Design Log DEC 033 dsi1 display connector provision for optional lid touchscreen - thepinkmile/Enigma-NG GitHub Wiki

DEC-033 - DSI1 Display Connector Provision for Optional Lid Touchscreen

  • Status: Decided
  • Date: 2026-04-14
  • Category: Electrical / HMI
  • Area: Controller Board - CM5 display interface; Main Enclosure lid
  • Author: Izzyonstage & GitHub Copilot

Summary

Add a DSI1 4-lane FPC connector (J9) to the Controller Board to provision for an optional lid-mounted touchscreen display, enabling the Enigma-NG to operate as a self-contained machine without an external monitor. The Display Add-on Board design is deferred.

Problem

The Enigma-NG currently relies on an external HDMI monitor and USB HID devices for system management and GUI display. With the CM5 virtual keyboard injection (DEC-031) and Settings Board (DEC-032) features, the machine is capable of autonomous operation. Adding a lid-mounted touchscreen completes the self-contained package - the only remaining external peripheral would be a keyboard and mouse for system administration tasks.

Decision

  1. Add J9 (Amphenol F52Q-1A7H1-11015, 15-pin 1.0mm pitch ZIF/FPC connector) to the Controller Board to expose the CM5 DSI1 4-lane interface (CLK +/-, D0-D3 +/-).
  2. Route DSI1 differential pairs on L3 (100 Ω differential stripline - same rule as HDMI/USB). Traces run from CM5 mezzanine connector pins to J9 near the CM5 socket.
  3. Touch I²C (capacitive touch controller SDA/SCL) is routed via the I²C-1 bus already present on the Controller Board.
  4. J9 is the only Controller-side display connector fixed in the current scope. Any future display power or auxiliary touch wiring remains deferred with the display add-on board definition.
  5. The HDMI port (J4) is retained and unaffected - J9 is additive.
  6. Display Add-on Board design (lid mounting frame, FPC cable assembly, backlight driver if required, touch controller interface) is deferred to a future design phase.

Display Interface

  • Interface: MIPI DSI1 (4-lane)
  • Differential impedance: 100 Ω (same as HDMI - no new routing rules)
  • Connector: Amphenol F52Q-1A7H1-11015 - 15-pin 1.0mm pitch right-angle ZIF/FPC
  • Cable: Thin FPC routed through lid hinge area (far superior to HDMI cable for hinge flexing)
  • Touch input: Capacitive touch controller I²C on I²C-1 bus (existing bus, no new wires required)
  • Supported sizes: DSI1 4-lane supports displays up to 10" at full resolution (e.g., RPi official 10" touchscreen)

Rationale

  • DSI1 FPC cable is thin and flexible - ideal for routing through a lid hinge without fatigue.
  • Retaining HDMI preserves external monitor compatibility for development and maintenance.
  • The 100 Ω differential rule is already established on L3 for HDMI - no new stackup or impedance work needed.
  • Provisioning the connector now (no extra cost, trivial PCB area) avoids a board revision later if the display add-on is adopted.
  • I²C-1 bus already has spare address space for a touch controller (e.g., FT5426 at 0x38 or similar).

Impact

  • Controller Board: J9 added; ~10 new 100 Ω diff traces on L3; small ZIF footprint on L1 near CM5
  • Main Enclosure: Lid requires provision for display mounting and FPC hinge routing (see Main_Enclosure/Design_Spec.md)
  • Display Add-on Board: Deferred - to be designed as a separate optional add-on
  • Firmware / OS: Standard RPi DSI1 display driver; no custom firmware changes needed for basic display output