nils
afb07dd1c0
- Power budget (BLE-only, never WiFi TX) with concrete mA table. - Bipolar CV section: ±12V rails (boost + inverter), DAC 0-4.096V -> ±10V via difference amp with 2.048V mid-ref, per-channel gain/offset trim, OPA4192 on ±12V, bidirectional TVS. Function+polarity decides mapping (pitch stays 0..+10V standard on a bipolar output). - Concrete BOM table (part numbers, packages, alternates). - Risks updated for bipolar calibration + ±12V rail noise. - Build order updated.
203 lines
11 KiB
Markdown
203 lines
11 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, **bipolar
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±10 V CV** via onboard ±12 V rails, 3.5 mm TRS MIDI, 4× stereo CV, 64×32 OLED +
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tact buttons (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 rails (boost + inverter) ── CV op-amps ── 4× stereo TRS (8 CV/gate)
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│
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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
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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** (RT6150B-33GQW) → RP2040, RM2, flash, DAC VDD,
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OLED, pull-ups.
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- VBUS_5V → **+12 V boost** (TPS61088 + LC filter) → +12 V CV op-amp rail.
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- VBUS_5V → **−12 V inverter** (TPS63700 + LC filter) → −12 V CV op-amp rail.
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Both rails feed the bipolar CV op-amps. Low current (~20 mA each), LC-filtered.
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- **4.096 V reference** (REF3040AIDBZ) for the DAC, and a **2.048 V mid-ref**
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(buffered divider or REF2040) for the bipolar offset — NOT the 3.3 V rail
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(too noisy for 1 V/oct pitch).
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**Power budget (BLE-only):** we use Bluetooth only, never WiFi TX. The cyw43
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driver must load the WiFi firmware to bring up BT (BT-only init isn't supported
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yet), but WiFi never transmits → power stays at BLE levels, not WiFi-TX levels.
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| Rail / consumer | Typical | Peak |
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|------------------------|-----------|----------|
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| RM2 (BLE TX, no WiFi TX)| ~30 mA @3.3 V | ~80 mA |
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| RP2040 / RP2350 | ~30 mA | ~50 mA |
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| DAC8568 | ~2 mA | ~2 mA |
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| OPA4192 ×2 (quiescent) | ~9 mA @±12 V | ~9 mA |
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| OLED SSD1306 | ~20 mA | ~40 mA |
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| ±12 V rails load (op-amps + DAC out) | ~20 mA @12 V → ~50 mA @5 V (×2 converters, ~85% eff) | ~60 mA @5 V |
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| MIDI out LED (brief) | — | ~15 mA @3.3 V |
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| **Total from 5 V** | **~180-230 mA** | **~330-380 mA** |
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Fits USB 2.0 (500 mA) with headroom; USB-C default (900 mA) is comfortable.
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If WiFi TX were ever enabled, RM2 spikes to ~350 mA and the total approaches
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~650 mA — stay BLE-only or require a USB-C 1.5 A/3 A source.
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Rationale: USB-only power (single cable, no external PSU) while delivering
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bipolar ±10 V CV via two small low-current rails. The LC filters + dedicated
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Vref keep pitch clean.
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## 4. CV analog section (4 stereo = 8 channels, bipolar ±10 V)
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- **DAC:** 8-channel 16-bit SPI (DAC8568IAPW), Vref = 4.096 V → 0-4.096 V out.
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16-bit over ±10 V (20 V span) ≈ 0.3 mV/step ≈ 0.003 semitone at 1 V/oct —
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plenty for pitch.
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- **Bipolar scaling:** each channel maps 0-4.096 V → −10 V…+10 V. Difference
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amp: `Vout = 4.88·(Vdac − 2.048)` where 2.048 V is the mid-reference
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(buffered divider from the 4.096 V ref, or REF2040). At Vdac=0 → −10 V,
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Vdac=2.048 → 0 V, Vdac=4.096 → +10 V. Per-channel gain/offset trimpot for
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pitch calibration (0.2 % ref → ~±20 mV untrimmed ≈ 0.02 semitone — trimmable
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to <1 mV).
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- **Op-amps:** OPA4192IPWR (quad, RRO, 25 µV offset) ×2 on the ±12 V rails.
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RRO needed to reach ±10 V cleanly on ±12 V (TL074 clips at ±~10.5 V — too
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close; OPA4192 swings to ±11.9 V).
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- **Per-channel model:** every CV channel is an independent
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`(midi_channel, function, polarity)` sink. The output stage is always
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bipolar-capable (±10 V); the function decides the mapping:
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- pitch → 0…+10 V (note/12, standard unipolar-positive on a bipolar output)
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- gate → 0/+10 V (or 0/+5 V)
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- velocity → 0…+10 V (vel/127·10)
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- CC (unipolar) → 0…+10 V; CC (bipolar) → −10…+10 V (cc/127·20 − 10)
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Gate = a DAC channel driven to 0/+10 V (keeps routing uniform).
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- **Output protection:** 1 kΩ series + **bidirectional TVS** per output (outputs
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swing negative) 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 (concrete parts)
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| Ref | Part | Package | Notes |
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|-----|------|---------|-------|
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| MCU | RP2040 (dev) / RP2350A (product) | QFN-56 / QFN-60 | dual target via embassy-rp features |
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| Flash | W25Q32JVSSIQ | SOIC-8 | 32 Mb (4 MB) — Rust firmware + blobs ~870 KB UF2, headroom for router code |
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| Radio | RM2 module RMC20452T | 16.5×14.5 mm, 21-pad | CYW43439 + PCB antenna, pre-certified |
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| DAC | DAC8568IAPW | TSSOP-16 | 8-ch 16-bit SPI, external Vref mode (AD5668BRUZ-1 alt) |
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| Op-amp | OPA4192IPWR ×2 | TSSOP-14 | quad RRO, 25 µV offset, on ±12 V rails — pitch-accurate |
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| Vref | REF3040AIDBZ | SOT-23 | 4.096 V DAC ref, 0.2%, 30 ppm/°C |
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| Mid-ref | REF2040AIDBZ (or buffered divider) | SOT-23 | 2.048 V bipolar offset reference |
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| Calib | 8× trimpot (e.g. 3296W) | THT/SMD | per-channel gain/offset trim |
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| MIDI opto | 6N138 | DIP-8 / SMD | MIDI in isolation |
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| 3.3 V reg | RT6150B-33GQW | QFN | buck-boost, 800 mA, Pico-proven (TPS62A01 alt) |
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| +12 V boost | TPS61088 | QFN | 5→12 V, low-noise + LC filter |
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| −12 V inverter | TPS63700 | SOT-23-6 / QFN | 5→−12 V, low-noise + LC filter (LM2611 alt) |
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| Display | SSD1306 64×32 OLED | 0.42" I2C | — |
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| Jacks | 6× 3.5 mm TRS | SMD | 2 MIDI + 4 CV stereo |
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| USB | USB-C receptacle | SMD | 5 V data + power |
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| UI | 4× low-profile tact | SMD | Up/Down/Select/Back (no encoder) |
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| Debug | 2×3 SWD header | THT | Picoprobe first-flash |
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| — | polyfuse, bidirectional TVS array, passives | — | bipolar output protection + rail protection |
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## 9. Risks / open items
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1. **Bipolar ±10 V offset/gain calibration** — the 2.048 V mid-ref and ×4.88
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gain set the 0 V center and ±10 V span; 0.2 % ref → ~±20 mV untrimmed
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(≈0.02 semitone). Per-channel trimpots trim gain + offset to <1 mV; or
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calibrate in firmware (measure + store correction) to avoid trimpots.
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2. **±12 V rail noise on pitch** — two switching converters (+12 V boost,
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−12 V inverter); LC filter both rails + 4.096 V/2.048 V refs are the pitch
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reference, not the rails, so op-amp PSRR + filtering keeps it clean.
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3. **Custom RP2040 bring-up** — needs a SWD header + Picoprobe; USB bootloader
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only works after first flash.
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4. **64×32 OLED UI** — tight; menu must be paginated and minimal.
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5. **Boost current budget** — RM2 TX spikes + 8 CV op-amps + two converters on
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a single USB 5 V/500 mA port is tight; BLE-only keeps it ~330-380 mA peak
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(fits), but specify USB-C 5 V/900 mA for headroom.
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6. **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, −12 V inverter, 4.096 V + 2.048 V refs).
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5. CV analog (DAC + difference-amp + offset + trim + bidir 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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