• Status: Review
• Portable, battery-powered MIDI utility/controller based on a Raspberry Pi Pico 2 (RP2350).
• Production intent: accessible assembly for prototypes through low-thousands volume, prioritizing available modules and conventional 2-layer PCB assembly.

• Handheld or desktop music device used between computers, synthesizers, and other MIDI equipment.
• Firmware already exists; hardware must expose reliable USB-MIDI and traditional DIN MIDI connections.

• Send and receive USB MIDI through USB-C.
• Receive galvanically isolated MIDI through a 5-pin DIN input.
• Transmit MIDI through a 5-pin DIN output.
• Accept four momentary button inputs.
• Show status and menus on a small OLED display.
• Operate from USB power or an internal rechargeable battery, including operation while charging.

• Raspberry Pi Pico 2 module with RP2350.
• USB-C for power, programming, and USB 2.0 device data.
• 5-pin DIN MIDI IN and MIDI OUT.
• Four front-panel tactile buttons.
• 0.96-inch 128x64 SSD1306-compatible I2C OLED.
• Protected 1-cell 1000 mAh LiPo battery.
• Power-path charging, power switch, and battery state monitoring.
• SWD/debug pads and production test access.

Diagram

• Raspberry Pi Pico 2 castellated module to reduce clock, flash, regulator, and RP2350 escape-layout risk.
• Use GPIO assignments compatible with the existing firmware; initial pin map will be documented after schematic capture.

• USB-C receptacle configured as a USB 2.0 device/sink.
• CC1 and CC2 pull-down resistors, ESD protection, and controlled routing for D+/D-.
• USB VBUS feeds the charger/power path; USB data connects to the Pico 2 USB pins.

• MIDI IN uses a standards-compatible current loop and optocoupler, preserving galvanic isolation.
• MIDI OUT uses a 3.3 V-compatible logic buffer and current-limiting network suitable for DIN MIDI receivers.

• Four normally-open tactile switches to GPIO with firmware pull-ups or external pull-ups as required.
• SSD1306-compatible I2C OLED on a 4-pin board connector/module footprint.

• 1-cell protected LiPo, nominal 3.7 V, proposed capacity 1000 mAh.
• USB charger with dynamic power path so the product can operate while charging.
• Battery monitor/fuel gauge on I2C.
• Pico 2 powered through VSYS; no separate 3.3 V regulator is planned unless the finalized load budget requires one.

• USB-C: USB 2.0 device data and 5 V input.
• MIDI IN: panel-mounted 5-pin DIN.
• MIDI OUT: panel-mounted 5-pin DIN.
• Battery: keyed 2-pin JST-PH connector, polarity clearly marked.
• Display: 4-pin I2C connection (GND, VCC, SCL, SDA).
• Debug: SWD and reset/test pads.

• Operates from USB with or without a battery fitted.
• Charges while operating.
• Target charge current: approximately 500 mA, subject to battery rating and thermal verification.
• Initial runtime target: at least 8 hours typical use from 1000 mAh.

• Battery worst-case design current: 250 mA plus power-conversion losses; battery and protection should support at least 500 mA continuous.
• Estimated runtime at 80 mA average from a 1000 mAh cell: roughly 10 hours after conversion and capacity margin; validate on hardware.
• USB input path and charger will be rated for at least 1 A even though default charging is approximately 500 mA.

• Two-layer custom carrier PCB where signal integrity and routing permit.
• Pico 2 castellated module avoids bare-RP2350 manufacturing complexity.
• Prefer 0603 passives and common SMT packages; DIN connectors, battery connector, buttons, and display connection may be through-hole for mechanical strength.
• Select active parts with multiple distributors and avoid obsolete components.
• Include accessible test pads for VBUS, VSYS, 3V3, GND, SWD, USB data, UART/MIDI signals, and button inputs.

• USB device connection must use the Pico 2 native USB interface.
• MIDI UART must support 31.25 kbaud.
• I2C bus for OLED and battery monitor.
• Four active-low button GPIOs.
• Battery-voltage/state information available to firmware.
• Boot, reset, and SWD access retained.

• Provisional enclosure: desktop/handheld rectangular case, approximately 100 x 60 x 25 mm.
• Provisional PCB envelope: maximum approximately 85 x 50 mm.
• USB-C centered on one short edge; two DIN connectors on the opposite long or short edge based on fit.
• OLED and four buttons accessible from the top face.
• Battery mounted below or beside the PCB with insulation and strain relief.
• Connector placement, mounting holes, component height, and final board outline remain provisional until the enclosure and exact off-board parts are approved.

• Use Pico 2 rather than bare RP2350 for fastest and lowest-risk path to production.
• Use USB-C for combined power, programming, and USB-MIDI.
• Use isolated DIN MIDI IN.
• Use a power-path charger rather than a charger-only IC.
• Use a replaceable 1-cell LiPo connected by JST-PH.
• Use an I2C OLED to minimize GPIO count.

• The existing firmware can accept a documented GPIO assignment or be configured to match it.
• Device is a USB peripheral, not a USB host.
• No USB Power Delivery is required.
• No wireless connectivity, audio I/O, rotary encoders, or external storage is required.
• Indoor consumer use, 0 to 40 C target environment.
• The provisional enclosure and PCB dimensions require user approval before PCB layout.

• Initial production-intent brief created from the agreed RP2350/Pico 2, USB-MIDI, DIN MIDI, OLED, four-button, and LiPo requirements.