DEC-086 - Cypher-Input Restructured into Common/Variant Files; 10-Numeric Variant Added; Single Shared I2C Address

Field	Value
Decision ID	DEC-086
Status	Decided
Date	2026-08-12
Author	Izzyonstage & GitHub Copilot
Amends	Cypher-Input Design_Spec.md §1, §3, §3a, §4, §5, §10, §11; Board_Layout.md §4; supersedes the per-variant I2C addressing introduced in the 2026-08-06 session (see handoff.md "What changed in this session (2026-08-06)")

Context

The Cypher-Input Board Design_Spec.md/Board_Layout.md had grown to document two board variants (26-Char Classic, 64-Char Extended) inline, with per-variant tables and BOM columns interleaved throughout the common circuit description. This diverged from the pattern already established for the Rotor board, which splits common circuit design (Rotor/Design_Spec.md) from per-variant detail (Rotor_26_Char_Design.md, Rotor_64_Char_Design.md).

A design review also identified that the Cypher-Input Board's on-board I2C GPIO expander (U4, PCA9534A) had been assigned a distinct I2C address per variant (0x38 Extended, 0x39 Classic) for board-type identification. This was redundant: the Cypher Board interconnect already carries a hardwired BOARD_ROLE_ID[2:0] strap (per Cypher/Board_Layout.md §4) whose sole purpose is variant identification, with 0b010 already reserved for a future 10-Numeric variant. A keyboard board only needs one identification mechanism.

Separately, the system was missing a 10-Numeric variant: a dedicated number-pad keyboard (0-9) alongside the existing Classic (26-letter) and Extended (64-character) variants.

Decision

1. Restructure Cypher-Input into common + per-variant files, mirroring the Rotor pattern:

   • Cypher-Input/Design_Spec.md and Board_Layout.md retain only content common to all variants (ENC module interface, LED/brightness/interconnect circuits, PCB fabrication, common BOM).
   • Cypher_Input_26_Char_Design.md, Cypher_Input_64_Char_Design.md, Cypher_Input_10_Numeric_Design.md each hold that variant's key layout, plain-bits allocation, BOARD_ROLE_ID[2:0] value, LED behaviour note, and variant-specific BOM (LED bank, current-limit resistors, hot-swap sockets, mechanical keyswitches).

2. Add the 10-Numeric variant: 10 digit keys (0-9) in a common number-pad grid layout (7 8 9 / 4 5 6 / 1 2 3 / Space 0 Enter), plus Space and Enter for CM5 UI input clarity (same non-cipher role as on the Extended variant). No Shift key - digits have no case distinction. BOARD_ROLE_ID[2:0] = 0b010 (already reserved in Cypher/Board_Layout.md §4).

3. Collapse U4 (PCA9534A) to a single fixed I2C address (0x38) across all Cypher-Input variants. Variant identification is carried solely by BOARD_ROLE_ID[2:0], not by I2C address. 0x39 is no longer reserved for the 26-Char Classic variant and reverts to the free pool (0x39-0x3E) for future board types (e.g. Cypher-Output, or a fully custom keyboard board outside this family) - not for further Cypher-Input variants, which use BOARD_ROLE_ID instead.

Rationale

• Splitting common/variant content keeps each variant file self-contained and avoids repeatedly interleaving 2-3 way conditional text through the common circuit description, consistent with the precedent set by the Rotor board.
• A single I2C address for U4 removes a redundant identification mechanism: BOARD_ROLE_ID[2:0] already exists on the shared Cypher Board interconnect and scales to any number of variants without consuming additional I2C address space.
• The 10-Numeric variant reuses the existing circuit topology (ENC module mount, LED bank, brightness control, board interconnect, I2C board-ID expander) with no new component types - only quantities, key layout, and the BOARD_ROLE_ID strap value change.

Impact

• design/Electronics/Cypher-Input/Design_Spec.md - §1 (variant table + 3-file split reference), §3 (plain-bits allocation moved to variant files), §3a (single I2C address, BOARD_ROLE_ID as sole identifier), §4/§5 (variant text generalised), §10 (10-Numeric data plate naming), §11 (BOM reduced to common components only).
• design/Electronics/Cypher-Input/Board_Layout.md §4 - PB[] usage note and BOARD_ROLE_ID note updated to include the 10-Numeric variant.
• New files: Cypher_Input_26_Char_Design.md, Cypher_Input_64_Char_Design.md, Cypher_Input_10_Numeric_Design.md.
• System-wide I2C address tables updated to reflect the single shared 0x38 address: design/Electronics/Boards_Overview.md, Electrical_Design.md, System_Architecture.md, Controller/Design_Spec.md §4.1.