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midi-router/docs/hardware-plan.md
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nils 9123f7691d Hardware plan: RM2 module, RP2040->RP2350 target 
Use the pre-certified Raspberry Pi Radio Module 2 (RM2, CYW43439 + PCB
antenna) instead of a bare CYW43439 — eliminates RF design + cert; only
the datasheet antenna keepout remains. Dev/test on RP2040 (Pico W),
product on RP2350; firmware dual-builds via embassy-rp features.
2026-08-21 10:54:24 +02:00

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Hardware Plan — flexible MIDI router (v1)

Status: draft 2026-08-21. Custom all-in-one PCB, desktop, USB-powered, 0-10 V CV via onboard boost, 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 boost ── CV op-amp rails (+12 V) ── 4× stereo TRS (8 CV/gate)
                  │
   4.096 V ref ── DAC ── op-amp gain ×2.44 ── 0-10 V ── 1 k series + 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 (RT6150 / TPS62A01) → RP2040, CYW43, flash, DAC VDD, OLED, pull-ups. Budget ~400 mA peak (CYW43 BT/WiFi TX spikes).
  • VBUS_5V → 12 V boost (MT3608 / TPS61088, ~80 mA) → +12 V CV op-amp rail, LC-filtered (pitch noise).
  • 4.096 V reference (REF3040 / LM4040-4.096) for the DAC — NOT the 3.3 V rail (too noisy for 1 V/oct pitch).

Rationale: USB-only power (single cable, no external PSU) while still delivering 0-10 V CV via a small boost. The boost + LC filter + dedicated Vref keeps pitch clean. If 0-5 V (5 octaves) is acceptable, drop the boost and use RRO op-amps on 5 V (simpler, fewer parts, but half the CV range).

4. CV analog section (4 stereo = 8 channels)

  • DAC: 8-channel 16-bit SPI (DAC8568 or AD5668), Vref = 4.096 V → 0-4.096 V out. 16-bit over 0-10 V ≈ 0.15 mV/step ≈ 0.002 semitone — plenty for 1 V/oct pitch.
  • Scaling: non-inverting op-amp, gain ×2.44 → 0-10 V from the 4.096 V DAC span, rail = +12 V. Needs rail-to-rail-output (TL074 is NOT RRO and can't do 0-10 V on a single +12 V rail). Use OPA4192 (quad RRO) ×2 for 8 channels, or TLV9154.
  • Per-channel model: every CV channel is an independent (midi_channel, function) sink (pitch/gate/velocity/CC). Gate = DAC channel driven to 0 or 10 V (keeps the routing uniform; no separate gate GPIOs).
  • Output protection: 1 kΩ series + TVS per output 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 sketch

  • RP2040 (dev) / RP2350A (product) MCU, W25Q16JV-equivalent flash, RM2 radio module (RMC20452T, CYW43439 + PCB antenna, pre-certified).
  • DAC8568IAPW (8-ch 16-bit SPI, TSSOP-16) — or AD5668.
  • OPA4192IPW ×2 (quad RRO op-amp, TSSOP-14).
  • REF3040AIDBZ (4.096 V ref, SOT-23).
  • 6N138 optocoupler (MIDI in).
  • USB-C receptacle; 6× 3.5 mm TRS jacks (2 MIDI + 4 CV stereo).
  • 3.3 V buck (RT6150/TPS62A01), 12 V boost (MT3608/TPS61088).
  • SSD1306 64×32 OLED; 4× low-profile SMD tact switches.
  • SWD 2×3 header (Picoprobe for first-flash).
  • Passives, polyfuse, TVS array.

9. Risks / open items

  1. 0-10 V CV noise from the boost — LC filter + dedicated 4.096 V Vref.
  2. Custom RP2040 bring-up — needs a SWD header + Picoprobe; USB bootloader only works after first flash.
  3. 64×32 OLED UI — tight; menu must be paginated and minimal.
  4. Boost current budget — RM2 TX spikes + 8 CV op-amps on a single USB 5 V/500 mA (USB 2.0) port is tight; specify USB-C 5 V/900 mA-3 A, or limit CV op-amp current.
  5. 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, Vref).
  5. CV analog (DAC + op-amps + 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.
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