Design Log DEC 044 fdc2114 internal oscillator selected clkin gnd - thepinkmile/Enigma-NG GitHub Wiki

DEC-044 - FDC2114 Internal Oscillator Selected (CLKIN → GND)

  • Status: Decided
  • Date: 2026-04-26
  • Category: Electrical / Component configuration
  • Area: Rotor Board, capacitive position sensing, FDC2114 clock source
  • Author: Izzyonstage & GitHub Copilot

Summary

The FDC2114 on every Rotor Board uses its internal oscillator as the conversion clock. The CLKIN pin is tied permanently to GND on the PCB. The CHx_FIN_SEL field in each channel's SETTLECOUNT register is set to 0b10 (internal oscillator reference).

Problem

The FDC2114 supports two clock sources: an external crystal/oscillator on CLKIN, or the internal ~43.35 MHz oscillator activated when CLKIN is tied to GND. A decision was required on which to use for the Rotor sensing application.

Decision

Use the FDC2114 internal oscillator (CLKIN → GND). This means:

  1. No external crystal or oscillator component on the Rotor PCB.
  2. CHx_FIN_SEL = 0b10 in all active channel SETTLECOUNT registers.
  3. Conversion reference clock frequency ≈ 43.35 MHz (internal specification).
  4. CPLD I²C master (VHDL bitstream) configures this field at power-up.

Rationale

  • Avoids an external crystal BOM cost and the associated PCB footprint on a space-constrained circular board (Ø92mm with CPLD, FDC2114, sensor electrodes, DIP switches, and connectors).
  • The internal oscillator frequency (~43.35 MHz) is well above the ~6.5 MHz nominal resonant frequency of the LC tank (18 µH + 33 pF), satisfying the FDC2114 requirement for the conversion clock to be >4x the sensor resonant frequency.
  • Absolute frequency accuracy of the conversion clock is not critical for this application. Gray-code position detection requires only relative threshold crossing detection (presence or absence of an aluminium shroud slot), not absolute frequency measurement.
  • Confirmed viable by TI FDC2114 datasheet §8 (internal oscillator specification) and TI FDC2114 application note guidance.

Alternatives Considered

Alternative Reason rejected
External crystal on CLKIN Additional BOM line, PCB footprint, and layout complexity; not needed for this sensing application
External clock oscillator module Same objections as crystal; adds cost and board area

Impact

  • Rotor Board PCB: CLKIN tied to GND; no crystal footprint required.
  • CPLD VHDL: set CHx_FIN_SEL = 0b10 in SETTLECOUNT register for all active channels at power-up initialisation.
  • design/Electronics/Rotor/Design_Spec.md: Resonant Front-End Topology subsection updated.
  • design/Software/CPLD_Logic/Rotor_Logic.md: FDC2114 register map documents this setting.

Cross-ref

design/Electronics/Rotor/Design_Spec.md, design/Software/CPLD_Logic/Rotor_Logic.md, design/Procedures/Lab_Tests.md (LT-001, LT-002).