20260928-1330-key-two-expansion-buses Give Key two expansion paths: I2C expansion units and an internal CAN bus

• status: completed
• priority: P1
• owner: claude/session-20260922-0815
• createdAt: 2026-09-28 13:30

Description

After comparing I2C with a multiplexer, RS-485 and a second CAN bus for Key expansion, the owner decided to build two paths ("用 A 扩展单元和 CAN 扩展两种方案来做"): I2C expansion units without an MCU for boards inside one assembly, and expansion units with a small CAN MCU on an internal CAN bus driven by the second TWAI controller of the ESP32-C6 for distant or numerous boards. The internal bus is separate from the external YBO CAN so that neither disturbs the other.

Acceptance criteria:

1. Spec, design, guide, brief, indexes, evaluation and governance pages describe both paths with their limits and costs; RS-485 is recorded as a compared alternative.
2. A numbered requirement keeps the internal CAN bus and the YBO CAN apart: no fault or traffic on one may affect the other, and expansion units are not X01 nodes.
3. bun run docs:lint && bun run docs:build && git diff --check passes.

ActiveForm

Adding the internal CAN expansion path to Key

Dependencies

• blocked by: (none)
• blocks: (none)

Notes

• Key R0.3 was committed as 68fc761 before this task finished; the change is recorded as spec R0.4, design R0.4 and guide R0.5.

• Plan: 20260928-1330-key-two-expansion-buses; approval given in the owner's decision.

• complete: Key pages amended for I2C and internal CAN expansion; KEY-22 and KEY-O-12 added; lint 229 pages 0 findings, build and git diff --check pass.