Design Log DEC 071 usm spdt switch dual terminated wiring and pull down resistor removal - thepinkmile/Enigma-NG GitHub Wiki
DEC-071 - USM SPDT Switch Dual-Terminated Wiring and Pull-Down Resistor Removal
| Field | Value |
|---|---|
| Decision ID | DEC-071 |
| Status | Confirmed |
| Date | 2026-05-14 |
| Author | Izzyonstage & Copilot |
| Amends | USM DR-USM-09 (switch wiring topology); USM DR-USM-07 (R11 renumbered R1) |
Context
A review of the SPDT toggle switch wiring for SW1-SW10 (E-Switch 200MSP1T2B4M2QE) raised the question of whether the opposite/unasserted throw, left unconnected in the prior design, could produce a floating GPIO input at U1. The three physical terminals of the 200MSP1T2B4M2QE are:
- Pin 1 - NC (Normally Closed): connected in the default/OFF switch position
- Pin 2 - COM (Common): centre lug, always present regardless of switch position
- Pin 3 - NO (Normally Open): connected in the asserted/ON switch position
Decision
D1 - Dual-terminated switch wiring adopted; pull-down resistors removed: Each toggle switch (SW1-SW10) is wired as follows:
- NC terminal (Pin 1) → GND
- COM terminal (Pin 2) → CFG_ GPIO input on U1*
- NO terminal (Pin 3) → 3V3_ENIG
Both throws are now hard-terminated: in the NO/ON position the COM net is driven HIGH by 3V3_ENIG; in the NC/OFF position the COM net is driven LOW by GND. Neither switch position leaves the COM terminal floating. The 10x 10kΩ 0603 pull-down resistors (R1-R10, DR-USM-09) are no longer required and are removed from the design. There is no shoot-through path because only one throw is connected at any time.
D2 - R11 renumbered to R1: With R1-R10 removed, R11 (10kΩ 0603 pull-up for CFG_APPLY_N, DR-USM-07) becomes the sole 10kΩ 0603 resistor on the board. It is renumbered R1 to eliminate a gap in the RefDes sequence. All cross-references to R11 in the USM Design_Spec.md, Board_Layout.md, and Consolidated_BOM.md are updated to R1.
D3 - LED driving architecture unchanged: The switch signal path (U1 @ 0x23) and LED output path (U2 @ 0x24, U3 @ 0x25) are fully independent. LED illumination remains under software control exclusively; there is no hardware tie between switch position and LED state.
Rationale
The dual-terminated topology is strictly superior to pull-resistor termination for panel toggle switches: it requires no additional components, eliminates both the floating-input risk and the power draw through pull resistors, and produces well-defined GPIO logic levels in both switch positions. The component reduction also simplifies board layout in the U1 vicinity.
R1-R10 were originally specified (DEC not recorded; reflected in the initial DR-USM-09 entry) as a defensive measure against floating inputs. With hard termination on both throws, this defensive measure is redundant.
Files Changed
- design/Electronics/User_Settings_Module/Design_Spec.md - DR-USM-09 rewritten (dual-terminated topology, pull-downs removed); DR-USM-07 R11 reference updated to R1; U1 pin table Pull column updated for GPA[3:0] and GPB[5:0]; silicon note updated; §6 CFG_APPLY_N pull-up reference updated R11→R1; BOM row updated R1-R11 Qty 11 → R1 Qty 1; component count row removed (switch pull-downs); component count misc resistor row updated R11→R1; total component count 166→156; Last Updated 2026-05-14
- design/Electronics/User_Settings_Module/Board_Layout.md - placement zone note updated R11→R1, switch pull-down reference removed; Last Updated 2026-05-14
- design/Electronics/Consolidated_BOM.md - 10kΩ row RefDes updated USM: R1-R11 → USM: R1; USM quantity updated 11→1; row total updated 21→11