--------------+-------------|
| crc16modbus_bit                 |    120/    0 |       11692 |
| crc16modbus_nibble              |    132/    0 |        5232 |
| crc16modbus_nibblem             |    134/   32 |        4912 |
| crc16modbus_byte                |    562/    0 |        1496 |
|---------------------------------+--------------+-------------|
| crc32_bit                       |    188/    0 |       18248 |
| crc32_nibble                    |    204/    0 |        7616 |
| crc32_nibblem                   |    208/   64 |        7104 |
| crc32_byte                      |   1106/    0 |        2272 |
|---------------------------------+--------------+-------------|
| CRC32                           |    208/    0 |        7688 |
| Arduino_CRC32                   |   1112/ 1024 |        2144 |
| FastCRC                         |   4262/    0 |        2160 |
+--------------------------------------------------------------+

ESP8266

+--------------------------------------------------------------+
| CRC algorithm                   |  flash/  ram |  micros/kiB |
|---------------------------------+--------------+-------------|
| crc8_bit                        |    128/    0 |        1513 |
| crc8_nibble                     |    144/    0 |         488 |
| crc8_nibblem                    |    112/   16 |         270 |
| crc8_byte                       |    336/    0 |         246 |
|---------------------------------+--------------+-------------|
| crc16ccitt_bit                  |    128/    0 |        1820 |
| crc16ccitt_nibble               |    160/    0 |         669 |
| crc16ccitt_nibblem              |    144/   32 |         283 |
| crc16ccitt_byte                 |    608/    0 |         375 |
|---------------------------------+--------------+-------------|
| crc16modbus_bit                 |    160/    0 |        1937 |
| crc16modbus_nibble              |    160/    0 |         642 |
| crc16modbus_nibblem             |    128/   32 |         244 |
| crc16modbus_byte                |    608/    0 |         375 |
|---------------------------------+--------------+-------------|
| crc32_bit                       |    144/    0 |        1567 |
| crc32_nibble                    |    176/    0 |         591 |
| crc32_nibblem                   |    160/   64 |         244 |
| crc32_byte                      |   1104/    0 |         340 |
|---------------------------------+--------------+-------------|
| CRC32                           |    224/    0 |         834 |
| Arduino_CRC32                   |   1120/ 1024 |         155 |
| FastCRC                         |   4656/    0 |         458 |
+--------------------------------------------------------------+

Comparing the different variants ("bit", "nibble" and "byte"), it seems that the "nibble" variants (4-bit lookup table) offer a good tradeoff between flash memory consumption and CPU speed in the following ways:

Compared to the "bit" versions, the "nibble" variants are about the same size but they can be up to ~2X (8-bit) to ~5X (32-bit) faster.
Compared to the "byte" versions, the "nibble" variants can be 4-5X smaller in flash size, but only about 3-4X (8-bit) to 1.5-2X (32-bit) slower.

The CRC-8 algorithm has the unfortunate property that arrays of zeros of different lengths (e.g. 1 zero or 2 zeros) have the exact same CRC (0). The other two (CRC-16-CCITT and CRC32) are able to distinguish strings of zeroes of different lengths. In terms of flash size and performance, the CRC-8 algorithm is not all that much faster than the CRC-16-CCITT, even on 8-bit processors. For these reasons, the CRC-8 algorithm is not recommended, unless you are really strapped for flash bytes.

The CRC-16-CCITT was selected for this library over the CRC-16 algorithm because CRC-16 suffers the same problem as CRC-8 with regards to arrays of zeros of different lengths. The CRC-16-CCITT uses a non-zero XOR-input which allows it to distinguish different lengths of zeros.

Between the CRC-16-CCITT and CRC-32 algorithms, if we look at the _nibble variants, there is very little difference in flash size and CPU speed, even on 8-bit processors. On 32-bit processors, the CRC-32 is actually faster. The advantage of CRC-32 over CRC-16-CCITT is that 32 bits will be able to detect longer sequences of errors than 16 bits.

The nibblem variant is the same as the nibble variant on all processors except for AVR (small speed improvement) and ESP8266 (1.9X-2.7X speed improvement). The cost is that the 16-64 bytes of static memory is consumed to store the lookup table, instead of keeping them in flash memory.

Putting all these together, here are my recommended algorithms in decreasing order of preference:

crc32_nibble in most situtations for a balance of flash size (~200 bytes) and speed
- Except on ESP8266, where crc32_nibblem should be used to gain a factor of 2.7X in performance, at a cost of only 64 byes of static RAM (out of a maximum size of 80kB).
crc16ccitt_nibble to save 60 bytes of flash on 8-bit AVR processors, with a reasonable amount of error detection
- Use crc16ccitt_nibblem if you want a 5% performance increase on an AVR, at a cost of 32 bytes of static memory. In most cases, this is probably not worth it.
crc32_byte if you need the fastest algorithm and you have 1100 bytes of flash to spare
- Except on ESP8266, where crc32_nibblem is even faster than crc32_byte, while consuming 1 kB of less flash memory, and costing only 64 bytes of extra static memory.
crc16ccitt_bit if you need to implement a very tiny CRC algorithm (90 bytes on an 8-bit AVR processor), and you are not worried about speed
crc8_bit if you need to implement the absolute smallest CRC algorithm (80 bytes on an 8-bit AVR processor), and you are not worried about speed, and you can tolerate high chances of corruption

On the ESP8266, the crc32_nibblem is the hands-down winner. It is faster than all other algorithms, while consuming only 160 bytes of flash and 64 bytes of static ram.

You can consult the results in examples/benchmarks to determine exactly how you want to make the space versus time tradeoff for your specific application.

Background and Motiviation

Before writing this library, I did not understand how CRC algorithms worked and how they were implemented. I just knew that they calculated a checksum. I had been using the crc32() algorithm from the FastCRC library (https://github.com/FrankBoesing/FastCRC) but when I dug in a little deeper, I discovered that it was configured to use "large tables" by default, which is why it is "fast". But that meant that the crc32() algorithm was pulling in a table of 1024 elements of 4 bytes each, for a total of 4kB.

The crc32() function was being called from my CrcEeprom class (https://github.com/bxparks/AceUtils), which I had included in various applications running on an Arduino Nano or Sparkfun Pro Micro with only 32kB of flash. Due to my ignorance, I was using at least 1/8 of my flash memory budget just to calculate the CRC32!

I decided to figure out how to write my own CRC routines. The best reference I found was A Painless Guide to CRC Error Detection Algorithms by Ross Williams written in 1993. Once I read that, I could understand how a code generator such as pycrc (https://pycrc.org/) worked. For each algorithm type (with a given polynomial generator), the pycrc code can generate multiple implementations. The 3 that seemed most useful in the context of small memory Arduino microcontrollers were:

bit-by-bit brute force algorithm
4-bit lookup table using 16 elements
8-bit lookup table using 256 elements

I calculated the flash and static memory consumption of each algorithm using examples/benchmarks/MemoryBenchmark. From those benchmarks, I can see that I am able to reduce the flash memory usage of the CrcEeprom class by a least 4kB by using a CRC algorithm that consumes only about 150-250 bytes (either the CRC16CCITT or CRC32 algorithm using a 4-bit lookup table).

Other CRC Libraries

I did a quick survey of existing CRC libraries for Arduino, before deciding to write my own. Part of the motiation was that I wanted to learn this stuff. The other part was that the existing libraries did not offer me enough control over space and time tradeoffs.

Arduino_CRC32 (https://github.com/arduino-libraries/Arduino_CRC32)
- uses pycrc to generate the CRC-32 with 8-bit lookup table
- equivalent to crc32_byte algorithm in the AceCRC library
- does not use PROGMEM, so the lookup table consumes 1024 bytes of static RAM
CRCx (https://github.com/hideakitai/CRCx)
- a thin abstraction layer on top of FastCRC (https://github.com/FrankBoesing/FastCRC) and CRCpp (https://github.com/d-bahr/CRCpp)
- FastCRC is used for Arduino
- FastCRC and CRCpp are vendored (copied) into that library
- FastCRC uses extra large tables by default and does not use PROGMEM, as described above
CRC32 (https://github.com/bakercp/CRC32)
- uses a 4-bit lookup table
- equivalent to crc32_nibble algorithm in the AceCRC library
- uses PROGMEM if available
FastCRC (https://github.com/FrankBoesing/FastCRC)
- uses extra large lookup tables (1024 elements, instead of usual 256 elements) by default
- does not use PROGMEM so lookup tables consume static RAM on AVR processors and ESP8266
- no longer availabe on Arduino Library Manager (see FrankBoesing/FastCRC#25)
uCRC16BPBLib (https://github.com/Naguissa/uCRC16BPBLib)
- calculates CRC-16-CCITT using bit-by-bit loop
- allows incremental CRC16 to be updated bit-by-bit
- does not use lookup table
uCRC16Lib (https://github.com/Naguissa/uCRC16Lib)
- I think the same bit-by-bit internal loop as above
- but exposes only byte-by-byte API
uCRC16XModemLib (https://github.com/Naguissa/uCRC16XModemLib)
- calculates CRC-16-XMODEM using bit-by-bit loop

Some platforms include an implementation of a CRC algorithm in a core header or an example file:

ESP8266
- #include <coredecls.h>
- uint32_t crc32 (const void* data, size_t length, uint32_t crc = 0xffffffff);
- performs a bit-by-bit loop with no lookup table
- equivalent to crc32_bit in the AceCRC library
AVR
- arduino/hardware/avr/1.8.3/libraries/EEPROM/examples/eeprom_crc/eeprom_crc.ino
- contains a CRC-32 implementation using a 4-bit lookup table
- equivalent to crc32_nibble in AceCRC

Bugs and Limitations

None that I know of right now.

System Requirements

Hardware

Tier 1: Fully supported

These boards are tested on each release:

Arduino Nano clone (16 MHz ATmega328P)
SparkFun Pro Micro clone (16 MHz ATmega32U4)
Seeeduino XIAO M0 (SAMD21, 48 MHz ARM Cortex-M0+)
STM32 Blue Pill (STM32F103C8, 72 MHz ARM Cortex-M3)
Adafruit ItsyBitsy M4 (SAMD51, 120 MHz ARM Cortex-M4)
NodeMCU 1.0 (ESP-12E module, 80MHz ESP8266)
WeMos D1 Mini (ESP-12E module, 80 MHz ESP8266)
ESP32 dev board (ESP-WROOM-32 module, 240 MHz dual core Tensilica LX6)

Tier 2: Should work

These boards should work but I don't test them as often:

ATtiny85 (8 MHz ATtiny85)
Arduino Pro Mini (16 MHz ATmega328P)
Mini Mega 2560 (Arduino Mega 2560 compatible, 16 MHz ATmega2560)
Teensy LC (48 MHz ARM Cortex-M0+)
Teensy 3.2 (72 MHz ARM Cortex-M4)

Tier 3: May work, but not supported

Any platform using the ArduinoCore-API (https://github.com/arduino/ArduinoCore-api), such as:
- Arduino Nano Every
- Arduino MKRZero
- Arduino UNO R4
- Raspberry Pi Pico RP2040

Tool Chain

It should work with PlatformIO but I have not tested it.

The library works on Linux or MacOS (using both g++ and clang++ compilers) using the EpoxyDuino emulation layer.

Operating System

I use Ubuntu 22.04 for the vast majority of my development. I expect that the library will work fine under MacOS and Windows, but I have not tested them.

The generator script in ./tools/generate.sh has only been tested under Ubuntu 20.04 and 22.04.

License

MIT License

Feedback and Support

If you have any questions, comments, or feature requests for this library, please use the GitHub Discussions for this project. If you have a bug report, please file a ticket in GitHub Issues. Feature requests should go into Discussions first because they often have alternative solutions which are useful to remain visible, instead of disappearing from the default view of the Issue tracker after the ticket is closed.

Please refrain from emailing me directly unless the content is sensitive. The problem with email is that I cannot reference the email conversation when other people ask similar questions later.

Authors

Created by Brian T. Park (brian@xparks.net).

bxparks/AceCRC