# Hardware Plan — flexible MIDI router (v1)

Status: draft 2026-08-21. Custom all-in-one PCB, desktop, USB-powered, **bipolar
±10 V CV** via onboard ±12 V rails, 3.5 mm TRS MIDI, 4× stereo CV, 64×32 OLED +
tact buttons (no encoder, low profile).

**MCU target:** develop/test on **RP2040** (Pico W dev board — the firmware
already runs on it), product on **RP2350**. The RM2 radio module is compatible
with both. The Rust firmware builds for either via embassy-rp features
(`rp2040` → `thumbv6m-none-eabi`; `rp235xa` for the RP2350A used in the Pico 2 W
→ `thumbv8m.main-none-eabihf`). Pin assignments below are RP2040/Pico-W; the
RP2350 product PCB keeps the Pico-2-W-compatible RM2 bus and reuses the same
peripheral choices.

See `midi-router-architecture` (session memory) for the software/firmware
architecture this hardware serves.

## 1. Block diagram

```
USB-C 5V ──┬── 3.3 V buck ── RP2040 + W25Q16 flash + RM2 module (CYW43439, BLE/WiFi)
           │                   │
           │                   ├── PIO0 SPI ── RM2 (CYW43439 gSPI, Pico-W pinout)
           │                   ├── USB DP/DM ── (USB-MIDI composite, CDC+MIDI)
           │                   ├── UART0 ──── MIDI TRS in (6N138) / out
           │                   ├── I2C0 ───── Pimoroni RGB Keypad (buttons)
           │                   ├── SPI0 ───── Pimoroni RGB Keypad (APA102 LEDs)
           │                   ├── SPI1 ───── 8-ch 16-bit DAC (CV outs)
           │                   ├── I2C1 ───── 64×32 SSD1306 OLED
           │                   └── GPIOs ──── 4× tact buttons (UI)
           │
           └── ±12 V rails (boost + inverter) ── CV op-amps ── 4× stereo TRS (8 CV/gate)
                  │
   4.096 V ref ── DAC (0-4.096 V) ── op-amp gain ×4.88, offset −10 V ── ±10 V ── 1 k + bidir TVS ── TRS tip/ring
```

## 2. RP2040 pin map

Peripherals follow the Pico W reference for the RM2/CYW43 bus (so the cyw43
driver and our firmware work unchanged) and our existing keypad wiring.

| Function           | RP2040 pins            | Notes                                   |
|--------------------|------------------------|-----------------------------------------|
| USB device         | GP14 (DM), GP15 (DP)   | USB-C; CDC + MIDI composite             |
| RM2 (CYW43) bus     | GP29 (CLK), GP24 (DAT), GP25 (CS-out), GP23 (PWR), PIO0, DMA CH0/CH1 | copy Pico W |
| RM2 onboard LED    | CYW43 WL_GPIO0         | status/heartbeat                        |
| Keypad buttons     | GP4 (SDA), GP5 (SCL)   | I2C0, expander @0x20                    |
| Keypad LEDs        | GP17 (CS), GP18 (SCK), GP19 (MOSI) | SPI0 tx-only, 4 MHz, APA102    |
| DAC (CV outs)      | GP10 (SCK), GP11 (MOSI), GP13 (CS) | SPI1 tx-only, 16-bit DAC; MISO unused |
| DAC LDAC/RESET     | GP20, GP21             | GPIO (optional, immediate-update)       |
| MIDI TRS UART      | GP0 (TX), GP1 (RX)     | UART0 @31250 baud                       |
| OLED               | GP2 (SDA), GP3 (SCL)   | I2C1, SSD1306 64×32                     |
| UI buttons         | GP6, GP7, GP8, GP9     | 4× low-profile tact: Up/Down/Select/Back|
| SWD debug header   | SWCLK, SWDIO           | 2×3 header for Picoprobe (first-flash)  |
| Spare              | GP16, GP22, GP26-28    | status LEDs / future analog in          |

No conflicts; GP14/15 (USB) and GP23-25/29 (CYW43) kept clear of everything else.

## 3. Power tree (USB-only, with CV boost)

- USB-C 5 V → polyfuse + ESD → **VBUS_5V**.
- VBUS_5V → **3.3 V buck** (RT6150B-33GQW) → RP2040, RM2, flash, DAC VDD,
  OLED, pull-ups.
- VBUS_5V → **+12 V boost** (TPS61088 + LC filter) → +12 V CV op-amp rail.
- VBUS_5V → **−12 V inverter** (TPS63700 + LC filter) → −12 V CV op-amp rail.
  Both rails feed the bipolar CV op-amps. Low current (~20 mA each), LC-filtered.
- **4.096 V reference** (REF3040AIDBZ) for the DAC, and a **2.048 V mid-ref**
  (buffered divider or REF2040) for the bipolar offset — NOT the 3.3 V rail
  (too noisy for 1 V/oct pitch).

**Power budget (BLE-only):** we use Bluetooth only, never WiFi TX. The cyw43
driver must load the WiFi firmware to bring up BT (BT-only init isn't supported
yet), but WiFi never transmits → power stays at BLE levels, not WiFi-TX levels.

| Rail / consumer        | Typical   | Peak     |
|------------------------|-----------|----------|
| RM2 (BLE TX, no WiFi TX)| ~30 mA @3.3 V | ~80 mA |
| RP2040 / RP2350        | ~30 mA    | ~50 mA   |
| DAC8568                | ~2 mA     | ~2 mA    |
| OPA4192 ×2 (quiescent) | ~9 mA @±12 V | ~9 mA |
| OLED SSD1306           | ~20 mA    | ~40 mA   |
| ±12 V rails load (op-amps + DAC out) | ~20 mA @12 V → ~50 mA @5 V (×2 converters, ~85% eff) | ~60 mA @5 V |
| MIDI out LED (brief)   | —         | ~15 mA @3.3 V |
| **Total from 5 V**     | **~180-230 mA** | **~330-380 mA** |

Fits USB 2.0 (500 mA) with headroom; USB-C default (900 mA) is comfortable.
If WiFi TX were ever enabled, RM2 spikes to ~350 mA and the total approaches
~650 mA — stay BLE-only or require a USB-C 1.5 A/3 A source.

Rationale: USB-only power (single cable, no external PSU) while delivering
bipolar ±10 V CV via two small low-current rails. The LC filters + dedicated
Vref keep pitch clean.

## 4. CV analog section (4 stereo = 8 channels, bipolar ±10 V)

- **DAC:** 8-channel 16-bit SPI (DAC8568IAPW), Vref = 4.096 V → 0-4.096 V out.
  16-bit over ±10 V (20 V span) ≈ 0.3 mV/step ≈ 0.003 semitone at 1 V/oct —
  plenty for pitch.
- **Bipolar scaling:** each channel maps 0-4.096 V → −10 V…+10 V. Difference
  amp: `Vout = 4.88·(Vdac − 2.048)` where 2.048 V is the mid-reference
  (buffered divider from the 4.096 V ref, or REF2040). At Vdac=0 → −10 V,
  Vdac=2.048 → 0 V, Vdac=4.096 → +10 V. Per-channel gain/offset trimpot for
  pitch calibration (0.2 % ref → ~±20 mV untrimmed ≈ 0.02 semitone — trimmable
  to <1 mV).
- **Op-amps:** OPA4192IPWR (quad, RRO, 25 µV offset) ×2 on the ±12 V rails.
  RRO needed to reach ±10 V cleanly on ±12 V (TL074 clips at ±~10.5 V — too
  close; OPA4192 swings to ±11.9 V).
- **Per-channel model:** every CV channel is an independent
  `(midi_channel, function, polarity)` sink. The output stage is always
  bipolar-capable (±10 V); the function decides the mapping:
  - pitch → 0…+10 V (note/12, standard unipolar-positive on a bipolar output)
  - gate → 0/+10 V (or 0/+5 V)
  - velocity → 0…+10 V (vel/127·10)
  - CC (unipolar) → 0…+10 V; CC (bipolar) → −10…+10 V (cc/127·20 − 10)
  Gate = a DAC channel driven to 0/+10 V (keeps routing uniform).
- **Output protection:** 1 kΩ series + **bidirectional TVS** per output (outputs
  swing negative) to TRS tip/ring.

## 5. MIDI TRS (3.5 mm) — 2 jacks (in + out), TRS-A wiring

- **In:** tip → 220 Ω → 6N138 optocoupler LED → cathode to ring. Transistor:
  collector to 3.3 V via 1 k pull-up → UART0 RX (GP1); emitter to GND; base
  speed-up (1N4148 + 1 k). Isolated; powered from the input loop.
- **Out:** UART0 TX (GP0) → 220 Ω → tip; ring to 3.3 V. (3.3 V/220 Ω ≈ 15 mA,
  fine for MIDI-out LED.)
- TRS-A: tip = signal, ring = return (Korg / Make Noise convention).

## 6. Display + UI

- **OLED:** SSD1306 64×32 (0.42"), I2C1. Small — UI is paginated (≈2 lines of
  small text); routing/CV assignments shown a few at a time.
- **Buttons:** 4× low-profile SMD tact (Up / Down / Select / Back) on GP6-9. No
  rotary encoder (height constraint).

## 7. RM2 radio module — risk largely eliminated

Use the **Raspberry Pi Radio Module 2 (RM2, part RMC20452T)**: a CYW43439-based
module (16.5 × 14.5 mm, 21 castellated pads) with integrated inverted-F PCB
antenna and internal PA/LNA/T-R switch. Full modular radio approval in US/EU/
UK/CA (FCC ID 2ABCB-RMC2GW4B52) — no radio design or certification needed on
the host board. Software-compatible with the Pico W SDK, so our cyw43 firmware
works unchanged.

Host PCB responsibilities (from the RM2 datasheet):
- Mount the 21 castellated pads; route the gSPI host pins to RP2040 (Pico W
  pinout: CLK=GP29, DAT=GP24, CS=GP25, PWR=GP23, on PIO0 + DMA CH0/CH1).
- 3.3 V supply (no other rails).
- **Antenna keepout area** must remain free of metal per the datasheet for
  the module's PCB antenna to perform.

## 8. BOM (concrete parts)

| Ref | Part | Package | Notes |
|-----|------|---------|-------|
| MCU | RP2040 (dev) / RP2350A (product) | QFN-56 / QFN-60 | dual target via embassy-rp features |
| Flash | W25Q32JVSSIQ | SOIC-8 | 32 Mb (4 MB) — Rust firmware + blobs ~870 KB UF2, headroom for router code |
| Radio | RM2 module RMC20452T | 16.5×14.5 mm, 21-pad | CYW43439 + PCB antenna, pre-certified |
| DAC | DAC8568IAPW | TSSOP-16 | 8-ch 16-bit SPI, external Vref mode (AD5668BRUZ-1 alt) |
| Op-amp | OPA4192IPWR ×2 | TSSOP-14 | quad RRO, 25 µV offset, on ±12 V rails — pitch-accurate |
| Vref | REF3040AIDBZ | SOT-23 | 4.096 V DAC ref, 0.2%, 30 ppm/°C |
| Mid-ref | REF2040AIDBZ (or buffered divider) | SOT-23 | 2.048 V bipolar offset reference |
| Calib | 8× trimpot (e.g. 3296W) | THT/SMD | per-channel gain/offset trim |
| MIDI opto | 6N138 | DIP-8 / SMD | MIDI in isolation |
| 3.3 V reg | RT6150B-33GQW | QFN | buck-boost, 800 mA, Pico-proven (TPS62A01 alt) |
| +12 V boost | TPS61088 | QFN | 5→12 V, low-noise + LC filter |
| −12 V inverter | TPS63700 | SOT-23-6 / QFN | 5→−12 V, low-noise + LC filter (LM2611 alt) |
| Display | SSD1306 64×32 OLED | 0.42" I2C | — |
| Jacks | 6× 3.5 mm TRS | SMD | 2 MIDI + 4 CV stereo |
| USB | USB-C receptacle | SMD | 5 V data + power |
| UI | 4× low-profile tact | SMD | Up/Down/Select/Back (no encoder) |
| Debug | 2×3 SWD header | THT | Picoprobe first-flash |
| — | polyfuse, bidirectional TVS array, passives | — | bipolar output protection + rail protection |

## 9. Risks / open items

1. **Bipolar ±10 V offset/gain calibration** — the 2.048 V mid-ref and ×4.88
   gain set the 0 V center and ±10 V span; 0.2 % ref → ~±20 mV untrimmed
   (≈0.02 semitone). Per-channel trimpots trim gain + offset to <1 mV; or
   calibrate in firmware (measure + store correction) to avoid trimpots.
2. **±12 V rail noise on pitch** — two switching converters (+12 V boost,
   −12 V inverter); LC filter both rails + 4.096 V/2.048 V refs are the pitch
   reference, not the rails, so op-amp PSRR + filtering keeps it clean.
3. **Custom RP2040 bring-up** — needs a SWD header + Picoprobe; USB bootloader
   only works after first flash.
4. **64×32 OLED UI** — tight; menu must be paginated and minimal.
5. **Boost current budget** — RM2 TX spikes + 8 CV op-amps + two converters on
   a single USB 5 V/500 mA port is tight; BLE-only keeps it ~330-380 mA peak
   (fits), but specify USB-C 5 V/900 mA for headroom.
6. **RM2 antenna keepout** — follow the datasheet keepout; the one layout
   constraint the radio imposes.

## 10. Build order (hardware)

1. Schematic (this doc is the spec).
2. RM2 module footprint + antenna keepout.
3. RP2040 + flash + USB + SWD.
4. Power (3.3 V buck, +12 V boost, −12 V inverter, 4.096 V + 2.048 V refs).
5. CV analog (DAC + difference-amp + offset + trim + bidir protection + TRS).
6. MIDI TRS circuit.
7. OLED + buttons.
8. Keypad header (the Pimoroni keypad connects via a header or is integrated).
9. PCB layout, DRC, review, fab.