Find counter signals in a capture

Goal

Identify likely counter fields inside a capture — small monotonically incrementing fields that often live in the upper or lower nibble of a byte.

Prerequisites

• CANarchy installed.
• A capture with at least a few hundred frames per ID. The examples use the in-tree fixture tests/fixtures/re_counter_nibble.candump.

Run

canarchy re counters tests/fixtures/re_counter_nibble.candump --text

The text output lists candidate counters with their ID, byte position, bit width, and stability score. JSON / JSONL output includes the same fields machine-readably:

canarchy re counters tests/fixtures/re_counter_nibble.candump --json

What "counter" means here

• The candidate field changes on most frames of the same ID.
• The change is usually +1, modulo the field width.
• Periodic rollover (for example a 4-bit counter wrapping from 0xF to 0x0) is recognised.

Validate with re entropy

Counters typically have high entropy. Cross-check with:

canarchy re entropy tests/fixtures/re_counter_nibble.candump --json

The combination is a good first pass before you start drafting signal definitions.

Where to go next

• Match an unknown capture against opendbc
• Command Spec — re