pw_checksum

Checksum calculation API

Stable C C++

The pw_checksum module provides functions for calculating checksums.

pw_checksum/crc8.h

pw::checksum::Crc8

pw_checksum/crc16_ccitt.h

pw::checksum::Crc16Ccitt

pw_checksum/crc32.h

pw::checksum::Crc32

Implementations

Pigweed provides 3 different CRC32 implementations with different size and runtime tradeoffs. The below table summarizes the variants. For more detailed size information see the Size report below. Instructions counts were calculated by hand by analyzing the assembly. Clock Cycle counts were measured using pw_perf_test on a STM32F429I-DISC1 development board.

Variant	Relative size (see Size Report below)	Speed	Lookup table size (entries)	Instructions/byte (M33/-Os)	Clock Cycles (123 char string)	Clock Cycles (9 bytes)
8 bits per iteration (default)	large	fastest	256	8	1538	170
4 bits per iteration	small	fast	16	13	2153	215
1 bit per iteration	smallest	slow	0	43	7690	622

The default implementation provided by the APIs above can be selected through Module Configuration Options. Additionally pw_checksum provides variants of the C++ API to explicitly use each of the implementations. These classes provide the same API as Crc32:

• Crc32EightBit

• Crc32FourBit

• Crc32OneBit

Size report

The CRC module currently optimizes for speed instead of binary size, by using pre-computed 256-entry tables to reduce the CPU cycles per byte CRC calculation.

Label	Segment	Delta
CRC16 with 256-entry table	FLASH -4 pw::checksum::RunChecksum() +8 vClearInterruptMaskFromISR DEL -4 pw::checksum::NoOpChecksum::value() DEL -2 pw::checksum::NoOpChecksum::Update() NEW +512 pw::checksum::(anonymous namespace)::kCrc16CcittTable NEW +40 pw_checksum_Crc16Ccitt NEW +10 pw::checksum::Crc16Ccitt::Calculate()	+560
CRC32: 8 bits per iteration, 256-entry table	FLASH +4 pw::checksum::RunChecksum() +2 main +8 vClearInterruptMaskFromISR DEL -4 pw::checksum::NoOpChecksum::value() DEL -2 pw::checksum::NoOpChecksum::Update() NEW +1,024 _pw_checksum_InternalCrc32EightBit::kCrc32Table NEW +36 _pw_checksum_InternalCrc32EightBit NEW +20 pw::checksum::Crc32Impl<>::Update()	+1,088
CRC32: 4 bit pers iteration, 16-entry table	FLASH +4 pw::checksum::RunChecksum() +2 main -4 vClearInterruptMaskFromISR DEL -4 pw::checksum::NoOpChecksum::value() DEL -2 pw::checksum::NoOpChecksum::Update() NEW +64 _pw_checksum_InternalCrc32FourBit::kCrc32Table NEW +48 _pw_checksum_InternalCrc32FourBit NEW +20 pw::checksum::Crc32Impl<>::Update()	+128
CRC32: 1 bit per iteration, no table	FLASH +4 pw::checksum::RunChecksum() +2 main -4 vClearInterruptMaskFromISR DEL -4 pw::checksum::NoOpChecksum::value() DEL -2 pw::checksum::NoOpChecksum::Update() NEW +48 _pw_checksum_InternalCrc32OneBit NEW +20 pw::checksum::Crc32Impl<>::Update()	+64
Fletcher16 (illustrative only)	FLASH +8 pw::checksum::RunChecksum() +2 main +8 vClearInterruptMaskFromISR DEL -4 pw::checksum::NoOpChecksum::value() DEL -2 pw::checksum::NoOpChecksum::Update() NEW +58 pw::checksum::Fletcher16::Update() NEW +24 __aeabi_uidivmod NEW +10 pw::checksum::Fletcher16::value()	+104

Compatibility

• C

• C++17

Dependencies

• pw_span

Module Configuration Options

The following configurations can be adjusted via compile-time configuration of this module, see the module documentation for more details.

PW_CHECKSUM_CRC32_DEFAULT_IMPL

Selects which of the Implementations the default CRC32 APIs use. Set to one of the following values:

• PW_CHECKSUM_CRC32_8BITS

• PW_CHECKSUM_CRC32_4BITS

• PW_CHECKSUM_CRC32_1BITS

Zephyr

To enable pw_checksum for Zephyr add CONFIG_PIGWEED_CHECKSUM=y to the project’s configuration.