---
title: "CRC"
canonical_url: https://developer.repebble.com/pebbleos/docs/subsystems/crc/
last_updated: 2026-10-21
sdk_version: 4.33.1
---
# CRC

Source: https://developer.repebble.com/pebbleos/docs/subsystems/crc/

`subsys/crc/` computes every checksum the firmware stores or exchanges. Its public API is `include/pbl/crc/crc.h`:

| 

Function

 | 

Algorithm

 | 

Used by

 |
| --- | --- | --- |
| 

`pbl_crc8()`, `pbl_crc8_reversed()`

 | 

CRC-8, polynomial 0x2F (CRC-8/OPENSAFETY)

 | 

PFS page chaining, settings file key hashes

 |
| 

`pbl_crc32()`

 | 

CRC-32/ISO-HDLC, zlib-compatible

 | 

PULSE2, flash CRC, shared PRF storage, boot bits

 |
| 

`pbl_crc32_legacy_*()`

 | 

CRC-32/MPEG-2 over little-endian words

 | 

PFS, app images, resources, data logging, PULSE, put bytes

 |

The legacy checksum reproduces what the STM32F2/F4 CRC unit computed when the firmware fed it 32-bit words, including the zero padding of a trailing partial word. It lives on in on-flash and on-wire formats, so its definition cannot change; `crc.h` spells it out.

## Layering

The layering follows Linux’s `lib/crc`. The public functions own the software implementations, which are always built, and decide when to hand work to hardware:

```
flowchart TD
  api["pbl_crc32() / pbl_crc32_legacy_update()<br/>subsys/crc"]
  hw["pbl_crc_hw_*()<br/>driver, optional"]
  sw["Software<br/>nybble-wide tables"]
  api -- "len ≥ CONFIG_CRC_HW_MIN_LEN" --> hw
  api -- "remainder" --> sw
  hw -. "what it did not process" .-> sw

  classDef app fill:#fa4a36,stroke:#fa4a36,color:#ffffff
  classDef system fill:#fd8a7c,stroke:#fd8a7c,color:#171717
  classDef base fill:#525252,stroke:#525252,color:#ffffff
  class api app
  class sw system
  class hw base
```

Hardware acceleration comes from a [CRC driver](https://developer.repebble.com/pebbleos/docs/drivers/crc.md), which may process any part of a buffer or none of it: the subsystem always finishes in software, so callers never see a failure.

`CONFIG_CRC_HW_MIN_LEN` keeps short buffers in software, where programming the peripheral costs more than it saves. CRC-8 inputs are a few bytes long and are never offloaded.

Generated from PebbleOS [3a7b71785](https://github.com/coredevices/pebbleos/commit/3a7b71785fd439666c4a8bc116f561140e8dec01). This page is maintained in the pebbleos repository: [docs/subsystems/crc.md](https://github.com/coredevices/pebbleos/blob/3a7b71785fd439666c4a8bc116f561140e8dec01/docs/subsystems/crc.md).