Design Log DEC 031 cm5 virtual keyboard key injection feature architecture - thepinkmile/Enigma-NG GitHub Wiki

DEC-031 - CM5 Virtual Keyboard (Key Injection) Feature Architecture

  • Status: Decided
  • Date: 2026-04-14
  • Category: Electrical / Firmware
  • Area: Stator Board - I²C Expanders, Servo, External Keyboard Source Mux
  • Author: Izzyonstage & GitHub Copilot

Summary

Design decision for enabling the CM5 to autonomously inject virtual keypresses into the Enigma cipher pipeline, enabling automated rotor configuration testing and fully autonomous cipher operation.

Problem

The Enigma machine requires a physical keypress to advance rotors and inject input into the cipher pipeline. For autonomous CM5 operation (testing, remote cipher operation), a mechanism is needed to inject keypresses without human intervention.

Decision

Add three I²C expanders to the Stator board on the shared I²C-1 bus:

  1. MCP23017 @ 0x20 (U6): 16-bit GPIO expander for ENC_IN/ENC_OUT monitoring. Replaces CM5 GPIO 4-15 (12 pins freed).

    • GPA[5:0] = ENC_IN[5:0] monitor (inputs)
    • GPB[5:0] = ENC_OUT[5:0] monitor (inputs)
    • GPA[6:7], GPB[6:7] = spare

  2. MCP23017 @ 0x21 (U7): 16-bit GPIO expander for virtual key data injection, external keyboard-source mux control, and SYS_RESET_N (replaces CM5 GPIO 26 - 1 pin freed).

    • GPA[5:0] = CM5_KEY_DATA[5:0] (outputs - 6-bit virtual key data bus)
    • GPA[6] = KEY_CM5_ACTIVE (output - mux select: 0=physical keyboard, 1=CM5 virtual)
    • GPA[7] = SYS_RESET_N (output - system-wide CPLD reset)
    • GPB[7:0] = spare / reserved

Rationale for MCP23017 (x2)

  • 32 GPIO total provides headroom for future I/O expansion.
  • SYS_RESET_N migration to expander reduces LINK-BETA pin count and frees CM5 GPIO 26.
  • All keyboard-source mux and reset control signals remain co-located with the Stator CPLD.
  • Only 2 wires (I²C SDA/SCL) needed on LINK-BETA instead of 13 discrete signal pins.

External Keyboard Source Mux

  • GPA[6] on U7 drives KEY_CM5_ACTIVE for the external keyboard-source mux at the Stator keyboard-source entry point.
  • KEY_CM5_ACTIVE=0: the physical keyboard 6-bit source bus is forwarded to the encryption pipeline.
  • KEY_CM5_ACTIVE=1: CM5_KEY_DATA[5:0] is forwarded instead.
  • The selected implementation uses U4 and U5, both 74HC157PW-Q100,118 quad 2:1 mux devices, to cover the 6-bit path, with both E pins tied to GND so the mux is always enabled while the board is powered.
  • GPA[7] now carries SYS_RESET_N, fully populating the U7 GPA port and leaving U7 GPB completely spare / reserved for future use.

Net Effect on LINK-BETA

  • Freed (13 pins): ENC_IN[5:0] (x6), ENC_OUT[5:0] (x6), SYS_RESET_N (x1).
  • Freed: pins 8 (SYS_RESET_N), 12-17 (ENC_IN[5:0]), and 19-24 (ENC_OUT[5:0]) were freed by this decision.
  • Later reuse: DEC-036 reallocated the former monitor block into grouped LINK-BETA power rails (5V_MAIN, 3V3_ENIG, and GND) once the Stator-side 5V needs were formalised.
  • R6 pull-up (10kΩ to 3V3_ENIG) on Stator keeps SYS_RESET_N HIGH at power-up (CPLDs out of reset).

Net Effect on CM5 GPIO

  • Freed (13 pins): GPIO 4-15 (ENC monitoring), GPIO 26 (SYS_RESET_N).
  • No new CM5 GPIO assignments required.

I²C Bus Address Map (no conflicts)

Address Device Location
0x09 LTC3350 Power Module
0x0B Smart Battery Power Module
0x20 MCP23017 (U6) Stator
0x21 MCP23017 (U7) Stator
0x22 MCP23017 (U8) Stator
0x23 MCP23017 (U1) Settings Board
0x24 MCP23017 (U2) Settings Board
0x25 MCP23017 (U3) Settings Board
0x28 STUSB4500 Power Module
0x40 INA219 (U12) Power Module
0x45 INA219 (U2) Stator