Design Log DEC 052 variant specific fdc2114 refdes use ab suffix convention - thepinkmile/Enigma-NG GitHub Wiki

DEC-052 - Variant-Specific FDC2114 RefDes Use A/B Suffix Convention

  • Status: Active
  • Date: 2025-07-10
  • Category: Architecture
  • Area: Rotor Board - Position Sensing (FDC2114 sensor ICs and associated passives)

Decision

Variant-specific FDC2114 sensor components on the Rotor board are assigned RefDes with an A or B suffix to indicate which PCB sub-board they are physically located on:

  • Board A (N=26 variant): U3A, C16A, C17A, C22A-C25A, L5A-L8A
  • Board B (N=64 variant): U3B, C16B, C17B, C22B-C25B, L5B-L8B

The B-suffix base numbers mirror the A-suffix base numbers exactly, making the pairing immediately readable. The always-fitted sensor (U2) and its bypass and resonant front-end components (C14, C15, C18-C21, L1-L4) retain plain RefDes with no suffix as they are fitted on all variants.

In KiCAD the A and B suffixes are unique RefDes. Two project variants are defined:

  • N26: U3A fitted; U3B (and associated B-suffix passives) marked DNP.
  • N64: U3B fitted; U3A (and associated A-suffix passives) marked DNP.

The Consolidated BOM System Qty column counts only the components fitted in a single variant: each mutually-exclusive sensor row carries qty 1 per rotor.

Rationale

KiCAD does not support duplicate RefDes within a single project. Using A/B suffixes allows both Board A and Board B sensor circuits to coexist in the same schematic and PCB layout file while remaining unique. Mirroring the base number across A and B (e.g. C16A / C16B) makes the pairing unambiguous without a separate mapping table. The overall capacitor and inductor sequence is kept consecutive with no gaps.

Alternatives Considered

  • U3 / U4 (no suffix): Used prior to this decision. Abandoned because schematic and PCB layout tools require unique RefDes; U3 and U4 appeared confusingly similar to Controller Board RefDes (U4-U6) documented elsewhere.
  • Separate schematic/layout per variant: Rejected; doubles maintenance burden and makes shared JTAG and data routing harder to manage.

Impact

  • design/Electronics/Rotor/Design_Spec.md - all occurrences of U3/U4 for the Rotor board updated to U3A/U3B; bypass and tank passive RefDes updated throughout.
  • design/Electronics/Rotor/Board_Layout.md - Board A and Board B tables updated.
  • design/Electronics/Rotor/Rotor_26_Char_Design.md - variant spec updated.
  • design/Electronics/Rotor/Rotor_64_Char_Design.md - variant spec updated.
  • design/Electronics/Consolidated_BOM.md - rows for C16/C17, C18-C21, C22-C25, L5-L8, and U3/U4 updated; System Qty corrected for mutually-exclusive rows.