Rotor CPLD Logic Requirements - FDC2114 Configuration

Status: Draft Project: Enigma-NG Author: Izzyonstage & GitHub Copilot Version: v.0.1.0 Associated Hardware Revision: Rev A Last Updated: 2026-04-26

1. Overview

This document defines the CPLD logic requirements and FDC2114 register configuration for the Rotor Board capacitive position sensing subsystem. The Intel MAX II EPM570T100I5N CPLD acts as the I²C master on the J9 bus and is responsible for configuring all FDC2114 devices at power-up and reading their conversion results to produce the Gray-coded position output.

Board-level connector ownership, physical placement, and BOM authority remain in:

• design/Electronics/Rotor/Design_Spec.md
• design/Electronics/Rotor/Board_Layout.md

1.1 FDC2114 Firmware Note

The FDC2114 is a hardware converter and has no user-programmable firmware. All register configuration is performed at runtime by the CPLD I²C master via VHDL bitstream. No UART programming header is present on the Rotor board. The JTAG daisy-chain programs the CPLD only.

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2. FDC2114 Register Configuration

The CPLD initialises each FDC2114 on the J9 bus immediately after power-up via I²C. All multi-channel devices (U2, and where populated U3/U4) receive the same baseline register map.

2.1 Register Map (Baseline)

Register	Address	Value (hex)	Field / Setting	Notes
DRIVE_CURRENT_CH0	0x1E	0x7C00	IDRIVE = 0b01111, INIT_DRIVE = 0b0	Baseline drive current - lab validate (LT-001)
DRIVE_CURRENT_CH1	0x1F	0x7C00	As above	All channels same baseline
DRIVE_CURRENT_CH2	0x20	0x7C00	As above	All channels same baseline
DRIVE_CURRENT_CH3	0x21	0x7C00	As above	All channels same baseline
MUX_CONFIG	0x1B	0x020D	DEGLITCH = 0b101 (10 MHz), AUTOSCAN_EN = 1, RR_SEQUENCE = 0b01 (CH0-CH1)	Adjust RR_SEQUENCE per active channel count; deglitch lab validate (LT-002)
CONFIG	0x1A	0x1601	ACTIVE_CHAN = 0b00 (start CH0), SLEEP_MODE_EN = 0, RP_OVERRIDE_EN = 0, SENSOR_ACTIVATE_SEL = 1	Continuous conversion, internal oscillator active
RESET_DEV	0x1C	0x8000	Full device reset	Issue once on power-up before writing other registers

CHx_FIN_SEL note: The FDC2114 CHx_FIN_SEL field in each channel's SETTLECOUNT register must be set to 0b10 (single-ended, internal oscillator as reference clock) for all channels. This routes the internal ~43.35 MHz oscillator as the conversion clock; CLKIN is tied to GND on the PCB.

2.2 Internal Oscillator Rationale

CLKIN is tied to GND on the Rotor PCB. The FDC2114 detects this condition and activates its internal oscillator (~43.35 MHz). Reasons for this choice:

• Avoids external crystal BOM cost and PCB footprint on a space-constrained circular board.
• Internal oscillator frequency is well above the ~6.5 MHz nominal resonant frequency of the LC tank (18 µH + 33 pF), satisfying the >4x oversampling requirement.
• Absolute frequency accuracy of the conversion clock is not critical; relative change detection (presence/absence of aluminium shroud segment) is sufficient for Gray-code position decoding.
• Confirmed viable by TI FDC2114 datasheet §8 (internal oscillator specification).

Design decision recorded in design/Design_Log/DEC-044_fdc2114-internal-oscillator-selected-clkin-gnd.md as DEC-044.

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3. J9 I²C Bus

The J9 bus spans Board A and Board B via the 1x5 inner-face header pair (J9 [Board A] male, J9 [Board B] female). Pin 3 = SCL, Pin 4 = SDA (3.3V logic levels, pulled up via R6/R7 on Board A). I²C addresses: U2 = 0x2A, U3 = 0x2B (Board A, N=26 only), U4 = 0x2B (Board B, N=64 only).

U3 and U4 share address 0x2B but reside on electrically separate boards. The CPLD manages Bus direction via the J9 header; U3 and U4 are never simultaneously addressable.

3.1 CPLD I²C Master Behaviour

1. On power-up, issue RESET_DEV (0x1C = 0x8000) to all FDC2114 devices.
2. Write baseline register map (§2.1) to each populated device.
3. Enter continuous polling loop: read DATA_CH0_MSB/DATA_CH0_LSB (addresses 0x00-0x07) for each active channel on each populated device.
4. Decode raw conversion values to detect sensor threshold crossing (presence of Al shroud slot).
5. Assemble Gray-coded bit vector and drive Encoder output lines.

Threshold calibration values are stored in CPLD configuration (loaded via JTAG). Initial baseline values require lab bench validation - see design/Procedures/Lab_Tests.md.

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4. JTAG Programming

The CPLD is programmed via the JTAG daisy-chain (J1 → J2 → J3 across a rotor stack, continuing to the Stator). Programming is performed using the Intel Quartus Prime Programmer via the Controller Board JTAG interface before final rotor assembly. Each Rotor board must have its CPLD image flashed as part of the manufacturing procedure before the aluminium shroud is closed.

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5. Lab Test Cross-References

ID	Title	File
LT-001	FDC2114 IDRIVE Baseline Calibration	design/Procedures/Lab_Tests.md
LT-002	fSENSOR Resonant Frequency Validation	design/Procedures/Lab_Tests.md