Custom Carrier Board for Linux ARM SoM

A 4–6 layer custom carrier board for a Linux ARM SoM, with on-board
audio output, MIDI output, an ESP32-driven LCD UI, and consumer-grade
power + service interfaces. A working v1 exists; v2 is a clean redesign
targeting production volumes.

## Required interfaces

1. **Linux ARM SoM**, socketed or soldered. We currently use Raspberry
Pi CM4; open to alternatives (CM5, Radxa CM3 / CM5, Pine64, custom)
— your call based on cost, availability, longevity.
2. **MIDI OUT** — 5-pin DIN and/or TRS-A 3.5 mm, 31250 baud, galvanically
isolated.
3. **Stereo line-level audio output** (RCA + 3.5 mm). Target ≥ 110 dB
SNR; ES9038-class optional for a higher-tier variant.
4. **LCD display** ~3–5", 600×170 to 800×480 range, driven by a
dedicated MCU (ESP32-S3 / ESP32-P4 / your choice). UART link to the
SoM at 115200 baud (protocol already defined on our side; you don't
need to know it).
5. **WiFi + Bluetooth** (typically in the SoM).
6. **microSD slot** on the carrier.
7. **Power**: single 5 V or 12 V input — barrel jack or USB-C PD; your
call. Must handle SoM peak boot current.
8. **Service**: USB-C for firmware / debug; optional UART debug header.
9. **Form factor**: slim bar, target ~120 × 60 × 25 mm, black anodized
aluminum enclosure or similar.

## Lessons from v1 to bake in

- **Don't put critical reset lines behind I2C expanders.** Route LCD
reset / touch reset / similar directly to MCU GPIO. v1 had cold-boot
black-screen failures when the I2C expander failed to come up.
- **Pull down any active-high enable** that sits on a high-Z pin during
MCU boot (e.g. backlight enable). v1 had visible backlight pulses
before firmware drove the pin.
- **Pick an audio jack with a proper mechanical mount** (through-hole or
mid-mount), not a pure SMD pad — v1 jack pad lifts when a TRS plug is
fully inserted.
- **Pick an LCD whose controller has its own framebuffer** — MIPI DSI
with embedded RAM, or QSPI. v1 used parallel RGB without on-chip FB
and suffered unavoidable scan-line tearing from PSRAM DMA contention.
We have a candidate panel (MIPI DSI 320×960 4.58") and are open to
alternatives.

(More detailed lessons-learned doc available once we're working
together.)

## Deliverables

1. **Architecture proposal** — block diagram + parts list, with
rationale for SoM choice, display interface, audio DAC tier.
2. **Schematic** (KiCad , EasyEDA preferred, Altium acceptable).
3. **PCB layout** (4-layer minimum; 6 layers if your audio approach
needs it).
4. **Mechanical drawing** for the enclosure (or off-the-shelf
recommendation).
5. **Fabrication output**: gerbers, BOM, pick-and-place, assembly notes.
6. **DFM review** with at least one fab quote (JLCPCB / PCBWay /
similar).

## What we want in your proposal

- Suggested **block architecture** (text + sketch is fine for the bid).
- **BOM cost target**
- **Schematic + layout time** in calendar weeks.
- **Examples** of similar boards you've designed (audio + MCU + Linux
SoM ideally).