midi-router/docs/hardware-plan.md

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.