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9123f7691d
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.
156 lines
8.0 KiB
Markdown
156 lines
8.0 KiB
Markdown
# Hardware Plan — flexible MIDI router (v1)
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Status: draft 2026-08-21. Custom all-in-one PCB, desktop, USB-powered, 0-10 V
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CV via onboard boost, 3.5 mm TRS MIDI, 4× stereo CV, 64×32 OLED + tact buttons
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(no encoder, low profile).
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**MCU target:** develop/test on **RP2040** (Pico W dev board — the firmware
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already runs on it), product on **RP2350**. The RM2 radio module is compatible
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with both. The Rust firmware builds for either via embassy-rp features
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(`rp2040` → `thumbv6m-none-eabi`; `rp235xa` for the RP2350A used in the Pico 2 W
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→ `thumbv8m.main-none-eabihf`). Pin assignments below are RP2040/Pico-W; the
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RP2350 product PCB keeps the Pico-2-W-compatible RM2 bus and reuses the same
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peripheral choices.
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See `midi-router-architecture` (session memory) for the software/firmware
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architecture this hardware serves.
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## 1. Block diagram
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```
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USB-C 5V ──┬── 3.3 V buck ── RP2040 + W25Q16 flash + RM2 module (CYW43439, BLE/WiFi)
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│ │
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│ ├── PIO0 SPI ── RM2 (CYW43439 gSPI, Pico-W pinout)
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│ ├── USB DP/DM ── (USB-MIDI composite, CDC+MIDI)
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│ ├── UART0 ──── MIDI TRS in (6N138) / out
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│ ├── I2C0 ───── Pimoroni RGB Keypad (buttons)
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│ ├── SPI0 ───── Pimoroni RGB Keypad (APA102 LEDs)
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│ ├── SPI1 ───── 8-ch 16-bit DAC (CV outs)
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│ ├── I2C1 ───── 64×32 SSD1306 OLED
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│ └── GPIOs ──── 4× tact buttons (UI)
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│
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└── 12 V boost ── CV op-amp rails (+12 V) ── 4× stereo TRS (8 CV/gate)
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│
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4.096 V ref ── DAC ── op-amp gain ×2.44 ── 0-10 V ── 1 k series + TVS ── TRS tip/ring
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```
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## 2. RP2040 pin map
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Peripherals follow the Pico W reference for the RM2/CYW43 bus (so the cyw43
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driver and our firmware work unchanged) and our existing keypad wiring.
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| Function | RP2040 pins | Notes |
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|--------------------|------------------------|-----------------------------------------|
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| USB device | GP14 (DM), GP15 (DP) | USB-C; CDC + MIDI composite |
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| RM2 (CYW43) bus | GP29 (CLK), GP24 (DAT), GP25 (CS-out), GP23 (PWR), PIO0, DMA CH0/CH1 | copy Pico W |
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| RM2 onboard LED | CYW43 WL_GPIO0 | status/heartbeat |
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| Keypad buttons | GP4 (SDA), GP5 (SCL) | I2C0, expander @0x20 |
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| Keypad LEDs | GP17 (CS), GP18 (SCK), GP19 (MOSI) | SPI0 tx-only, 4 MHz, APA102 |
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| DAC (CV outs) | GP10 (SCK), GP11 (MOSI), GP13 (CS) | SPI1 tx-only, 16-bit DAC; MISO unused |
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| DAC LDAC/RESET | GP20, GP21 | GPIO (optional, immediate-update) |
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| MIDI TRS UART | GP0 (TX), GP1 (RX) | UART0 @31250 baud |
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| OLED | GP2 (SDA), GP3 (SCL) | I2C1, SSD1306 64×32 |
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| UI buttons | GP6, GP7, GP8, GP9 | 4× low-profile tact: Up/Down/Select/Back|
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| SWD debug header | SWCLK, SWDIO | 2×3 header for Picoprobe (first-flash) |
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| Spare | GP16, GP22, GP26-28 | status LEDs / future analog in |
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No conflicts; GP14/15 (USB) and GP23-25/29 (CYW43) kept clear of everything else.
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## 3. Power tree (USB-only, with CV boost)
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- USB-C 5 V → polyfuse + ESD → **VBUS_5V**.
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- VBUS_5V → **3.3 V buck** (RT6150 / TPS62A01) → RP2040, CYW43, flash, DAC VDD,
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OLED, pull-ups. Budget ~400 mA peak (CYW43 BT/WiFi TX spikes).
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- VBUS_5V → **12 V boost** (MT3608 / TPS61088, ~80 mA) → +12 V CV op-amp rail,
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LC-filtered (pitch noise).
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- **4.096 V reference** (REF3040 / LM4040-4.096) for the DAC — NOT the 3.3 V
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rail (too noisy for 1 V/oct pitch).
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Rationale: USB-only power (single cable, no external PSU) while still delivering
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0-10 V CV via a small boost. The boost + LC filter + dedicated Vref keeps pitch
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clean. If 0-5 V (5 octaves) is acceptable, drop the boost and use RRO op-amps
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on 5 V (simpler, fewer parts, but half the CV range).
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## 4. CV analog section (4 stereo = 8 channels)
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- **DAC:** 8-channel 16-bit SPI (DAC8568 or AD5668), Vref = 4.096 V → 0-4.096 V out.
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16-bit over 0-10 V ≈ 0.15 mV/step ≈ 0.002 semitone — plenty for 1 V/oct pitch.
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- **Scaling:** non-inverting op-amp, gain ×2.44 → 0-10 V from the 4.096 V DAC
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span, rail = +12 V. Needs rail-to-rail-output (TL074 is NOT RRO and can't do
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0-10 V on a single +12 V rail). Use **OPA4192** (quad RRO) ×2 for 8 channels,
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or TLV9154.
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- **Per-channel model:** every CV channel is an independent
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`(midi_channel, function)` sink (pitch/gate/velocity/CC). Gate = DAC channel
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driven to 0 or 10 V (keeps the routing uniform; no separate gate GPIOs).
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- **Output protection:** 1 kΩ series + TVS per output to TRS tip/ring.
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## 5. MIDI TRS (3.5 mm) — 2 jacks (in + out), TRS-A wiring
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- **In:** tip → 220 Ω → 6N138 optocoupler LED → cathode to ring. Transistor:
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collector to 3.3 V via 1 k pull-up → UART0 RX (GP1); emitter to GND; base
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speed-up (1N4148 + 1 k). Isolated; powered from the input loop.
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- **Out:** UART0 TX (GP0) → 220 Ω → tip; ring to 3.3 V. (3.3 V/220 Ω ≈ 15 mA,
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fine for MIDI-out LED.)
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- TRS-A: tip = signal, ring = return (Korg / Make Noise convention).
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## 6. Display + UI
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- **OLED:** SSD1306 64×32 (0.42"), I2C1. Small — UI is paginated (≈2 lines of
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small text); routing/CV assignments shown a few at a time.
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- **Buttons:** 4× low-profile SMD tact (Up / Down / Select / Back) on GP6-9. No
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rotary encoder (height constraint).
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## 7. RM2 radio module — risk largely eliminated
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Use the **Raspberry Pi Radio Module 2 (RM2, part RMC20452T)**: a CYW43439-based
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module (16.5 × 14.5 mm, 21 castellated pads) with integrated inverted-F PCB
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antenna and internal PA/LNA/T-R switch. Full modular radio approval in US/EU/
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UK/CA (FCC ID 2ABCB-RMC2GW4B52) — no radio design or certification needed on
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the host board. Software-compatible with the Pico W SDK, so our cyw43 firmware
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works unchanged.
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Host PCB responsibilities (from the RM2 datasheet):
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- Mount the 21 castellated pads; route the gSPI host pins to RP2040 (Pico W
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pinout: CLK=GP29, DAT=GP24, CS=GP25, PWR=GP23, on PIO0 + DMA CH0/CH1).
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- 3.3 V supply (no other rails).
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- **Antenna keepout area** must remain free of metal per the datasheet for
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the module's PCB antenna to perform.
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## 8. BOM sketch
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- RP2040 (dev) / **RP2350A** (product) MCU, W25Q16JV-equivalent flash,
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**RM2 radio module (RMC20452T, CYW43439 + PCB antenna, pre-certified)**.
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- DAC8568IAPW (8-ch 16-bit SPI, TSSOP-16) — or AD5668.
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- OPA4192IPW ×2 (quad RRO op-amp, TSSOP-14).
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- REF3040AIDBZ (4.096 V ref, SOT-23).
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- 6N138 optocoupler (MIDI in).
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- USB-C receptacle; 6× 3.5 mm TRS jacks (2 MIDI + 4 CV stereo).
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- 3.3 V buck (RT6150/TPS62A01), 12 V boost (MT3608/TPS61088).
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- SSD1306 64×32 OLED; 4× low-profile SMD tact switches.
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- SWD 2×3 header (Picoprobe for first-flash).
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- Passives, polyfuse, TVS array.
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## 9. Risks / open items
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1. **0-10 V CV noise from the boost** — LC filter + dedicated 4.096 V Vref.
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2. **Custom RP2040 bring-up** — needs a SWD header + Picoprobe; USB bootloader
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only works after first flash.
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3. **64×32 OLED UI** — tight; menu must be paginated and minimal.
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4. **Boost current budget** — RM2 TX spikes + 8 CV op-amps on a single USB
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5 V/500 mA (USB 2.0) port is tight; specify USB-C 5 V/900 mA-3 A, or limit
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CV op-amp current.
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5. **RM2 antenna keepout** — follow the datasheet keepout; the one layout
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constraint the radio imposes.
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## 10. Build order (hardware)
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1. Schematic (this doc is the spec).
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2. RM2 module footprint + antenna keepout.
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3. RP2040 + flash + USB + SWD.
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4. Power (3.3 V buck, 12 V boost, Vref).
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5. CV analog (DAC + op-amps + protection + TRS).
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6. MIDI TRS circuit.
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7. OLED + buttons.
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8. Keypad header (the Pimoroni keypad connects via a header or is integrated).
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9. PCB layout, DRC, review, fab.
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