midi-router/docs/hardware-plan.md

Hardware Plan — flexible MIDI router (v1)

Status: draft 2026-08-21. Custom all-in-one PCB, RP2040 + CYW43439, 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).

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 + CYW43439 (BLE/WiFi)
           │                   │
           │                   ├── PIO0 SPI ── CYW43439 (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 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
CYW43 PIO-SPI	GP29 (CLK), GP24 (DAT), GP25 (CS-out), GP23 (PWR), PIO0, DMA CH0/CH1	copy Pico W
CYW43 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. CYW43439 RF section — the risk

Bare CYW43439 on a custom PCB is the single hardest part:

• QFN routing (0.4 mm pitch), 32.768 kHz + 37.4 MHz crystals, 50 Ω controlled- impedance RF trace, chip-antenna keepout (no copper under antenna).
• BLE transmits → FCC/CE intentional-radiator cert for commercial sale (less critical for personal use, but the radio must still work).
• Mitigation: Raspberry Pi open-sourced the Pico W schematic + PCB — lift the CYW43439 RF section verbatim (chip, crystals, RF match, antenna, keepout).

8. BOM sketch

• RP2040 (QFN-56), W25Q16JV flash (16 Mb), CYW43439, 32.768 kHz + 37.4 MHz crystals, chip antenna (Pico W part or equivalent).
• 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. CYW43439 RF + cert — biggest risk; copy the open Pico W RF section.
2. 0-10 V CV noise from the boost — LC filter + dedicated 4.096 V Vref.
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 — CYW43 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.

10. Build order (hardware)

1. Schematic (this doc is the spec).
2. CYW43439 RF section — copied from Pico W reference.
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.